1 hooks使命
#逻辑组件复用
逻辑与UI组件分离
React 官方推荐在开发中将逻辑部分与视图部分结耦，便于定位问题和职责清晰
函数组件拥有state
在函数组件中如果要实现类似拥有state的状态，必须要将组件转成class组件
逻辑组件复用
社区一直致力于逻辑层面的复用，像 render props / HOC，不过它们都有对应的问题，Hooks是目前为止相对完美的解决方案
#hooks 解决的问题
render props
Avator 组件是一个渲染头像的组件，里面包含其中一些业务逻辑，User组件是纯ui组件，用于展示用户昵称
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">export</span> <span style="color:#cc99cd">default</span> funtion <span style="color:#f8c555">APP</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span> <span style="color:#e2777a">className</span><span style="color:#7ec699"><span style="color:#cccccc">=</span><span style="color:#cccccc">"</span>App<span style="color:#cccccc">"</span></span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span><span style="color:#f8c555">Avatar</span></span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">{</span>data<span style="color:#67cdcc">=></span> <span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span><span style="color:#f8c555">User</span></span> <span style="color:#e2777a">name</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span>data<span style="color:#cccccc">}</span><span style="color:#cccccc">/></span></span><span style="color:#cccccc">}</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span><span style="color:#f8c555">Avatar</span></span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>- 通过渲染props来实现逻辑组件复用
- render props 通过嵌套组件实现，在真实的业务中，会出现嵌套多层，以及梭理props不清晰的问题
Hoc
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">class</span> <span style="color:#f8c555">Avatar</span> <span style="color:#cc99cd">extends</span> <span style="color:#f8c555">Component</span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">render</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span><span style="color:#cccccc">{</span><span style="color:#cc99cd">this</span><span style="color:#cccccc">.</span>props<span style="color:#cccccc">.</span>name<span style="color:#cccccc">}</span><span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
funtion <span style="color:#f08d49">HocAvatar</span><span style="color:#cccccc">(</span>Component<span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span> <span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span><span style="color:#f8c555">Component</span></span> <span style="color:#e2777a">name</span><span style="color:#7ec699"><span style="color:#cccccc">=</span><span style="color:#cccccc">'</span>王艺瑾<span style="color:#cccccc">'</span></span><span style="color:#cccccc">/></span></span>
<span style="color:#cccccc">}</span>
</code></span></span></span>- 通过对现有组件进行扩展、增强的方式来实现复用，通常采用包裹方法来实现
- 高阶组件的实现会额外地增加元素层级，使得页面元素的数量更加臃肿
Hooks
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">import</span> React<span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useState<span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">function</span> <span style="color:#f08d49">HooksAvatar</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>name<span style="color:#cccccc">,</span>setName<span style="color:#cccccc">]</span><span style="color:#67cdcc">=</span><span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'王一瑾'</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span></span><span style="color:#cccccc">></span></span><span style="color:#cccccc">{</span>name<span style="color:#cccccc">}</span><span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span></span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">}</span>
</code></span></span></span>- React 16.8引入的Hooks，使得实现相同功能而代码量更少成为现实
- 通过使用Hooks，不仅在编码层面减少代码的数量，同样在编译之后的代码也会更少
#2 hooks实践
#Hook官方APi（大概率用到的）
- useState 函数组件中的state方法
- useEffect 函数组件处理副作用的方法，什么是副作用？异步请求、订阅原生的dom实事件、setTimeoutd等
- useContext 接受一个context对象（React.createContext的返回值）并返回该context的当前值，当前的context由上层组件中距离最近的
<Mycontext.provider></Mycontext.provider>的value prop决定 - useReducer 另一种"useState"，跟redux有点类似
- useRef 返回一个突变的ref对象，对象在函数的生命周期内一直存在
- useMemo 缓存数值
- useCallback 缓存函数
- useCustom 自定义Hooks组件
- useState
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">import</span> React<span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useState<span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">HooksTest</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 声明一个count的state变量，useState可以给一个默认值</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">,</span>setCount<span style="color:#cccccc">]</span><span style="color:#67cdcc">=</span><span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">0</span><span style="color:#cccccc">)</span>
<span style="color:#999999">/*
useState也可以传递一个函数，
const [count,setCount]=useState(()=>{
return 2
})
setCount也可以传递一个函数
这个函数第一个参数可以拿到上一次的值，
在可以在函数里做一些操作
setCount((preState)=>{
return {...preState,..updatedValues}
})
*/</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">{</span><span style="color:#999999">/*通过setCount来改变count的值*/</span><span style="color:#cccccc">}</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>button</span> <span style="color:#e2777a">onClick</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span>count<span style="color:#67cdcc">+</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">}</span>
<span style="color:#cccccc">></span></span>Add<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>button</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">{</span>count<span style="color:#cccccc">}</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
)
}
</code></span></span></span>- useEffect
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">import</span> React<span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useEffect<span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// 我们可以把useEffect 看做componentDidmount、componentDidUpdate、componntWillUnmount</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">HooksTest</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">,</span> setCount<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">0</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// useEffect可以让你在第一个参数的函数中执行副作用操作，就是请求数据，dom操作之类的</span>
<span style="color:#999999">// useEffect返回一个函数，函数里表示要清除的副作用，例如清除定时器,返回的函数会在卸载组件时执行</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
document<span style="color:#cccccc">.</span>title <span style="color:#67cdcc">=</span> <span style="color:#7ec699">`</span><span style="color:#7ec699">You clicked </span><span style="color:#cccccc">${</span>count<span style="color:#cccccc">}</span><span style="color:#7ec699"> times</span><span style="color:#7ec699">`</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">clearInterval</span><span style="color:#cccccc">(</span>timer<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
<span style="color:#999999">/*
useEffect的第二个参数，通过在数组中传递值，例如只有count变化时才调用Effect，达到
不用每次渲染后都执行清理或执行effect导致的性能问题
*/</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
document<span style="color:#cccccc">.</span>title <span style="color:#67cdcc">=</span> <span style="color:#7ec699">`</span><span style="color:#7ec699">You clicked </span><span style="color:#cccccc">${</span>count<span style="color:#cccccc">}</span><span style="color:#7ec699"> times</span><span style="color:#7ec699">`</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span>count<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#999999">/*
如果想执行只运行一次的effect（仅在组件挂载和卸载时执行），可以传递一个空数组，
告诉React你的Effect不依赖与props或state中任何值
*/</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
document<span style="color:#cccccc">.</span>title <span style="color:#67cdcc">=</span> <span style="color:#7ec699">`</span><span style="color:#7ec699">You clicked </span><span style="color:#cccccc">${</span>count<span style="color:#cccccc">}</span><span style="color:#7ec699"> times</span><span style="color:#7ec699">`</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#999999">/*
可以使用多个Effect，将不相关的逻辑分离到不同的effect中
*/</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
axios<span style="color:#cccccc">.</span><span style="color:#f08d49">get</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'login'</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span><span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>p</span><span style="color:#cccccc">></span></span>You clicked <span style="color:#cccccc">{</span>count<span style="color:#cccccc">}</span> times<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>p</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>button</span> <span style="color:#e2777a">onClick</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span>count <span style="color:#67cdcc">+</span> <span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">></span></span>
Click me
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>button</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>- useContext
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">// 1. 创建一个上下文管理组件context-manager.js，用于统一导出context实例</span>
<span style="color:#cc99cd">import</span> React <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">const</span> ItemsContext <span style="color:#67cdcc">=</span> React<span style="color:#cccccc">.</span><span style="color:#f08d49">createContext</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span> name<span style="color:#67cdcc">:</span> <span style="color:#7ec699">''</span> <span style="color:#cccccc">}</span><span style="color:#cccccc">)</span> <span style="color:#999999">//接受一个默认值</span>
<span style="color:#999999">// 2. 父组件提供数据</span>
<span style="color:#cc99cd">import</span> React <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">import</span> Child <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'./child'</span>
<span style="color:#cc99cd">import</span> <span style="color:#cccccc">{</span> ItemsContext <span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'./context-manager'</span>
<span style="color:#cc99cd">import</span> <span style="color:#7ec699">'./index.scss'</span>
<span style="color:#cc99cd">const</span> items <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span> name<span style="color:#67cdcc">:</span> <span style="color:#7ec699">'测试'</span> <span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">Father</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span> <span style="color:#e2777a">className</span><span style="color:#7ec699"><span style="color:#cccccc">=</span><span style="color:#cccccc">'</span>father<span style="color:#cccccc">'</span></span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span><span style="color:#f8c555">ItemsContext.Provider</span></span> <span style="color:#e2777a">value</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span>items<span style="color:#cccccc">}</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span><span style="color:#f8c555">Child</span></span><span style="color:#cccccc">></span></span><span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span><span style="color:#f8c555">Child</span></span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span><span style="color:#f8c555">ItemsContext.Provider</span></span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">default</span> Father
<span style="color:#999999">// 3.子组件用useContext解析上下文</span>
<span style="color:#cc99cd">import</span> React <span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useContext<span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">import</span> <span style="color:#cccccc">{</span> ItemsContext <span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'./context-manager'</span>
<span style="color:#cc99cd">import</span> <span style="color:#7ec699">'./index.scss'</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">Child</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> items<span style="color:#67cdcc">=</span><span style="color:#f08d49">useContext</span><span style="color:#cccccc">(</span>ItemsContext<span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span> <span style="color:#e2777a">className</span><span style="color:#7ec699"><span style="color:#cccccc">=</span><span style="color:#cccccc">'</span>child<span style="color:#cccccc">'</span></span><span style="color:#cccccc">></span></span>
子组件
<span style="color:#cccccc">{</span>items<span style="color:#cccccc">.</span>name<span style="color:#cccccc">}</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">default</span> Child
</code></span></span></span>- useReducer
useReducer是useState的替代方案，它接受一个形如(state,action)=>newState的reducer，并返回当前的state以及与其配套的dispatch方法
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">import</span> React<span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useReducer<span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">const</span> initialState<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>count<span style="color:#67cdcc">:</span><span style="color:#f08d49">0</span><span style="color:#cccccc">}</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">reducer</span> <span style="color:#cccccc">(</span>state<span style="color:#cccccc">,</span>action<span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">switch</span> <span style="color:#cccccc">(</span>action<span style="color:#cccccc">.</span>type<span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">case</span> <span style="color:#7ec699">'increment'</span><span style="color:#67cdcc">:</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">{</span>count<span style="color:#67cdcc">:</span>state<span style="color:#cccccc">.</span>count<span style="color:#67cdcc">+</span><span style="color:#f08d49">1</span><span style="color:#cccccc">}</span>
<span style="color:#cc99cd">case</span> <span style="color:#7ec699">'decrement'</span><span style="color:#67cdcc">:</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">{</span>count<span style="color:#67cdcc">:</span>state<span style="color:#cccccc">.</span>count<span style="color:#67cdcc">-</span><span style="color:#f08d49">1</span><span style="color:#cccccc">}</span>
<span style="color:#cc99cd">default</span><span style="color:#67cdcc">:</span>
<span style="color:#cc99cd">throw</span> <span style="color:#cc99cd">new</span> <span style="color:#f8c555">Error</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>state<span style="color:#cccccc">.</span>dispatch<span style="color:#cccccc">]</span><span style="color:#67cdcc">=</span><span style="color:#f08d49">useReducer</span><span style="color:#cccccc">(</span>reducer<span style="color:#cccccc">,</span>initialState<span style="color:#cccccc">)</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">HooksTest</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">{</span>state<span style="color:#cccccc">.</span>count<span style="color:#cccccc">}</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>button</span> <span style="color:#e2777a">onClick</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">dispatch</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span>type<span style="color:#67cdcc">:</span><span style="color:#7ec699">'increment'</span><span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">}</span><span style="color:#cccccc">></span></span>increment<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>button</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>button</span> <span style="color:#e2777a">onClick</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">dispatch</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span>type<span style="color:#67cdcc">:</span><span style="color:#7ec699">'decrement'</span><span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">}</span><span style="color:#cccccc">></span></span>increment<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>button</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
)
}
</code></span></span></span>- useRef
- 获取dom
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">import</span> React<span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useRef<span style="color:#cccccc">}</span> <span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">HooksTest</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> inputEl<span style="color:#67cdcc">=</span><span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">onButtion</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// inputEl.current 就是我们获取的dom对象</span>
inputEl<span style="color:#cccccc">.</span>current<span style="color:#cccccc">.</span><span style="color:#f08d49">focus</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>input</span> <span style="color:#e2777a">type</span><span style="color:#7ec699"><span style="color:#cccccc">=</span><span style="color:#cccccc">'</span>text<span style="color:#cccccc">'</span></span> <span style="color:#e2777a">ref</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span>inputEl<span style="color:#cccccc">}</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>button</span> <span style="color:#e2777a">onClick</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span>onButtion<span style="color:#cccccc">}</span>
<span style="color:#cccccc">></span></span>Add<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>button</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">{</span>count<span style="color:#cccccc">}</span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
)
}
</code></span></span></span>- 存变量
因为在函数式组件里没有this来存放一些实例的变量，所以React建议使用useRef来存放有一些会发生变化的值，useRef 不单是为了DOM的ref，同时也是为了存放实例属性
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">const</span> intervalRef<span style="color:#67cdcc">=</span><span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
intervalRef<span style="color:#cccccc">.</span>current<span style="color:#67cdcc">=</span><span style="color:#f08d49">setInterVal</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span><span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">clearInterval</span><span style="color:#cccccc">(</span>intervalRef<span style="color:#cccccc">.</span>current<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
</code></span></span></span>- useImperativeHandle
可以让你在使用ref时自定义暴露给父组件的实例值,useImperativeHandle 应当与forwardRef 一起使用，这样可以父组件可以调用子组件的方法
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">// 父组件</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">Father</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> modelRef <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#999999">/* 确定 */</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">sureBtn</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 调用子组件的方法</span>
inputRef<span style="color:#cccccc">.</span>current<span style="color:#cccccc">.</span><span style="color:#f08d49">model</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#67cdcc"><</span><span style="color:#67cdcc">></span>
<span style="color:#67cdcc"><</span>Button onClick<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>sureBtn<span style="color:#cccccc">}</span><span style="color:#67cdcc">></span>确定<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>Button<span style="color:#67cdcc">></span>
<span style="color:#67cdcc"><</span>Children ref<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>modelRef<span style="color:#cccccc">}</span><span style="color:#67cdcc">></span><span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>Children<span style="color:#67cdcc">></span>
<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span><span style="color:#67cdcc">></span>
<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 子组件</span>
<span style="color:#cc99cd">const</span> Children <span style="color:#67cdcc">=</span> React<span style="color:#cccccc">.</span><span style="color:#f08d49">forwardRef</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span>props<span style="color:#cccccc">,</span>ref<span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>visible<span style="color:#cccccc">,</span> setVisible<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">false</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">useImperativeHandle</span><span style="color:#cccccc">(</span>ref<span style="color:#cccccc">,</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">(</span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">model</span><span style="color:#67cdcc">:</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">setVisible</span><span style="color:#cccccc">(</span><span style="color:#f08d49">true</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
</code></span></span></span>- useMemo
useMemo的理念和memo差不多，都是根据判断是否满足当前的有限条件来决定是否执行useMemo的callback函数，第二个参数是一个deps数组，数组里的参数变化决定了useMemo是否更新回调函数。
useMemo和useCallback参数一样，区别是useMemo的返回的是缓存的值，useCallback返回的是函数。
- useMemo减少不必要的渲染
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">// 用 useMemo包裹的list可以限定当且仅当list改变的时候才更新此list，这样就可以避免List重新循环 </span>
<span style="color:#cccccc">{</span><span style="color:#f08d49">useMemo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">(</span>
<span style="color:#67cdcc"><</span>div<span style="color:#67cdcc">></span><span style="color:#cccccc">{</span>
list<span style="color:#cccccc">.</span><span style="color:#f08d49">map</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span>i<span style="color:#cccccc">,</span> v<span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">(</span>
<span style="color:#67cdcc"><</span>span
key<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>v<span style="color:#cccccc">}</span> <span style="color:#67cdcc">></span>
<span style="color:#cccccc">{</span>i<span style="color:#cccccc">.</span>patentName<span style="color:#cccccc">}</span>
<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>span<span style="color:#67cdcc">></span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span>
<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>div<span style="color:#67cdcc">></span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>list<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span>
</code></span></span></span>- useMemo减少子组件的渲染次数
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code> <span style="color:#f08d49">useMemo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">(</span>
<span style="color:#cccccc">{</span> <span style="color:#999999">/* 减少了PatentTable组件的渲染 */</span> <span style="color:#cccccc">}</span>
<span style="color:#67cdcc"><</span>PatentTable
getList<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>getList<span style="color:#cccccc">}</span>
selectList<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>selectList<span style="color:#cccccc">}</span>
cacheSelectList<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>cacheSelectList<span style="color:#cccccc">}</span>
setCacheSelectList<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>setCacheSelectList<span style="color:#cccccc">}</span> <span style="color:#67cdcc">/</span><span style="color:#67cdcc">></span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>listshow<span style="color:#cccccc">,</span> cacheSelectList<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
</code></span></span></span>- useMemo避免很多不必要的计算开销
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">Demo</span><span style="color:#67cdcc">=</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#999999">/* 用useMemo 包裹之后的log函数可以避免了每次组件更新再重新声明 ，可以限制上下文的执行 */</span>
<span style="color:#cc99cd">const</span> newLog <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useMemo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">log</span> <span style="color:#67cdcc">=</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#999999">// 大量计算 </span>
<span style="color:#999999">// 在这里面不能获取实时的其他值</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> log
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#999999">// or</span>
<span style="color:#cc99cd">const</span> log2 <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useMemo（</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#999999">// 大量计算 </span>
<span style="color:#cc99cd">return</span> <span style="color:#999999">// 计算后的值</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span>list<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#67cdcc"><</span>div onClick<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#f08d49">newLog</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span> <span style="color:#67cdcc">></span><span style="color:#cccccc">{</span>log2<span style="color:#cccccc">}</span><span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>div<span style="color:#67cdcc">></span>
<span style="color:#cccccc">}</span>
</code></span></span></span>- useCallback
useMemo和useCallback接收的参数都是一样，都是依赖项发生变化后才会执行；useMemo返回的是函数运行结果，useCallback返回的是函数；父组件传递一个函数 给子组件的时候，由于函数组件每一次都会生成新的props函数，这就使的每次一个传递给子组件的函数都发生的变化，这样就会触发子组件的更新，有些更新是没有必要的。
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">Father</span><span style="color:#67cdcc">=</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span> id <span style="color:#cccccc">}</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> getInfo <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useCallback</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span>sonName<span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
console<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span>sonName<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span>id<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#67cdcc"><</span>div<span style="color:#67cdcc">></span>
<span style="color:#cccccc">{</span><span style="color:#999999">/* 点击按钮触发父组件更新 ，但是子组件没有更新 */</span><span style="color:#cccccc">}</span>
<span style="color:#67cdcc"><</span>button onClick<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#f08d49">setNumber</span><span style="color:#cccccc">(</span>number<span style="color:#67cdcc">+</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">}</span> <span style="color:#67cdcc">></span>增加<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>button<span style="color:#67cdcc">></span>
<span style="color:#67cdcc"><</span>DemoChildren getInfo<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>getInfo<span style="color:#cccccc">}</span> <span style="color:#67cdcc">/</span><span style="color:#67cdcc">></span>
<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>div<span style="color:#67cdcc">></span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">/* 用react.memo */</span>
<span style="color:#cc99cd">const</span> Children <span style="color:#67cdcc">=</span> React<span style="color:#cccccc">.</span><span style="color:#f08d49">memo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span>props<span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#999999">/* 只有初始化的时候打印了 子组件更新 */</span>
console<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'子组件更新'</span><span style="color:#cccccc">,</span>props<span style="color:#cccccc">.</span><span style="color:#f08d49">getInfo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#67cdcc"><</span>div<span style="color:#67cdcc">></span>子组件<span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>div<span style="color:#67cdcc">></span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span>
</code></span></span></span>useCallback必须配合 react.memo pureComponent，否则不但不会提升性能，还有可能降低性能。
react-hooks的诞生，也不是说它能够完全代替class声明的组件，对于业务比较复杂的组件，class组件还是首选，只不过我们可以把class组件内部拆解成funciton组件，根据业务需求，哪些负责逻辑交互，哪些需要动态渲染，然后配合usememo等api，让性能提升起来。react-hooks使用也有一些限制条件，比如说不能放在流程控制语句中，执行上下文也有一定的要求。
#扩展资料
#2 React-hooks原理解析
#2.1 前言
WARNING
阅读以下内容之前先了解一下，hooks出现的动机,同时也要熟悉hooks的用法，可以参考上一篇文章；看完useState、useEffect源码，我相信你已经基本掌握了hooks；其它的很简单。
废话不多说，我首先克隆一份代码下来
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#f08d49">git</span> clone --branch v17.0.2 https://github.com/facebook/react.git
</code></span></span></span>hooks导出部分在react/packages/react/src/ReactHooks.js，虽然在react导出，但是真正实现在react-reconciler这个包里面。
前置知识点:
- fiber
Fiber是一种数据结构，React使用链表把VirtualDOM节点表示一个Fiber，Fiber是一个执行单元，每次执行完一个执行单元，React会检查现在还剩多少时间，如果没有时间就将控制权让出去，去执行一些高优先级的任务。
- 循环链表
- 是一种链式存储结构，整个链表形成一个环
- 它的特点是最后一个节点的指针指向头节点
读源码，我们逐个击破的方式:
useState
useEffect
useRef
useCallback
useMemo
hooks不是一个新api也不是一个黑魔法，就是单纯的一个数组，看下面的例子hooks api返回一个数组，一个是当前值，一个是设置当前值的函数。
#hooks中的useState
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">import</span> React <span style="color:#cccccc">,</span><span style="color:#cccccc">{</span>useState<span style="color:#cccccc">}</span><span style="color:#cc99cd">from</span> <span style="color:#7ec699">'react'</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">App</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>name<span style="color:#cccccc">,</span>setName<span style="color:#cccccc">]</span><span style="color:#67cdcc">=</span><span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'王艺瑾'</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span><span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>div</span><span style="color:#cccccc">></span></span><span style="color:#cccccc">{</span>name<span style="color:#cccccc">}</span><span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"><</span>button</span>
<span style="color:#e2777a">onClick</span><span style="color:#cccccc">=</span><span style="color:#cccccc">{</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span> <span style="color:#f08d49">setName</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'张艺凡'</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span>
<span style="color:#cccccc">></span></span>切换<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>button</span><span style="color:#cccccc">></span></span>
<span style="color:#e2777a"><span style="color:#e2777a"><span style="color:#cccccc"></</span>div</span><span style="color:#cccccc">></span></span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">default</span> App<span style="color:#cccccc">;</span>
</code></span></span></span>上边是一个非常简单的Hook API，创建了name和setName，在页面上展示name，按钮的点击事件修改name
那么在这个过程中setState是如何实现的呢？
#react 包中导出的useState
源码出处：react/packages/react/src/ReactHooks.js
react包中导出的usesate，其实没什么东西，大致看一下就能明白
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">export</span> <span style="color:#cc99cd">function</span> useState<span style="color:#67cdcc"><</span><span style="color:#f8c555">S</span><span style="color:#67cdcc">></span><span style="color:#cccccc">(</span>
initialState<span style="color:#67cdcc">:</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#f8c555">S</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">|</span> <span style="color:#f8c555">S</span><span style="color:#cccccc">,</span> <span style="color:#999999">// flow类型注解</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> dispatcher <span style="color:#67cdcc">=</span> <span style="color:#f08d49">resolveDispatcher</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> dispatcher<span style="color:#cccccc">.</span><span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span>initialState<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>在ReactHooks.js搜索到了useState，函数里先执行了resolveDispatcher,我们先看看resolveDispatcher函数做了写什么？ resolveDispatcher函数的执行，获取了ReactCurrentDispatcher的current，那我们在看看ReactCurrentDispatcher是什么？
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">resolveDispatcher</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> dispatcher <span style="color:#67cdcc">=</span> ReactCurrentDispatcher<span style="color:#cccccc">.</span>current<span style="color:#cccccc">;</span>
<span style="color:#f08d49">invariant</span><span style="color:#cccccc">(</span>
dispatcher <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span>
<span style="color:#7ec699">'Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">' one of the following reasons:\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'1. You might have mismatching versions of React and the renderer (such as React DOM)\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'2. You might be breaking the Rules of Hooks\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'3. You might have more than one copy of React in the same app\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'See https://reactjs.org/link/invalid-hook-call for tips about how to debug and fix this problem.'</span><span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> dispatcher<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>源码出处：react/packages/react/src/ReactCurrentDispatcher.js
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* Keeps track of the current dispatcher.
*/</span>
<span style="color:#cc99cd">const</span> ReactCurrentDispatcher <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">/**
* @internal
* @type {ReactComponent}
*/</span>
current<span style="color:#67cdcc">:</span> <span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">|</span> Dispatcher<span style="color:#cccccc">)</span><span style="color:#cccccc">,</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">default</span> ReactCurrentDispatcher<span style="color:#cccccc">;</span>
</code></span></span></span>ReactCurrentDispatcher现在是null，到这里我们线索好像中断了，因为current要有个hooks方法才行；我们可以断点的形式，去看看在mount阶段，react执行了什么？也就是在mount阶段ReactCurrentDispatcher.current挂载的hooks，蓝色部分就是react在初始化阶段执行的函数
下面才是正文，千万不要放弃
源码出处：react/packages/react-reconciler/src/ReactFiberHooks.new.js
renderWithHooks
- 为什么从renderWithhooks讲起？
因为renderWithhooks是调用函数组件的主要函数，所有的函数组件执行，都会执行这个方法。
下面我说的hooks代表组件中的hooks，例如：useState；hook对象是每次执行hooks所创建的对象
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">// 挂载和更新页面的时候，用的是不同的hooks，hooks在不同的阶段有不同的实现</span>
<span style="color:#999999">/*
举个例子，页面在初始化阶段我们在页面中调用的useSate实际调用的是mountState，
在更新阶段调用的是updateState；其他的hooks也是同理
*/</span>
<span style="color:#cc99cd">const</span> HooksDispatcherOnMount <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span> <span style="color:#999999">// 存储初次挂载的hook</span>
useState<span style="color:#67cdcc">:</span> mountState<span style="color:#cccccc">,</span>
useEffect<span style="color:#67cdcc">:</span>mountEffect
<span style="color:#67cdcc">...</span><span style="color:#67cdcc">...</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> HooksDispatcherOnUpdate <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span> <span style="color:#999999">// 存储更新时候的hook</span>
useState<span style="color:#67cdcc">:</span> updateState<span style="color:#cccccc">,</span>
useEffect<span style="color:#67cdcc">:</span>updateEffect
<span style="color:#67cdcc">...</span><span style="color:#67cdcc">...</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">let</span> currentlyRenderingFiber<span style="color:#cccccc">;</span> <span style="color:#999999">//当前正在使用的fiber</span>
<span style="color:#cc99cd">let</span> workInProgressHook <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span> <span style="color:#999999">// 存储当前最新的hook，跟链表有关系，往下看会明白</span>
<span style="color:#cc99cd">let</span> currentHook<span style="color:#67cdcc">=</span><span style="color:#cc99cd">null</span> <span style="color:#999999">// 在组件更新阶段对应是老的hook</span>
<span style="color:#999999">/**
* @param {*} current 上一个fiber 初次挂载 的时候null
* @param {*} workInProgress 这一次正在构建中的fiber树
* @param {*} Component 当前组件
*/</span>
<span style="color:#cc99cd">export</span> <span style="color:#cc99cd">function</span> <span style="color:#f08d49">renderWithHooks</span><span style="color:#cccccc">(</span>
current<span style="color:#cccccc">,</span>
workInProgress<span style="color:#cccccc">,</span>
Component<span style="color:#cccccc">,</span>
props<span style="color:#cccccc">,</span>
secondArg<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// currentlyRenderingFiber指向本次要构建的fiber（workInProgress）</span>
<span style="color:#999999">// 要区分一下workInProgress和workInProgressHook，不要搞混了</span>
currentlyRenderingFiber <span style="color:#67cdcc">=</span> workInProgress<span style="color:#cccccc">;</span>
<span style="color:#999999">//在执行组件方法之前，要清空hook链表 因为你肯定要创建新的hook链表，要把新的信息挂载到这2个属性上</span>
<span style="color:#999999">//在函数组件中 memoizedState以链表的形式存放hook信息，如果在class组件中，memoizedState存放state信息</span>
workInProgress<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// updateQueue存 effect对象，阅读完useEffect源码就会明白</span>
workInProgress<span style="color:#cccccc">.</span>updateQueue <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// current === null || current.memoizedState === null 说明是mount阶段，否则是update阶段</span>
<span style="color:#999999">// 我们就在这里给ReactCurrentDispatcher.current赋值了</span>
ReactCurrentDispatcher<span style="color:#cccccc">.</span>current <span style="color:#67cdcc">=</span>
current <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">||</span> current<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span>
<span style="color:#67cdcc">?</span> HooksDispatcherOnMount
<span style="color:#67cdcc">:</span> HooksDispatcherOnUpdate<span style="color:#cccccc">;</span>
<span style="color:#999999">// 调用我们的组件函数，然后我们组件里的hooks才会被依次执行</span>
<span style="color:#cc99cd">let</span> children <span style="color:#67cdcc">=</span> <span style="color:#f08d49">Component</span><span style="color:#cccccc">(</span>props<span style="color:#cccccc">,</span>secondArg<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#999999">/*
我们的hooks必须写在组件函数的内部，当上面组件里的hooks执行完后，
我们又给ReactCurrentDispatcher.current赋值了，ContextOnlyDispatcher会报错的形式提示，hooks不能函数外面；
在不同的阶段赋值不同的hooks对象，判断hooks执行是否在函数组件内部
*/</span>
ReactCurrentDispatcher<span style="color:#cccccc">.</span>current <span style="color:#67cdcc">=</span> ContextOnlyDispatcher<span style="color:#cccccc">;</span>
currentlyRenderingFiber <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span><span style="color:#999999">//渲染结束 后把currentlyRenderingFiber清空</span>
workInProgressHook <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// 指向当前调度的hooks节点,主要用于update阶段</span>
currentHook <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> children<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span><span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">// 不在函数内写的hooks指向的函数</span>
<span style="color:#cc99cd">const</span> ContextOnlyDispatcher <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
useState<span style="color:#67cdcc">:</span>throwInvalidHookError
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">throwInvalidHookError</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">invariant</span><span style="color:#cccccc">(</span>
<span style="color:#f08d49">false</span><span style="color:#cccccc">,</span>
<span style="color:#7ec699">'Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">' one of the following reasons:\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'1. You might have mismatching versions of React and the renderer (such as React DOM)\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'2. You might be breaking the Rules of Hooks\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'3. You might have more than one copy of React in the same app\n'</span> <span style="color:#67cdcc">+</span>
<span style="color:#7ec699">'See https://fb.me/react-invalid-hook-call for tips about how to debug and fix this problem.'</span><span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>renderWithHooks主要做的事情：
- 判断是mount阶段还是update阶段给ReactCurrentDispatcher.current赋值。
- 执行组件函数，执行hooks。
- 清空在执行hooks所赋值的全局对象，下一次更新函数需要再次用到。
有几个memoizedState，需要注意：
currentlyRenderingFiber.memoizedState是存整个链表，就是每次执行hooks就会创建hook对象，多个hooks所形成的链表。hook.memoizedState用于存当前执行的hooks的一些信息。
workInProgress和workInProgressHook：
workInProgress正在构建的fiberworkInProgressHook正在构建的hook对象
currentHook和workInProgressHook
currentHook主要用于更新阶段，在mount阶段创建了hook对象，在更新阶段我们需要取出来，需要复用上一次存的信息，currentHook就是正在执行的这个hooks上一次存的信息。workInProgressHook正在创建的hook对象，在mount和update阶段都会创建。
current：初始化阶段为null，当第一次渲染之后会产生一个fiber树，最终会换成真实的dom树
workInProgress：正在构建的fiber树，更新过程中会从current赋值给workInProgress，更新完毕后将当前的 workInProgress树赋值给current。
#2.2 useState
# mount阶段 重要
#1. mountState
初次挂载的时候，useState对应的函数是mountState
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">basicStateReducer</span><span style="color:#cccccc">(</span>state<span style="color:#cccccc">,</span> action<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#cc99cd">typeof</span> action <span style="color:#67cdcc">===</span> <span style="color:#7ec699">'function'</span> <span style="color:#67cdcc">?</span> <span style="color:#f08d49">action</span><span style="color:#cccccc">(</span>state<span style="color:#cccccc">)</span> <span style="color:#67cdcc">:</span> action<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountState</span><span style="color:#cccccc">(</span>
initialState
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 返回当前正在运行的hook对象,构建hook单项链表，下面会详细讲解</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#999999">/*
初始值如果是函数，就执行函数拿到初始值
useState((preState)=> return '初始值')
*/</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span><span style="color:#cc99cd">typeof</span> initialState <span style="color:#67cdcc">===</span> <span style="color:#7ec699">'function'</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
initialState <span style="color:#67cdcc">=</span> <span style="color:#f08d49">initialState</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 把初始值赋值给 hook.baseState和hook.memoizedState</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">.</span>baseState <span style="color:#67cdcc">=</span> initialState<span style="color:#cccccc">;</span>
<span style="color:#999999">// 定义一个队列</span>
<span style="color:#cc99cd">const</span> queue <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span>hook<span style="color:#cccccc">.</span>queue <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
pending<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> <span style="color:#999999">// 存放update对象</span>
dispatch<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> <span style="color:#999999">// 放hooks更新函数</span>
lastRenderedReducer<span style="color:#67cdcc">:</span> basicStateReducer<span style="color:#cccccc">,</span> <span style="color:#999999">//它是一个函数， 用于得到最新的 state</span>
lastRenderedState<span style="color:#67cdcc">:</span> initialState<span style="color:#cccccc">,</span> <span style="color:#999999">// 最后一次得到的 state</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#999999">/*
dispatchAction 是负责更新的函数,就是代表下面的setState函数
const [state,setState]=useState()
*/</span>
<span style="color:#cc99cd">const</span> dispatch <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span>queue<span style="color:#cccccc">.</span>dispatch <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#f08d49">dispatchAction</span><span style="color:#cccccc">.</span><span style="color:#f08d49">bind</span><span style="color:#cccccc">(</span>
<span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span>
currentlyRenderingFiber<span style="color:#cccccc">,</span>
queue<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// 2个值以数值的形式返回</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">[</span>hook<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">,</span> dispatch<span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>mountState主要做的事情：
- 创建hook对象，在上面存上hooks信息，下次更新的时候可以从对象上获取。
- 返回一个数组，包括初始化的值和更新函数
#2. mountWorkInProgressHook
构建hooks单向链表，将组件中的hooks函数以链表的形式串连起来，并赋值给workInProgress的memoizedState；
例子：
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">work</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>name<span style="color:#cccccc">,</span>setName<span style="color:#cccccc">]</span><span style="color:#67cdcc">=</span><span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'h'</span><span style="color:#cccccc">)</span> <span style="color:#999999">// hooks1</span>
<span style="color:#cc99cd">const</span> age<span style="color:#67cdcc">=</span><span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#f08d49">20</span><span style="color:#cccccc">)</span> <span style="color:#999999">// hooks2</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span> <span style="color:#999999">// hooks3</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 构建单向链表</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>memoizedState<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>
memoizedState<span style="color:#67cdcc">:</span><span style="color:#7ec699">'h'</span><span style="color:#cccccc">,</span>
next<span style="color:#67cdcc">:</span><span style="color:#cccccc">{</span>
memoizedState<span style="color:#67cdcc">:</span><span style="color:#7ec699">'20'</span><span style="color:#cccccc">,</span>
next<span style="color:#67cdcc">:</span><span style="color:#cccccc">{</span>
memoizedState<span style="color:#67cdcc">:</span>effect<span style="color:#cccccc">,</span>
next<span style="color:#67cdcc">:</span><span style="color:#cc99cd">null</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// hooks1的next指向hooks2，hooks2的next指向hooks3</span>
</code></span></span></span>
为什么构建一个单向链表？
因为我们在组件更新阶段，需要拿到上次的值，拿到上次的值与本次设置的值做对比来判断是否更新
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">//创建一个hooks对象</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
memoizedState<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> <span style="color:#999999">// useState中保存state信息，useEffect中保存Effect对象，useMemo中保存缓存的值和依赖；useRef保存的是ref对象</span>
baseState<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> <span style="color:#999999">// useState和useReducer中保存最新的state</span>
baseQueue<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span><span style="color:#999999">// useState和useReducer中保存最新的更新队列</span>
queue<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> <span style="color:#999999">// 自己的更新队列，形成环状链表</span>
next<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> <span style="color:#999999">// 下一个更新，就是我们下的页面中下一个hooks</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>workInProgressHook <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">//说明这是我们的第一个hook</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> workInProgressHook <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span> <span style="color:#cc99cd">else</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 说明函数组件中不止一个hooks</span>
workInProgressHook <span style="color:#67cdcc">=</span> workInProgressHook<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> workInProgressHook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>如果上面构建hooks单向链表没有看懂，请看下面解析
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code> <span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>workInProgressHook <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">//说明这是我们的第一个hook</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> workInProgressHook <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span> <span style="color:#cc99cd">else</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 说明函数组件中不止一个hooks</span>
workInProgressHook <span style="color:#67cdcc">=</span> workInProgressHook<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>- 第一次我们创建了hook对象，在堆内存中开辟了一块空间，
currentlyRenderingFiber.memoizedState、workInProgressHook都指向了这个值，对象是引用类型值；我们称这个值为hooks1吧。
currentlyRenderingFiber.memoizedState = hooks1
- 第二次我们再次创建了hook对象，在堆内存中又开辟了一块空间，我们称这个值为hooks2吧，
workInProgressHook.next指向了hooks2，也就是hooks1.next指向了hook2；因为当前的workInProgressHook和hooks1指向同一个地址，只要有一个修改内存里的值，其他变量只要引用该值了，也会随之发生变化；最后又把hooks2又赋值给workInProgressHook，那么workInProgressHook又指向了hooks2。
hooks1.next= hooks2
workInProgressHook=hooks2
- 第三次我们再次创建了hook对象，在堆内存中又开辟了一块空间，我们称这个值为hooks3吧，hooks3又赋值给了
workInProgressHook.next，现在的workInProgressHook和hooks2指向是同一个地址，那么我改变workInProgressHook.next就是改变hooks2的next。
hooks2.next= hooks3
workInProgressHook=hooks3
workInProgressHook始终和最新hook对象指向同一个地址，这样就方便修改上一个hook对象的next
#3. dispatchAction
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {*} fiber 当前正在使用的fiber
* @param {*} queue 队列的初始对象
* @param {*} action 更新函数或者要更新的值
*
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">dispatchAction</span><span style="color:#cccccc">(</span>fiber<span style="color:#cccccc">,</span> queue<span style="color:#cccccc">,</span> action<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 创建一个update对象</span>
<span style="color:#cc99cd">const</span> update<span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
action<span style="color:#cccccc">,</span>
eagerReducer<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span>
eagerState<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span>
next<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> pending <span style="color:#67cdcc">=</span> queue<span style="color:#cccccc">.</span>pending<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>pending <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span> <span style="color:#999999">// 证明第一次更新</span>
update<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> update<span style="color:#cccccc">;</span><span style="color:#999999">//让自己和自己构建成一个环状链表</span>
<span style="color:#cccccc">}</span> <span style="color:#cc99cd">else</span> <span style="color:#cccccc">{</span> <span style="color:#999999">// 不是第一次更新</span>
update<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> pending<span style="color:#cccccc">.</span>next<span style="color:#cccccc">;</span>
pending<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> update<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
queue<span style="color:#cccccc">.</span>pending <span style="color:#67cdcc">=</span> update<span style="color:#cccccc">;</span>
<span style="color:#999999">// queue.pending`永远指向最后一个更新，pending.next 永远指向第一个更新</span>
<span style="color:#cc99cd">const</span> currentState <span style="color:#67cdcc">=</span> queue<span style="color:#cccccc">.</span>lastRenderedState<span style="color:#cccccc">;</span><span style="color:#999999">// 上一次的state</span>
<span style="color:#cc99cd">const</span> eagerState <span style="color:#67cdcc">=</span> <span style="color:#f08d49">lastRenderedReducer</span><span style="color:#cccccc">(</span>currentState<span style="color:#cccccc">,</span> action<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span><span style="color:#999999">//获取最新的state</span>
update<span style="color:#cccccc">.</span>eagerState <span style="color:#67cdcc">=</span> eagerState<span style="color:#cccccc">;</span>
<span style="color:#999999">// 判断上一次的值和当前的值是否一样，是同一个值或同一个引用就return，不进行更新</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span><span style="color:#f08d49">is</span><span style="color:#cccccc">(</span>eagerState<span style="color:#cccccc">,</span> currentState<span style="color:#cccccc">)</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 调度渲染当前fiber，scheduleUpdateOnFiber是react渲染更新的主要函数。</span>
<span style="color:#f08d49">scheduleUpdateOnFiber</span><span style="color:#cccccc">(</span>fiber<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>类组件更新调用setState,函数组件hooks更新调用dispatchAction,都会产生一个update对象，里面记录此处更新的信息； 把update对象放在queue.pending上。
为什么创建update对象？
每次创建update对象，是希望形成一个环状链表。我们看下面一个例子，三次setCount的update对象会暂时放在queue.pending上，组件里的state不会立即更新，在下一次函数组件执行的时候，三次update会被合并到baseQueue上，我们要获取最新的状态，会一次执行update上的每一个action，得到最新的state。
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">work</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">,</span>setCount<span style="color:#cccccc">]</span><span style="color:#67cdcc">=</span><span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">0</span><span style="color:#cccccc">)</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">add</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span>
<span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span><span style="color:#f08d49">2</span><span style="color:#cccccc">)</span>
<span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span><span style="color:#f08d49">3</span><span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">(</span>
<span style="color:#67cdcc"><</span>button onClick<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>add<span style="color:#cccccc">}</span><span style="color:#67cdcc">></span><span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>button<span style="color:#67cdcc">></span>
<span style="color:#cccccc">)</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>为什么不是直接执行最后一个setCount？
如果setCount((state)=>{state+1})参数是函数，那么需要依赖state，下一个要依赖上一个的state；所以需要都执行一遍才能 拿到准确的值。
# update阶段 重要
#1.updateState
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">basicStateReducer</span><span style="color:#cccccc">(</span>state<span style="color:#cccccc">,</span> action<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// $FlowFixMe: Flow doesn't like mixed types</span>
<span style="color:#cc99cd">return</span> <span style="color:#cc99cd">typeof</span> action <span style="color:#67cdcc">===</span> <span style="color:#7ec699">'function'</span> <span style="color:#67cdcc">?</span> <span style="color:#f08d49">action</span><span style="color:#cccccc">(</span>state<span style="color:#cccccc">)</span> <span style="color:#67cdcc">:</span> action<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 可以看出updateState其实调用的是updateReducer</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateState</span><span style="color:#cccccc">(</span>
initialState
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#f08d49">updateReducer</span><span style="color:#cccccc">(</span>basicStateReducer<span style="color:#cccccc">,</span> initialState<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateReducer</span><span style="color:#cccccc">(</span>reducer<span style="color:#cccccc">,</span> initialArg<span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">let</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">updateWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// 构建新的链表</span>
<span style="color:#cc99cd">const</span> queue <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">.</span>queue<span style="color:#cccccc">;</span><span style="color:#999999">//hooks自己的更新队列</span>
<span style="color:#999999">// lastRenderedReducer用于得到最新state，它是一个函数</span>
queue<span style="color:#cccccc">.</span>lastRenderedReducer <span style="color:#67cdcc">=</span> reducer<span style="color:#cccccc">;</span>
<span style="color:#999999">// currentHook记录了当前这个hooks上一次存在链表上的memoizedState、queue、next等信息</span>
<span style="color:#cc99cd">const</span> current <span style="color:#67cdcc">=</span> currentHook<span style="color:#cccccc">;</span>
<span style="color:#999999">// pendingQueue就是更新队列的最后一个update对象</span>
<span style="color:#cc99cd">const</span> pendingQueue <span style="color:#67cdcc">=</span> queue<span style="color:#cccccc">.</span>pending<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span><span style="color:#cccccc">(</span>pendingQueue<span style="color:#67cdcc">!==</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">let</span> first <span style="color:#67cdcc">=</span> pendingQueue<span style="color:#cccccc">.</span>next<span style="color:#cccccc">;</span><span style="color:#999999">//第一个更新对象</span>
<span style="color:#cc99cd">let</span> newState <span style="color:#67cdcc">=</span> current<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">;</span><span style="color:#999999">//拿到老状态</span>
<span style="color:#cc99cd">let</span> update <span style="color:#67cdcc">=</span> first<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">do</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> action <span style="color:#67cdcc">=</span> update<span style="color:#cccccc">.</span>action<span style="color:#cccccc">;</span><span style="color:#999999">//action：就是传的参数，例如setState('参数')</span>
newState <span style="color:#67cdcc">=</span> <span style="color:#f08d49">reducer</span><span style="color:#cccccc">(</span>newState<span style="color:#cccccc">,</span>action<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span><span style="color:#999999">//计算新状态，因为如果传的是函数，要依赖老状态</span>
update <span style="color:#67cdcc">=</span> update<span style="color:#cccccc">.</span>next<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cc99cd">while</span><span style="color:#cccccc">(</span>update <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">&&</span> update <span style="color:#67cdcc">!==</span> first<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
queue<span style="color:#cccccc">.</span>pending <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span><span style="color:#999999">//更新过了可以清空更新环形链表</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> newState<span style="color:#cccccc">;</span><span style="color:#999999">//让新的hook对象的memoizedState等于计算的新状态 </span>
queue<span style="color:#cccccc">.</span>lastRenderedState <span style="color:#67cdcc">=</span> newState<span style="color:#cccccc">;</span><span style="color:#999999">//把新状态也赋值给lastRenderedState一份</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> dispatch <span style="color:#67cdcc">=</span> <span style="color:#f08d49">dispatchAction</span><span style="color:#cccccc">.</span><span style="color:#f08d49">bind</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span> currentlyRenderingFiber<span style="color:#cccccc">,</span> queue<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">[</span>hook<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">,</span> dispatch<span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2. updateWorkInProgressHook
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#cc99cd">let</span> nextCurrentHook<span style="color:#cccccc">;</span>
<span style="color:#999999">//currentHook为null，说明执行的是第一个hooks；currentHook就是老的hook对象</span>
<span style="color:#cc99cd">if</span><span style="color:#cccccc">(</span>currentHook <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
<span style="color:#999999">// current:老的fiber、workInProgress:正在构建的fiber</span>
<span style="color:#cc99cd">let</span> current <span style="color:#67cdcc">=</span> currentlyRenderingFiber<span style="color:#cccccc">.</span>alternate<span style="color:#cccccc">;</span><span style="color:#999999">//alternate属性 对应的是老的fiBer</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>current <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 老的fiber的memoizedState对应的是链表的第一个节点</span>
nextCurrentHook <span style="color:#67cdcc">=</span> current<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span> <span style="color:#cc99cd">else</span> <span style="color:#cccccc">{</span>
nextCurrentHook <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span><span style="color:#cc99cd">else</span><span style="color:#cccccc">{</span>
<span style="color:#999999">// 不是第一个hooks，那么指向下一个 hooks</span>
nextCurrentHook<span style="color:#67cdcc">=</span>currentHook<span style="color:#cccccc">.</span>next<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
currentHook<span style="color:#67cdcc">=</span>nextCurrentHook<span style="color:#cccccc">;</span>
<span style="color:#999999">//创建新的hook对象</span>
<span style="color:#cc99cd">const</span> newHook <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
memoizedState<span style="color:#67cdcc">:</span>currentHook<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">,</span>
queue<span style="color:#67cdcc">:</span>currentHook<span style="color:#cccccc">.</span>queue<span style="color:#cccccc">,</span>
next<span style="color:#67cdcc">:</span><span style="color:#cc99cd">null</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 创建新链表</span>
<span style="color:#cc99cd">if</span><span style="color:#cccccc">(</span>workInProgressHook <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">{</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> workInProgressHook <span style="color:#67cdcc">=</span> newHook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cc99cd">else</span><span style="color:#cccccc">{</span>
workInProgressHook <span style="color:#67cdcc">=</span> workInProgressHook<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> newHook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> workInProgressHook<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2.3 useEffect
# mount阶段 重要
#1. mountEffect
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {function} create - 回调函数
* @param {Array} deps - 依赖数组
*
*/</span>
<span style="color:#cc99cd">const</span> PassiveEffect <span style="color:#67cdcc">=</span> <span style="color:#f08d49">0b000000001000000000</span><span style="color:#cccccc">;</span> <span style="color:#999999">// useEffect</span>
<span style="color:#cc99cd">const</span> PassiveStaticEffect <span style="color:#67cdcc">=</span> <span style="color:#f08d49">0b001000000000000000</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountEffect</span><span style="color:#cccccc">(</span>
create<span style="color:#cccccc">,</span>
deps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 如果在代码中看见 __DEV__，可以不用关心，开发环境才会执行里面的代码，生产会tree shaking</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>__DEV__<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span><span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> <span style="color:#f08d49">mountEffectImpl</span><span style="color:#cccccc">(</span>
PassiveEffect <span style="color:#67cdcc">|</span> PassiveStaticEffect<span style="color:#cccccc">,</span> <span style="color:#999999">// 按位操作</span>
HookPassive<span style="color:#cccccc">,</span>
create<span style="color:#cccccc">,</span>
deps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2. mountEffectImpl
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code>
<span style="color:#999999">// 位操作 ：| 、&</span>
<span style="color:#cc99cd">const</span> HookHasEffect<span style="color:#67cdcc">=</span> <span style="color:#f08d49">0b001</span><span style="color:#cccccc">;</span>
hookFlags <span style="color:#67cdcc">=</span> <span style="color:#f08d49">0b100</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountEffectImpl</span><span style="color:#cccccc">(</span>fiberFlags<span style="color:#cccccc">,</span> hookFlags<span style="color:#cccccc">,</span> create<span style="color:#cccccc">,</span> deps<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// 构建单向链表</span>
<span style="color:#cc99cd">const</span> nextDeps <span style="color:#67cdcc">=</span> deps <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">undefined</span> <span style="color:#67cdcc">?</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">:</span> deps<span style="color:#cccccc">;</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>flags <span style="color:#67cdcc">|=</span> fiberFlags<span style="color:#cccccc">;</span>
<span style="color:#999999">/*
每个hooks都会创建个hook对象，memoizedState在useState中保存的是state
在useEffect中保存的effect对象
*/</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#f08d49">pushEffect</span><span style="color:#cccccc">(</span>
HookHasEffect <span style="color:#67cdcc">|</span> hookFlags<span style="color:#cccccc">,</span>
create<span style="color:#cccccc">,</span>
<span style="color:#cc99cd">undefined</span><span style="color:#cccccc">,</span>
nextDeps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#3. pushEffect
pushEffect 创建effec对象，并形成环状链表存值与updateQueue上
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">createFunctionComponentUpdateQueue</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#cccccc">{</span>
lastEffect<span style="color:#67cdcc">:</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">,</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">pushEffect</span><span style="color:#cccccc">(</span>tag<span style="color:#cccccc">,</span> create<span style="color:#cccccc">,</span> destroy<span style="color:#cccccc">,</span> deps<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 创建effect对象</span>
<span style="color:#cc99cd">const</span> effect <span style="color:#67cdcc">=</span> <span style="color:#cccccc">{</span>
tag<span style="color:#cccccc">,</span>
create<span style="color:#cccccc">,</span>
destroy<span style="color:#cccccc">,</span>
deps<span style="color:#cccccc">,</span>
next<span style="color:#67cdcc">:</span><span style="color:#cc99cd">null</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">let</span> componentUpdateQueue <span style="color:#67cdcc">=</span> currentlyRenderingFiber<span style="color:#cccccc">.</span>updateQueue<span style="color:#cccccc">;</span>
<span style="color:#999999">// 第一个useEffect</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>componentUpdateQueue <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// componentUpdateQueue : {lastEffect:null}</span>
componentUpdateQueue <span style="color:#67cdcc">=</span> <span style="color:#f08d49">createFunctionComponentUpdateQueue</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>updateQueue <span style="color:#67cdcc">=</span> componentUpdateQueue
<span style="color:#999999">// effect 赋值给effect.next；它们指向了内存中同一个地址</span>
<span style="color:#999999">// componentUpdateQueue.lastEffect指向effect 也就是componentUpdateQueue.updateQueue.lastEffect指向了 Effect</span>
componentUpdateQueue<span style="color:#cccccc">.</span>lastEffect <span style="color:#67cdcc">=</span> effect<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> effect<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span> <span style="color:#cc99cd">else</span> <span style="color:#cccccc">{</span> <span style="color:#999999">// 存在多个useEffect</span>
<span style="color:#999999">// componentUpdateQueue.lastEffect 就是上一个Effect对象</span>
<span style="color:#cc99cd">const</span> lastEffect <span style="color:#67cdcc">=</span> componentUpdateQueue<span style="color:#cccccc">.</span>lastEffect<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> firstEffect <span style="color:#67cdcc">=</span> lastEffect<span style="color:#cccccc">.</span>next<span style="color:#cccccc">;</span>
lastEffect<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> effect<span style="color:#cccccc">;</span>
effect<span style="color:#cccccc">.</span>next <span style="color:#67cdcc">=</span> firstEffect<span style="color:#cccccc">;</span>
componentUpdateQueue<span style="color:#cccccc">.</span>lastEffect <span style="color:#67cdcc">=</span> effect<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> effect<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#999999">// componentUpdateQueue.lastEffect 永远指向最新的</span>
</code></span></span></span>
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">2</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">3</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">,</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span>
<span style="color:#999999">// 执行第一个effect</span>
<span style="color:#cc99cd">const</span> effect1<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">create</span><span style="color:#67cdcc">:</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">,</span>
deps<span style="color:#67cdcc">:</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span>
next<span style="color:#67cdcc">:</span>effect1
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 执行第二个effect</span>
<span style="color:#cc99cd">const</span> effect1<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">create</span><span style="color:#67cdcc">:</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">,</span>
deps<span style="color:#67cdcc">:</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span>
next<span style="color:#67cdcc">:</span>effect2
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> effect2<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">create</span><span style="color:#67cdcc">:</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">}</span><span style="color:#cccccc">,</span>
deps<span style="color:#67cdcc">:</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span>
next<span style="color:#67cdcc">:</span>effect1
<span style="color:#cccccc">}</span>
<span style="color:#999999">// 执行第三个effect</span>
<span style="color:#cc99cd">const</span> effect2<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">create</span><span style="color:#67cdcc">:</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">,</span>
deps<span style="color:#67cdcc">:</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span>
next<span style="color:#67cdcc">:</span>effect3
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> effect3<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>
<span style="color:#f08d49">create</span><span style="color:#67cdcc">:</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#cccccc">{</span>consoe<span style="color:#cccccc">.</span><span style="color:#f08d49">log</span><span style="color:#cccccc">(</span><span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">,</span>
deps<span style="color:#67cdcc">:</span><span style="color:#cccccc">[</span><span style="color:#cccccc">]</span>
next<span style="color:#67cdcc">:</span>effect1 <span style="color:#999999">// effect1指向的是effect2</span>
<span style="color:#cccccc">}</span>
</code></span></span></span># update阶段 重要
#1. updateEffect
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateEffect</span><span style="color:#cccccc">(</span>
create<span style="color:#cccccc">,</span>
deps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> <span style="color:#f08d49">updateEffectImpl</span><span style="color:#cccccc">(</span>PassiveEffect<span style="color:#cccccc">,</span> HookPassive<span style="color:#cccccc">,</span> create<span style="color:#cccccc">,</span> deps<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2. updateEffectImpl
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">areHookInputsEqual</span><span style="color:#cccccc">(</span>
nextDeps<span style="color:#cccccc">,</span>
prevDeps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">for</span> <span style="color:#cccccc">(</span><span style="color:#cc99cd">let</span> i <span style="color:#67cdcc">=</span> <span style="color:#f08d49">0</span><span style="color:#cccccc">;</span> i <span style="color:#67cdcc"><</span> prevDeps<span style="color:#cccccc">.</span>length <span style="color:#67cdcc">&&</span> i <span style="color:#67cdcc"><</span> nextDeps<span style="color:#cccccc">.</span>length<span style="color:#cccccc">;</span> i<span style="color:#67cdcc">++</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span><span style="color:#f08d49">is</span><span style="color:#cccccc">(</span>nextDeps<span style="color:#cccccc">[</span>i<span style="color:#cccccc">]</span><span style="color:#cccccc">,</span> prevDeps<span style="color:#cccccc">[</span>i<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">continue</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> <span style="color:#f08d49">false</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">return</span> <span style="color:#f08d49">true</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateEffectImpl</span><span style="color:#cccccc">(</span>fiberFlags<span style="color:#cccccc">,</span> hookFlags<span style="color:#cccccc">,</span> create<span style="color:#cccccc">,</span> deps<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// updateWorkInProgressHook可以往上看，就是创建新的hook对象，不过会复用上一次存的一些信息</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">updateWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> nextDeps <span style="color:#67cdcc">=</span> deps <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">undefined</span> <span style="color:#67cdcc">?</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">:</span> deps<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">let</span> destroy <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">undefined</span><span style="color:#cccccc">;</span>
<span style="color:#999999">// currentHook 可以说是老的hook</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>currentHook <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 拿到上一次存的effect对象</span>
<span style="color:#cc99cd">const</span> prevEffect <span style="color:#67cdcc">=</span> currentHook<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">;</span>
destroy <span style="color:#67cdcc">=</span> prevEffect<span style="color:#cccccc">.</span>destroy<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>nextDeps <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> prevDeps <span style="color:#67cdcc">=</span> prevEffect<span style="color:#cccccc">.</span>deps<span style="color:#cccccc">;</span>
<span style="color:#999999">// 对比依赖对象，是否发生更新，没有更新就复用nextDeps</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span><span style="color:#f08d49">areHookInputsEqual</span><span style="color:#cccccc">(</span>nextDeps<span style="color:#cccccc">,</span> prevDeps<span style="color:#cccccc">)</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">pushEffect</span><span style="color:#cccccc">(</span>hookFlags<span style="color:#cccccc">,</span> create<span style="color:#cccccc">,</span> destroy<span style="color:#cccccc">,</span> nextDeps<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
currentlyRenderingFiber<span style="color:#cccccc">.</span>flags <span style="color:#67cdcc">|=</span> fiberFlags<span style="color:#cccccc">;</span>
<span style="color:#999999">// deps里发生更新，就创建新的effect对象</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#f08d49">pushEffect</span><span style="color:#cccccc">(</span>
HookHasEffect <span style="color:#67cdcc">|</span> hookFlags<span style="color:#cccccc">,</span>
create<span style="color:#cccccc">,</span>
destroy<span style="color:#cccccc">,</span>
nextDeps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2.3 useRef
#mountRef (mount阶段)
看起来很简单，就是把initialValue 赋值给hook.memoizedState， 所以说只要弄懂useState、useEffect ，其他的看一眼就明白
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {any} initialValue - 初始化值
*
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountRef</span><span style="color:#cccccc">(</span>initialValue<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> ref <span style="color:#67cdcc">=</span> initialValue<span style="color:#cccccc">;</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> ref<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> ref<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#updateRef (update阶段)
拿到上一次的值并返回
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {any} initialValue - 初始化值
*
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateRef</span><span style="color:#cccccc">(</span>initialValue<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> ref <span style="color:#67cdcc">=</span> initialValue<span style="color:#cccccc">;</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> ref<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> ref<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2.4 useCallback
#mountCallback (mount阶段)
把函数和依赖数组存到hook.memoizedState，并返回函数
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {function} callback - 函数
* @param {Array} deps - 依赖数组
* @return {function} callback
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountCallback</span><span style="color:#cccccc">(</span>callback<span style="color:#cccccc">,</span> deps<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> nextDeps <span style="color:#67cdcc">=</span> deps <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">undefined</span> <span style="color:#67cdcc">?</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">:</span> deps<span style="color:#cccccc">;</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#cccccc">[</span>callback<span style="color:#cccccc">,</span> nextDeps<span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> callback<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#updateCallback (update阶段)
对比依赖是否变化，变化就返回最新的函数，没有变化就返回上一个函数
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {function} callback - 函数
* @param {Array} deps - 依赖数组
* @return {function} callback
*
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateCallback</span><span style="color:#cccccc">(</span>callback<span style="color:#cccccc">,</span> deps<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">updateWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> nextDeps <span style="color:#67cdcc">=</span> deps <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">undefined</span> <span style="color:#67cdcc">?</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">:</span> deps<span style="color:#cccccc">;</span>
<span style="color:#999999">// prevState：[callback, nextDeps]</span>
<span style="color:#cc99cd">const</span> prevState <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>prevState <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>nextDeps <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> prevDeps <span style="color:#67cdcc">=</span> prevState<span style="color:#cccccc">[</span><span style="color:#f08d49">1</span><span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span><span style="color:#f08d49">areHookInputsEqual</span><span style="color:#cccccc">(</span>nextDeps<span style="color:#cccccc">,</span> prevDeps<span style="color:#cccccc">)</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> prevState<span style="color:#cccccc">[</span><span style="color:#f08d49">0</span><span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#cccccc">[</span>callback<span style="color:#cccccc">,</span> nextDeps<span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> callback<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#2.5 useMemo
#mountMemo (mount阶段)
调用传入函数拿到返回值，把值和依赖数组存到hook.memoizedState，并返回值
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {function} nextCreate - 函数
* @param {Array} deps - 依赖数组
* @return {any} nextValue
*
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">mountMemo</span><span style="color:#cccccc">(</span>
nextCreate<span style="color:#cccccc">,</span>
deps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">mountWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> nextDeps <span style="color:#67cdcc">=</span> deps <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">undefined</span> <span style="color:#67cdcc">?</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">:</span> deps<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> nextValue <span style="color:#67cdcc">=</span> <span style="color:#f08d49">nextCreate</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#cccccc">[</span>nextValue<span style="color:#cccccc">,</span> nextDeps<span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> nextValue<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#updateMemo (update阶段)
对比依赖是否变化，变化就返回最新的值，没有变化就返回上一个值
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#999999">/**
* @param {function} callback - 函数
* @param {Array} deps - 依赖数组
* @return {any} nextValue
*
*/</span>
<span style="color:#cc99cd">function</span> <span style="color:#f08d49">updateMemo</span><span style="color:#cccccc">(</span>
nextCreate<span style="color:#cccccc">,</span>
deps<span style="color:#cccccc">,</span>
<span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> hook <span style="color:#67cdcc">=</span> <span style="color:#f08d49">updateWorkInProgressHook</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> nextDeps <span style="color:#67cdcc">=</span> deps <span style="color:#67cdcc">===</span> <span style="color:#cc99cd">undefined</span> <span style="color:#67cdcc">?</span> <span style="color:#cc99cd">null</span> <span style="color:#67cdcc">:</span> deps<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> prevState <span style="color:#67cdcc">=</span> hook<span style="color:#cccccc">.</span>memoizedState<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>prevState <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// Assume these are defined. If they're not, areHookInputsEqual will warn.</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span>nextDeps <span style="color:#67cdcc">!==</span> <span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> prevDeps <span style="color:#67cdcc">=</span> prevState<span style="color:#cccccc">[</span><span style="color:#f08d49">1</span><span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">if</span> <span style="color:#cccccc">(</span><span style="color:#f08d49">areHookInputsEqual</span><span style="color:#cccccc">(</span>nextDeps<span style="color:#cccccc">,</span> prevDeps<span style="color:#cccccc">)</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">return</span> prevState<span style="color:#cccccc">[</span><span style="color:#f08d49">0</span><span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
<span style="color:#cc99cd">const</span> nextValue <span style="color:#67cdcc">=</span> <span style="color:#f08d49">nextCreate</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
hook<span style="color:#cccccc">.</span>memoizedState <span style="color:#67cdcc">=</span> <span style="color:#cccccc">[</span>nextValue<span style="color:#cccccc">,</span> nextDeps<span style="color:#cccccc">]</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> nextValue<span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#3 使用hooks会遇到的问题
在工程中必须引入lint插件，并开启相应规则，避免踩坑。
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cccccc">{</span>
<span style="color:#7ec699">"plugins"</span><span style="color:#67cdcc">:</span> <span style="color:#cccccc">[</span><span style="color:#7ec699">"react-hooks"</span><span style="color:#cccccc">]</span><span style="color:#cccccc">,</span>
<span style="color:#7ec699">"rules"</span><span style="color:#67cdcc">:</span> <span style="color:#cccccc">{</span>
<span style="color:#7ec699">"react-hooks/rules-of-hooks"</span><span style="color:#67cdcc">:</span> <span style="color:#7ec699">"error"</span><span style="color:#cccccc">,</span>
<span style="color:#7ec699">"react-hooks/exhaustive-deps"</span><span style="color:#67cdcc">:</span> <span style="color:#7ec699">"warn"</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>这2条规则，对于新手，这个过程可能是比较痛苦的，如果你觉得这2个规则对你编写代码造成了困扰，说明你还未完全掌握hooks，对于某写特殊场景，确实不需要「exhaustive-deps」，可在代码处加eslint-disable-next-line react-hooks/exhaustive-deps；切记只能禁止本处代码，不能偷懒把整个文件都禁了。
#3.1 useEffect相关问题
- 依赖变量问题
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">ErrorDemo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">,</span> setCount<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">0</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> dom <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
dom<span style="color:#cccccc">.</span>current<span style="color:#cccccc">.</span><span style="color:#f08d49">addEventListener</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'click'</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span>count <span style="color:#67cdcc">+</span> <span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> <span style="color:#67cdcc"><</span>div ref<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>dom<span style="color:#cccccc">}</span><span style="color:#67cdcc">></span><span style="color:#cccccc">{</span>count<span style="color:#cccccc">}</span><span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>div<span style="color:#67cdcc">></span><span style="color:#cccccc">;</span>
</code></span></span></span>像这种情况，每次count变化都会重新绑定一次事件，那我们怎么解决呢？
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">ErrorDemo</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">,</span> setCount<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">0</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> dom <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
dom<span style="color:#cccccc">.</span>current<span style="color:#cccccc">.</span><span style="color:#f08d49">addEventListener</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'click'</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span>count <span style="color:#67cdcc">+</span> <span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> <span style="color:#67cdcc"><</span>div ref<span style="color:#67cdcc">=</span><span style="color:#cccccc">{</span>dom<span style="color:#cccccc">}</span><span style="color:#67cdcc">></span><span style="color:#cccccc">{</span>count<span style="color:#cccccc">}</span><span style="color:#67cdcc"><</span><span style="color:#67cdcc">/</span>div<span style="color:#67cdcc">></span><span style="color:#cccccc">;</span>
</code></span></span></span>把依赖count变量去掉吗?如果把依赖去掉的话，意味着hooks只在组件挂载的时候运行一次，count的值永远不会超过1；因为在effect 执行时，我们会创建一个闭包，并将count的值保存在闭包当中，且初始值为0
#思路1:消除依赖
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code> <span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 在这不依赖于外部的 `count` 变量</span>
dom<span style="color:#cccccc">.</span>current<span style="color:#cccccc">.</span><span style="color:#f08d49">addEventListener</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'click'</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span>precount<span style="color:#cccccc">)</span><span style="color:#67cdcc">=></span><span style="color:#67cdcc">++</span>precount<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span> <span style="color:#999999">// 我们的 effect 不使用组件作用域中的任何变量</span>
</code></span></span></span>setCount也可以接收一个函数，这样就不用依赖count了
#思路1: 重新绑定事件
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code> <span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> $dom <span style="color:#67cdcc">=</span> dom<span style="color:#cccccc">.</span>current<span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> <span style="color:#f08d49">event</span> <span style="color:#67cdcc">=</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span>count<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">;</span>
$dom<span style="color:#cccccc">.</span><span style="color:#f08d49">addEventListener</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'click'</span><span style="color:#cccccc">,</span> event<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">return</span> $dom<span style="color:#cccccc">.</span><span style="color:#f08d49">removeEventListener</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'click'</span><span style="color:#cccccc">,</span> event<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
</code></span></span></span>#思路2:ref
你可以 使用一个 ref 来保存一个可变的变量。然后你就可以对它进行读写了
当你实在找不到更好的办法的时候，才这么做，因为依赖的变更使组件变的难以预测
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code> <span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>count<span style="color:#cccccc">,</span> setCount<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#f08d49">0</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> dom <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> countRef<span style="color:#67cdcc">=</span><span style="color:#f08d49">useRef</span><span style="color:#cccccc">(</span>count<span style="color:#cccccc">)</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
countRef<span style="color:#cccccc">.</span>current<span style="color:#67cdcc">=</span>count
<span style="color:#cccccc">}</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 在任何时候读取最新的 count</span>
dom<span style="color:#cccccc">.</span>current<span style="color:#cccccc">.</span><span style="color:#f08d49">addEventListener</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'click'</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#f08d49">setCount</span><span style="color:#cccccc">(</span>countRef<span style="color:#cccccc">.</span>current <span style="color:#67cdcc">+</span> <span style="color:#f08d49">1</span><span style="color:#cccccc">)</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// 这个 effect 从不会重新执行</span>
</code></span></span></span>- 依赖函数问题
只有 当函数（以及它所调用的函数）不引用 props、state 以及由它们衍生而来的值时，你才能放心地把它们从依赖列表中省略。下面这个案例有一个 Bug：
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">ProductPage</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span> productId <span style="color:#cccccc">}</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>product<span style="color:#cccccc">,</span> setProduct<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">async</span> <span style="color:#cc99cd">function</span> <span style="color:#f08d49">fetchProduct</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> response <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">await</span> <span style="color:#f08d49">fetch</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'http://myapi/product/'</span> <span style="color:#67cdcc">+</span> productId<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// 使用了 productId prop</span>
<span style="color:#cc99cd">const</span> json <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">await</span> response<span style="color:#cccccc">.</span><span style="color:#f08d49">json</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">setProduct</span><span style="color:#cccccc">(</span>json<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">fetchProduct</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span><span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// 🔴 这样是无效的，因为 `fetchProduct` 使用了 `productId`</span>
<span style="color:#999999">// ...</span>
</code></span></span></span>#思路1:推荐的修复方案是把那个函数移动到你的 effect 内部
这样就能很容易的看出来你的 effect 使用了哪些 props 和 state，并确保它们都被声明了：
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">ProductPage</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span> productId <span style="color:#cccccc">}</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>product<span style="color:#cccccc">,</span> setProduct<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#999999">// 把这个函数移动到 effect 内部后，我们可以清楚地看到它用到的值。</span>
<span style="color:#cc99cd">async</span> <span style="color:#cc99cd">function</span> <span style="color:#f08d49">fetchProduct</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> response <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">await</span> <span style="color:#f08d49">fetch</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'http://myapi/product/'</span> <span style="color:#67cdcc">+</span> productId<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> json <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">await</span> response<span style="color:#cccccc">.</span><span style="color:#f08d49">json</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">setProduct</span><span style="color:#cccccc">(</span>json<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#f08d49">fetchProduct</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>productId<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// ✅ 有效，因为我们的 effect 只用到了 productId</span>
<span style="color:#999999">// ...</span>
<span style="color:#cccccc">}</span>
</code></span></span></span>#思路2: useCallback
把函数加入 effect 的依赖但 把它的定义包裹 进 useCallback Hook。这就确保了它不随渲染而改变，除非 它自身 的依赖发生了改变
<span style="background-color:#282c34"><span style="color:#2c3e50"><span style="color:#cccccc"><code><span style="color:#cc99cd">function</span> <span style="color:#f08d49">ProductPage</span><span style="color:#cccccc">(</span><span style="color:#cccccc">{</span> productId <span style="color:#cccccc">}</span><span style="color:#cccccc">)</span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> <span style="color:#cccccc">[</span>product<span style="color:#cccccc">,</span> setProduct<span style="color:#cccccc">]</span> <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useState</span><span style="color:#cccccc">(</span><span style="color:#cc99cd">null</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cc99cd">const</span> fetchProduct <span style="color:#67cdcc">=</span> <span style="color:#f08d49">useCallback</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#cc99cd">const</span> response <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">await</span> <span style="color:#f08d49">fetch</span><span style="color:#cccccc">(</span><span style="color:#7ec699">'http://myapi/product/'</span> <span style="color:#67cdcc">+</span> productId<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> <span style="color:#999999">// 使用了 productId prop</span>
<span style="color:#cc99cd">const</span> json <span style="color:#67cdcc">=</span> <span style="color:#cc99cd">await</span> response<span style="color:#cccccc">.</span><span style="color:#f08d49">json</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#f08d49">setProduct</span><span style="color:#cccccc">(</span>json<span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>productId<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span>
<span style="color:#f08d49">useEffect</span><span style="color:#cccccc">(</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span> <span style="color:#67cdcc">=></span> <span style="color:#cccccc">{</span>
<span style="color:#f08d49">fetchProduct</span><span style="color:#cccccc">(</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span>
<span style="color:#cccccc">}</span><span style="color:#cccccc">,</span> <span style="color:#cccccc">[</span>fetchProduct<span style="color:#cccccc">]</span><span style="color:#cccccc">)</span><span style="color:#cccccc">;</span> </code></span></span></span>
