React18原理: React核心对象之ReactElement对象和Fiber对象

React中的核心对象

在React应用中，有很多特定的对象或数据结构.了解这些内部的设计，可以更容易理解react运行原理
列举从react启动到渲染过程出现频率较高，影响范围较大的对象，它们贯穿整个react运行时
- 如 ReactElement 对象
- 如 Fiber 对象
其他过程的重要对象
- 如事件对象（位于react-dom/events保障react应用能够响应ui交互）
- 如 ReactContext, ReactProvider, ReactConsumer对象等

ReactElement对象

入口函数 ReactDOM.render(<App />, document.getElementByld('root');
可以简单的认为，包括 <App/> 及其所有子节点都是ReactElement对象（在render之后才会生成子节点)
每个ReactElement对象的区别在于type不同，其type定义在shared包中
所有采用jsx语法书写的节点，都会被编译器转换，最终会以React.createElement(..)的方式
创建出来一个与之对应的ReactElement对象

ReactElement对象的数据结构如下，定义在: packages/shared/ReactElementType.js

/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

export type ReactElement = {
     
  $$typeof: any,
  type: any,
  key: any,
  ref: any,
  props: any,
  // ReactFiber
  _owner: any,

  // __DEV__
  _store: {
     validated: boolean, ...},
};

特别关注 key 和 type 这两个属性
- key 属性在reconciler阶段会用到
  - 所有的ReactElement对象都有key属性
  - 且其默认值是null
  - 后续在diff算法中会使用到
- type 属性决定了节点的种类
  - 它的值可以是字符串（代表div,span等dom节点），函数（代表function,class等节点）
  - 或者react内部定义的节点类型(portal,context,fragment等)
  - 在reconciler阶段，会根据type执行不同的逻辑
    - 如type是一个字符串类型，则直接使用.
    - 如type是一个ReactComponent类型，则会调用其render方法获取节点.
    - 如type是一个function类型，则会调用该方法获取子节点

ReactElement 的工厂方法如下，定义在：packages/react/src/ReactElementProd.js

/**
 * Factory method to create a new React element. This no longer adheres to
 * the class pattern, so do not use new to call it. Also, instanceof check
 * will not work. Instead test $$typeof field against Symbol.for('react.element') to check
 * if something is a React Element.
 *
 * @param {*} type
 * @param {*} props
 * @param {*} key
 * @param {string|object} ref
 * @param {*} owner
 * @param {*} self A *temporary* helper to detect places where `this` is
 * different from the `owner` when React.createElement is called, so that we
 * can warn. We want to get rid of owner and replace string `ref`s with arrow
 * functions, and as long as `this` and owner are the same, there will be no
 * change in behavior.
 * @param {*} source An annotation object (added by a transpiler or otherwise)
 * indicating filename, line number, and/or other information.
 * @internal
 */
function ReactElement(type, key, ref, owner, props) {
     
  const element = {
     
    // This tag allows us to uniquely identify this as a React Element
    $$typeof: REACT_ELEMENT_TYPE,

    // Built-in properties that belong on the element
    type: type,
    key: key,
    ref: ref,
    props: props,

    // Record the component responsible for creating this element.
    _owner: owner,
  };

  if (__DEV__) {
     
    // The validation flag is currently mutative. We put it on
    // an external backing store so that we can freeze the whole object.
    // This can be replaced with a WeakMap once they are implemented in
    // commonly used development environments.
    element._store = {
     };

    // To make comparing ReactElements easier for testing purposes, we make
    // the validation flag non-enumerable (where possible, which should
    // include every environment we run tests in), so the test framework
    // ignores it.
    Object.defineProperty(element._store, 'validated', {
     
      configurable: false,
      enumerable: false,
      writable: true,
      value: false,
    });
    // debugInfo contains Server Component debug information.
    Object.defineProperty(element, '_debugInfo', {
     
      configurable: false,
      enumerable: false,
      writable: true,
      value: null,
    });
    if (Object.freeze) {
     
      Object.freeze(element.props);
      Object.freeze(element);
    }
  }

  return element;
}

举一个App组件的例子

class App extends React.Component {
  render() {
    return (
      <div className="app">
        <header>header</header>
        <Content />
        <footer>footer</footer>
      </div>
    )
  }
}

class Content extends React.Component {
  render() {
    return (
      <React.Fragment>
        <p>1</p>
        <p>2</p>
        <p>3</p>
      </React.Fragment>
      );
    }
}

export default App;

这个代码，对应着 ReactElement 树结构如下

class和function类型的组件，其子节点是在render之后(reconciler阶段)才生成的
此处只是单独表示 ReactElement 的数据结构
父级对象和子级对象之间是通过props.children属性进行关联的（与fiber树不同）
ReactElement虽然不能算是一个严格的树，也不能算是一个严格的链表
- 它的生成过程是自顶向下的，是所有组件节点的总和
- ReactElement树（暂且用树来表述）和fiber树是以props.children为单位先后交替生成的
- 当ReactElement树构造完毕，fiber树也随后构造完毕
reconciler阶段会根据ReactElement的类型生成对应的fiber节点
- 注意，不是一一对应
- 比如，Fragment类型的组件在生成fiber节点的时候会略过

Fiber对象

react-reconciler包是react应用的中枢
连接渲染器(react-dom)和调度中心(scheduler)，同时自身也负责fiber树的构造

先看数据结构，其type类型的定义在 ReactInternalTypes.js 中

// A Fiber is work on a Component that needs to be done or was done. There can
// be more than one per component.
export type Fiber = {
     
  // These first fields are conceptually members of an Instance. This used to
  // be split into a separate type and intersected with the other Fiber fields,
  // but until Flow fixes its intersection bugs, we've merged them into a
  // single type.

  // An Instance is shared between all versions of a component. We can easily
  // break this out into a separate object to avoid copying so much to the
  // alternate versions of the tree. We put this on a single object for now to
  // minimize the number of objects created during the initial render.

  // Tag identifying the type of fiber.
  tag: WorkTag,

  // Unique identifier of this child.
  key: null | string,

  // The value of element.type which is used to preserve the identity during
  // reconciliation of this child.
  elementType: any,

  // The resolved function/class/ associated with this fiber.
  type: any,

  // The local state associated with this fiber.
  stateNode: any,

  // Conceptual aliases
  // parent : Instance -> return The parent happens to be the same as the
  // return fiber since we've merged the fiber and instance.

  // Remaining fields belong to Fiber

  // The Fiber to return to after finishing processing this one.
  // This is effectively the parent, but there can be multiple parents (two)
  // so this is only the parent of the thing we're currently processing.
  // It is conceptually the same as the return address of a stack frame.
  return: Fiber | null,

  // Singly Linked List Tree Structure.
  child: Fiber | null,
  sibling: Fiber | null,
  index: number,

  // The ref last used to attach this node.
  // I'll avoid adding an owner field for prod and model that as functions.
  ref:
    | null
    | (((handle: mixed) => void) & {
     _stringRef: ?string, ...})
    | RefObject,

  refCleanup: null | (() => void),

  // Input is the data coming into process this fiber. Arguments. Props.
  pendingProps: any, // This type will be more specific once we overload the tag.
  memoizedProps: any, // The props used to create the output.

  // A queue of state updates and callbacks.
  updateQueue: mixed,

  // The state used to create the output
  memoizedState: any,

  // Dependencies (contexts, events) for this fiber, if it has any
  dependencies: Dependencies | null,

  // Bitfield that describes properties about the fiber and its subtree. E.g.
  // the ConcurrentMode flag indicates whether the subtree should be async-by-
  // default. When a fiber is created, it inherits the mode of its
  // parent. Additional flags can be set at creation time, but after that the
  // value should remain unchanged throughout the fiber's lifetime, particularly
  // before its child fibers are created.
  mode: TypeOfMode,

  // Effect
  flags: Flags,
  subtreeFlags: Flags,
  deletions: Array<Fiber> | null,

  // Singly linked list fast path to the next fiber with side-effects.
  nextEffect: Fiber | null,

  // The first and last fiber with side-effect within this subtree. This allows
  // us to reuse a slice of the linked list when we reuse the work done within
  // this fiber.
  firstEffect: Fiber | null,
  lastEffect: Fiber | null,

  lanes: Lanes,
  childLanes: Lanes,

  // This is a pooled version of a Fiber. Every fiber that gets updated will
  // eventually have a pair. There are cases when we can clean up pairs to save
  // memory if we need to.
  alternate: Fiber | null,

  // Time spent rendering this Fiber and its descendants for the current update.
  // This tells us how well the tree makes use of sCU for memoization.
  // It is reset to 0 each time we render and only updated when we don't bailout.
  // This field is only set when the enableProfilerTimer flag is enabled.
  actualDuration?: number,

  // If the Fiber is currently active in the "render" phase,
  // This marks the time at which the work began.
  // This field is only set when the enableProfilerTimer flag is enabled.
  actualStartTime?: number,

  // Duration of the most recent render time for this Fiber.
  // This value is not updated when we bailout for memoization purposes.
  // This field is only set when the enableProfilerTimer flag is enabled.
  selfBaseDuration?: number,

  // Sum of base times for all descendants of this Fiber.
  // This value bubbles up during the "complete" phase.
  // This field is only set when the enableProfilerTimer flag is enabled.
  treeBaseDuration?: number,

  // Conceptual aliases
  // workInProgress : Fiber ->  alternate The alternate used for reuse happens
  // to be the same as work in progress.
  // __DEV__ only

  _debugInfo?: ReactDebugInfo | null,
  _debugOwner?: Fiber | null,
  _debugIsCurrentlyTiming?: boolean,
  _debugNeedsRemount?: boolean,

  // Used to verify that the order of hooks does not change between renders.
  _debugHookTypes?: Array<HookType> | null,
};

fiber.tag
- 表示fiber类型，根据 ReactElement 组件的type进行生成，在react内部共定义了25种tag
fiber.key
- ReactElement 组件的key一致
fiber.elementType
- 一般来讲和ReactElement组件的type一致
fiber.type
- 一般来讲和fiber.elementType一致.一些特殊情形下，比如在开发环境下为了兼容热更新 (HotReloading)
- 会对 function, class, ForwardRef类型的 ReactElement做一定的处理
- 这种情况会区别于fiber.elementType
fiber.stateNode
- 与fiber关联的局部状态节点
- 比如：HostComponent 类型指向与fiber节点对应的dom节点
- 根节点 fiber.stateNode 指向的是 FiberRoot
- class 类型节点其 stateNode 指向的是 class 实例
fiber.return
- 指向父节点
fiber.child
- 指向第一个子节点
fiber.sibling
- 指向下一个兄弟节点
fiber.index
- fiber在兄弟节点中的索引，如果是单节点默认为0.
fiber.ref
- 指向在ReactElement组件上设置的ref
- string类型的ref除外，这种类型的ref已经不推荐使用
- reconciler阶段会将string类型的ref转换成一个function类型
fiber.pendingProps
- 输入属性，从 ReactElement对象传的props
- 用于和 fiber.memoizedProps 比较可以得出属性是否变动
fiber.memoizedProps
- 上一次生成子节点时用到的属性，生成子节点之后保持在内存中
- 向下生成子节点之前叫做pendingProps
- 生成子节点之后会把pendingProps赋值给 memoizedProps用于下一次比较
- pendingProps和memoizedProps比较可以出属性是否变动
fiber.updateQueue
- 存储update更新对象的队列，每一次发起更新，都需要在该队列上创建一个update对象
fiber.memoizedState
- 上一次生成子节点之后保持在内存中的局部状态
fiber.dependencies
- 该fiber节点所依赖的(contexts, events)等
fiber.mode
- 二进制位 Bitfield, 继承至父节点，影响本fiber节点及其子树中所有节点
- 与react应用的运行模式有关（有ConcurrentMode, NoMode等选项）
fiber.flags
- 标志位，副作用标记（在16.x版本中叫做effectTag）
- 在 ReactFiberFlags.js中定义了所有的标志位
- reconciler阶段会将所有拥有flags标记的节点添加到副作用链表中，等待commit阶段的处理
fiber.subtreeFlags
- subtreeFlags是一个二进制形式的属性，它代表了Fiber节点子树的操作依据
- 换句话说，它包含了有关该节点及其所有子节点的更新信息。这是React内部用于优化和协调更新的重要机制之一
- 具体来说，subtreeFlags可能包含各种标志，这些标志指示了哪些子节点需要更新、哪些操作正在进行中、哪些操作已经完成等
- React使用这些标志来有效地管理组件的更新过程，确保只更新实际发生变化的部分，从而提高性能和效率
- 需要注意的是，subtreeFlags是React内部使用的属性，对于大多数开发者来说，通常不需要直接与之交互
- React的公开API和抽象层使得开发者可以专注于编写组件逻辑，而无需关心底层的更新和渲染机制
fiber.deletions
- 存储将要被删除的子节点.默认未开启，
fiber.nextEffect
- 单向链表，指向下一个有副作用的fiber节点
fiber.firstEffect
- 指向副作用链表中的第一个fiber节点
fiber.lastEffect
- 指向副作用链表中的最后一个fiber节点.
fiber.lanes
- 本fiber节点所属的优先级，创建fiber的时候设置
fiber.childLanes
- 子节点所属的优先级
fiber.alternate
- 指向内存中的另一个fiber,每个被更新过fiber节点在内存中都是成对出现(current 和 workInProgress)