使用generator实现async函数

我们先来看一下async函数是怎么使用的

const getData = (sec) => new Promise((resolve) => {
      setTimeout(() => resolve(sec * 2), sec * 1000);
    })

// aim to get this asycnFun by generator
async function asyncFun() {
	const data1 = await getData(1);
	const data2 = await getData(data1);
	return data2;
}
// 可以看到asyncFun执行的返回结果是一个Promise，value为data2
// 3s后输出了4
asyncFun().then(console.log);

使用generator实现async函数

function* generatorFun() {
	const data1 = yield getData(1);
	const data2 = yield getData(2);
	return data2;
}

// 将generator函数转换为async函数
function generator2Async(generatorFun) {
	// 返回async函数，该函数的返回值应为Promise
	return function() {
		const gen = generatorFun.apply(this, arguments);
		return new Promise((resolve, reject) => {
			function step(key, arg) {
				try {
					const {value, done} = gen[key](arg);
					if (done) {
						return resolve(value);
					} else {
						Promise.resolve(value)
								.then((res) => step('next', res), err => step('throw', err));
					}
				} catch (err) {
					return reject(err);
				}
			}
			// execute
			step('next');
		})
	}
}

const asyncFunFromGenrator = generator2Async(generatorFun);
asyncFunFromGenerator().then(console.log);