30 # 宏任务和微任务的区分

可以通过 vue 的源码 nextTick 方法：里面描述了浏览器中的常见的宏任务和微任务

• 宏任务：script、ui、setTimeout、setInterval、requestFrameAnimation、setImmediate（ie 10以上、node 中支持）、MessageChannel（消息通道）都是异步的，还有 click，ajax
• 微任务：语言本身提供的 promise.then、mutationObserver、nextTick

【nextTick 源码：我这里用 2.6.11 版本的】https://github.com/vuejs/vue/blob/v2.6.11/src/core/util/next-tick.js

Vue 在内部对异步队列判断当前环境优先支持的异步方法，优先选择微任务，其顺序如下：

1. 先判断当前环境是否原生支持 promise
2. 如果不支持 promise, 就判断是否支持 MutationObserver，不是IE环境，并且原生支持 MutationObserver
3. 判断当前环境是否原生支持 setImmediate
4. 以上三种都不支持就选择 setTimeout（setTimeout 可能产生一个 4ms 的延迟，而 setImmediate 会在主线程执行完后立刻执行）

优先级: Promise —> MutationObserver —> setImmediate—> setTimeout

/* @flow */
/* globals MutationObserver */
// noop 表示一个无操作空函数，用作函数默认值，防止传入 undefined 导致报错
import { noop } from "shared/util";
// 错误处理函数
import { handleError } from "./error";
// 环境判断函数，isNative 判断某个属性或方法是否原生支持，如果不支持或通过第三方实现支持都会返回false
import { isIE, isIOS, isNative } from "./env";
// 标记 nextTick 最终是否以微任务执行
export let isUsingMicroTask = false;
// 存放调用 nextTick 时传入的回调函数
const callbacks = [];
// 标记是否已经向任务队列中添加了一个任务，如果已经添加了就不能再添加了
// 当向任务队列中添加了任务时，将 pending 置为true, 当任务被执行时将 pending 置为 false
let pending = false;

// flushCallbacks 函数用来遍历 callbacks 数组的拷贝并执行其中的回调
function flushCallbacks() {
    pending = false;
    // 拷贝一份 callbacks
    const copies = callbacks.slice(0);
    // 然后清空 callbacks
    callbacks.length = 0;
    // 遍历执行传入的回调
    for (let i = 0; i < copies.length; i++) {
        copies[i]();
    }
}

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
let timerFunc;

// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */

if (typeof Promise !== "undefined" && isNative(Promise)) {
    const p = Promise.resolve();
    timerFunc = () => {
        // 用 promise.then 把 flushCallbacks 函数包裹成个异步微任务
        p.then(flushCallbacks);
        // In problematic UIWebViews, Promise.then doesn't completely break, but
        // it can get stuck in a weird state where callbacks are pushed into the
        // microtask queue but the queue isn't being flushed, until the browser
        // needs to do some other work, e.g. handle a timer. Therefore we can
        // "force" the microtask queue to be flushed by adding an empty timer.
        // 因为在 ios 下 promise.then 后面没有宏任务的话，微任务队列不会刷新，这里的 setTimeout 是用来强制刷新微任务队列
        if (isIOS) setTimeout(noop);
    };
    // 标记当前 nextTick 使用的微任务
    isUsingMicroTask = true;
} else if (
    !isIE &&
    typeof MutationObserver !== "undefined" &&
    (isNative(MutationObserver) ||
        // PhantomJS and iOS 7.x
        MutationObserver.toString() === "[object MutationObserverConstructor]")
) {
    // Use MutationObserver where native Promise is not available,
    // e.g. PhantomJS, iOS7, Android 4.4
    // (#6466 MutationObserver is unreliable in IE11)
    let counter = 1;
    // new 一个 MutationObserver 类
    const observer = new MutationObserver(flushCallbacks);
    // 创建一个文本节点
    const textNode = document.createTextNode(String(counter));
    // 监听这个文本节点，当数据发生变化就执行 flushCallbacks
    observer.observe(textNode, {
        characterData: true
    });
    timerFunc = () => {
        counter = (counter + 1) % 2;
        // 更新数据
        textNode.data = String(counter);
    };
    // 标记当前 nextTick 使用的微任务
    isUsingMicroTask = true;
} else if (typeof setImmediate !== "undefined" && isNative(setImmediate)) {
    // Fallback to setImmediate.
    // Technically it leverages the (macro) task queue,
    // but it is still a better choice than setTimeout.
    timerFunc = () => {
        setImmediate(flushCallbacks);
    };
} else {
    // Fallback to setTimeout.
    timerFunc = () => {
        setTimeout(flushCallbacks, 0);
    };
}

// 声明 nextTick 函数，接收一个回调函数和一个执行上下文作为参数
export function nextTick(cb?: Function, ctx?: Object) {
    let _resolve;
    // 将传入的回调函数存放到数组中，后面会遍历执行其中的回调
    callbacks.push(() => {
        if (cb) {
            try {
                cb.call(ctx);
            } catch (e) {
                handleError(e, ctx, "nextTick");
            }
        } else if (_resolve) {
            _resolve(ctx);
        }
    });
    // 当在同一轮事件循环中多次调用 nextTick 时，timerFunc 只会执行一次
    if (!pending) {
        pending = true;
        timerFunc();
    }
    // 如果没有传入回调，并且当前环境支持 Promise，就返回一个 Promise，在返回的这个 promise.then 中 DOM 已经更新好了
    // $flow-disable-line
    if (!cb && typeof Promise !== "undefined") {
        return new Promise((resolve) => {
            _resolve = resolve;
        });
    }
}