从vue.nextTick看宏任务和微任务

    Vue 是异步执行 DOM 更新的，其中nextTick就是等待操做完成再执行dom操做，vue.nextTick是vue实现生命周期的重要API函数，官方文档对nextTick的介绍以下：

将回调延迟到下次 DOM 更新循环以后执行。在修改数据以后当即使用它，而后等待 DOM 更新。

    vue.nextTick的实现就是根据宏任务、微任务，咱们能够中github中查看nextTick的源码：github.com/vuejs/vue/b…
    nextTick源码包括注释只有117行，以下。

/* @flow */
/* globals MessageChannel */

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIOS, isNative } from './env'

const callbacks = []
let pending = false

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

// Here we have async deferring wrappers using both microtasks and (macro) tasks.
// In < 2.4 we used microtasks everywhere, but there are some scenarios where
// microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690) or even between bubbling of the same
// event (#6566). However, using (macro) tasks everywhere also has subtle problems
// when state is changed right before repaint (e.g. #6813, out-in transitions).
// Here we use microtask by default, but expose a way to force (macro) task when
// needed (e.g. in event handlers attached by v-on).
let microTimerFunc
let macroTimerFunc
let useMacroTask = false

// Determine (macro) task defer implementation.
// Technically setImmediate should be the ideal choice, but it's only available // in IE. The only polyfill that consistently queues the callback after all DOM // events triggered in the same loop is by using MessageChannel. /* istanbul ignore if */ if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) { macroTimerFunc = () => { setImmediate(flushCallbacks) } } else if (typeof MessageChannel !== 'undefined' && ( isNative(MessageChannel) || // PhantomJS MessageChannel.toString() === '[object MessageChannelConstructor]' )) { const channel = new MessageChannel() const port = channel.port2 channel.port1.onmessage = flushCallbacks macroTimerFunc = () => { port.postMessage(1) } } else { /* istanbul ignore next */ macroTimerFunc = () => { setTimeout(flushCallbacks, 0) } } // Determine microtask defer implementation. /* istanbul ignore next, $flow-disable-line */ if (typeof Promise !== 'undefined' && isNative(Promise)) { const p = Promise.resolve() microTimerFunc = () => { 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. if (isIOS) setTimeout(noop) } } else { // fallback to macro microTimerFunc = macroTimerFunc } /** * Wrap a function so that if any code inside triggers state change, * the changes are queued using a (macro) task instead of a microtask. */ export function withMacroTask (fn: Function): Function { return fn._withTask || (fn._withTask = function () { useMacroTask = true const res = fn.apply(null, arguments) useMacroTask = false return res }) } export function nextTick (cb?: Function, ctx?: Object) { let _resolve callbacks.push(() => { if (cb) { try { cb.call(ctx) } catch (e) { handleError(e, ctx, 'nextTick') } } else if (_resolve) { _resolve(ctx) } }) if (!pending) { pending = true if (useMacroTask) { macroTimerFunc() } else { microTimerFunc() } } // $flow-disable-line if (!cb && typeof Promise !== 'undefined') { return new Promise(resolve => { _resolve = resolve }) } } 复制代码

    这里面的宏任务：setTimeout，ie中特有的setImmediate、MessageChannel（port1.postMessage，port2.onmessage）。
    这里面的微任务：Promise，从源码中能够看到if (typeof Promise !== 'undefined' && isNative(Promise)) 而后不符合的时候，就直接把微任务转变成了宏任务。而这里面没有提到的另外一种微任务的方法是：MutationObsever。