nextTick 在 vue 2.5 和 vue 2.6 之间有什么不同-白红宇

简单了解 $nextTick

我们知道对于 Vue 来说，从数据变化到执行 DOM 更新，这个过程是异步的，发生在下一个 tick 里。

它会创建一个更新队列 queue，队列中维护着各个属性的 watcher，在需要时执行、更新它们。

在下次 DOM 更新循环结束之后执行延迟回调。在修改数据之后立即使用这个方法，获取更新后的 DOM。

Vue.nextTick()    .then(function () {    	// DOM 更新了	})复制代码

那么针对这样一个核心功能，Vue 2.5 与 Vue 2.6 的实现有什么不同呢？

可能需要你简单了解下 js 的 event loop。

Vue 2.5 nextTick 实现

在 Vue 2.5 中，nextTick 的实现是 microTimerFunc、macroTimerFunc 组合实现的，具体见源码。

import { noop } from 'shared/util'import { handleError } from './error'import { isIOS, isNative } from './env'const callbacks = []let pending = falsefunction 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 microTimerFunclet macroTimerFunclet 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

比较关键的几行

let useMacroTask = falseif (!pending) {    pending = true    if (useMacroTask) {    	macroTimerFunc()    } else {    	microTimerFunc()    }}复制代码

这里说明，Vue 2.5 会优先使用 microTimerFunc，如果存在兼容性问题，则降级为 macroTimerFunc。

microTimerFunc 的实现：原生的 Promise；

macroTimerFunc 的实现：setImmediate || MessageChannel || setTimeout。

同时 Vue 2.5 的 next-tick 还对外暴露了两个函数：nextTick 以及 withMacroTask （用于处理一些 DOM 交互事件，如 v-on 绑定的事件回调函数的处理，会强制走 macro task）。

补充，更详细的可以参考 @marsprince 完成的。

通读源码，发现逻辑很清晰，也完成优雅渐进，那么发什么了导致 Vue 在 2.6 对其进行了 fix 呢？来看一段 Vue 2.6 的注释

// 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).复制代码

主要阐明了两个问题：

在重绘之前状态发生改变会有轻微的问题；

利用 macro task 处理事件时，会产生一系列无法规避的诡异问题。

简单地描述下这两个问题：

第一个问题，具体见图

，具体描述见，本质上就是在 css 中定义了 @media 媒体查询，js 中 window 监听了 resize 事件，那么当触发固定阈值时，state 发生了变化、样式也需要重绘，这就产生了问题。

第二个问题，一般可以概括为由于使用 macroTask 处理 DOM 操作，会使得有些时候触发和执行之间间隔太大，例如在移动端，单击的 handler 和音频播放功能不在同一 tick 里。

Vue 2.6 nextTick 实现

由于以上问题，所以在 Vue 2.6 实现过程中，利用 microtasks 代替之前的解决方案，具体见源码。

/* @flow *//* globals MutationObserver */import { noop } from 'shared/util'import { handleError } from './error'import { isIE, isIOS, isNative } from './env'const callbacks = []let pending = falsefunction 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 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 = () => {    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 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  const observer = new MutationObserver(flushCallbacks)  const textNode = document.createTextNode(String(counter))  observer.observe(textNode, {    characterData: true  })  timerFunc = () => {    counter = (counter + 1) % 2    textNode.data = String(counter)  }} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {  // Fallback to setImmediate.  // Techinically 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)  }}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    timerFunc()  }  // $flow-disable-line  if (!cb && typeof Promise !== 'undefined') {    return new Promise(resolve => {      _resolve = resolve    })  }}复制代码

Vue 2.6 利用最典型的两个 microTask，promise.then 以及 Mutation observers，并添加 setImmediate 、setTimeout，作为降级方案。

只对外暴露了一个接口 next-tick，同时用 microTask 来处理 event handler。这种实现方法解决了上述问题，但是也有一个很明显的弊端。由于 microTask 的优先级太高，导致当连续触发 event 事件时产生问题，具体见注释。