示例 {#%E7%A4%BA%E4%BE%8B}

先写一个示例了解下关于 Vue中的DOM更新以及nextTick的作用。

<template>
  <div id="app">
    <div ref="msg">{{msg}}</div>
    <div>
      <button @click="change()">
        请点击
      </button>
    </div>
    <div v-if="msg1">msg1: {{msg1}}</div>
    <div v-if="msg2">msg2: {{msg2}}</div>
    <div v-if="msg3">msg3: {{msg3}}</div>
  </div>
</template>
`<script>
export default {
data() {
return {
msg: "你好吗？",
msg1: "",
msg2: "",
msg3: ""
};
},
methods: {
change() {
this.msg = "我不好！"; //改变msg的数据
this.msg1 = this.$refs.msg.innerHTML; //拿到msg的数据给msg1
this.$nextTick(() => {
this.msg2 = this.$refs.msg.innerHTML; //使用$nextTick把msg的数据给msg2
});
this.msg3 = this.$refs.msg.innerHTML; //拿到msg的数据给msg3
}
}
};
</script>
`

运行效果：

vue-nexttick-1

从图中可以得知：msg1和msg3显示的内容还是变换之前的，而msg2显示的内容是变换之后的。

根本原因是因为Vue中DOM更新是异步的（下面说）。

原理 {#%E5%8E%9F%E7%90%86}

Vue 官方对nextTick这个API的描述：

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。

因为Vue实现响应式并不是数据发生变化之后 DOM 立即变化，而是按一定的策略进行 DOM 的更新。

在 Vue 的文档中，说明Vue是异步执行 DOM 更新的。

简单来说，Vue在修改数据后，视图不会立刻更新，而是等同一事件循环中的所有数据变化完成之后，再统一进行视图更新。

用途 {#%E7%94%A8%E9%80%94}

应用场景：需要在视图更新之后，基于新的视图进行操作。

在Vue生命周期中的created()钩子函数进行的DOM操作一定要放在Vue.nextTick()的回调函数中

因为在created()钩子函数执行的时候DOM其实并未进行任何渲染，而此时进行DOM操作无异于徒劳，所以此处一定要将DOM操作的js代码放进Vue.nextTick()的回调函数中。

与之对应的就是mounted()钩子函数，因为该钩子函数执行时所有的DOM挂载和渲染都已完成，此时在该钩子函数中进行任何DOM操作都不会有问题。
但是需要注意一点：mounted不会承诺所有的子组件也都一起被挂载。如果你希望等到整个视图都渲染完毕，可以用 vm.$nextTick 替换掉 mounted

mounted: function () {
  this.$nextTick(function () {
    // Code that will run only after the
    // entire view has been rendered
  })
}

在数据变化后要执行的某个操作，而这个操作需要使用随数据改变而改变的DOM结构的时候，这个操作都应该放进Vue.nextTick()的回调函数中。（正如上面的代码示例所演示的那样）

官方说明：

Vue 异步执行 DOM 更新。只要观察到数据变化，Vue 将开启一个队列，并缓冲在同一事件循环中发生的所有数据改变。如果同一个 watcher 被多次触发，只会被推入到队列中一次。这种在缓冲时去除重复数据对于避免不必要的计算和 DOM 操作上非常重要。然后，在下一个的事件循环"tick"中，Vue 刷新队列并执行实际 (已去重的) 工作。Vue 在内部尝试对异步队列使用原生的 Promise.then 和 MessageChannel，如果执行环境不支持，会采用 setTimeout(fn, 0) 代替。例如，当你设置vm.someData = 'new value'，该组件不会立即重新渲染。当刷新队列时，组件会在事件循环队列清空时的下一个"tick"更新。多数情况我们不需要关心这个过程，但是如果你想在 DOM 状态更新后做点什么，这就可能会有些棘手。虽然 Vue.js 通常鼓励开发人员沿着"数据驱动"的方式思考，避免直接接触 DOM，但是有时我们确实要这么做。为了在数据变化之后等待 Vue 完成更新 DOM ，可以在数据变化之后立即使用Vue.nextTick(callback) 。这样回调函数在 DOM 更新完成后就会调用。

源码 {#%E6%BA%90%E7%A0%81}

Vue.nextTick用于延迟执行一段代码，它接受2个参数（回调函数和执行回调函数的上下文环境），如果没有提供回调函数，那么将返回promise对象。

2.5版本 {#2.5%E7%89%88%E6%9C%AC}

/* @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
}
}


// 在2.4中使用了microtasks ，但是还是存在问题，
// 在2.5版本中组合使用macrotasks和microtasks，组合使用的方式是对外暴露withMacroTask函数
// 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).


// 2.5版本在nextTick中对于调用microtask（微任务）还是macrotask（宏任务）声明了两个不同的变量
let microTimerFunc
let macroTimerFunc


// 默认使用microtask（微任务）
let useMacroTask = false


// 这里主要定义macrotask（宏任务）函数
// macrotask（宏任务）的执行优先级
// setImmediate -\> MessageChannel -\> setTimeout
// setImmediate是最理想的选择
// 最Low的状况是降级执行setTimeout


// 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)
}
}


// 这里主要定义microtask（微任务）函数
// microtask（微任务）的执行优先级
// Promise -\> macroTimerFunc
// 如果原生不支持Promise，那么执行macrotask（宏任务）函数


// 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
}


// 对外暴露withMacroTask 函数
// 触发变化执行nextTick时强制执行macrotask（宏任务）函数


/\*\*


   


    
* 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
  try {
  return fn.apply(null, arguments)
  } finally {
  useMacroTask = false

  }
  })
  }


   
`// 这里需要注意pending
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
})
}
}
`

2.6版本 {#2.6%E7%89%88%E6%9C%AC}

/* @flow */
/* globals MutationObserver */

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


export let isUsingMicroTask = false


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
}
}


// 在2.5版本中组合使用microtasks 和macrotasks，但是重绘的时候还是存在一些小问题，而且使用macrotasks在任务队列中会有几个特别奇怪的行为没办法避免，So又回到了之前的状态，在任何地方优先使用microtasks 。
// 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 \*/


// task的执行优先级
// Promise -\> MutationObserver -\> setImmediate -\> setTimeout


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)
}
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
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)
}
isUsingMicroTask = true
} 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
})
}
}
`