Vue.nextTick的应用场景
Vue 是采用异步的方式执行 DOM 更新。只要观察到数据变化，Vue 将开启一个队列，并缓冲同一事件循环中发生的所有数据改变。然后，在下一个的事件循环中，Vue 刷新队列并执行页面渲染工作。所以修改数据后DOM并不会立刻被重新渲染，如果想在数据更新后对页面执行DOM操作，可以在数据变化之后立即使用 Vue.nextTick(callback)。
下面这一段摘自vue官方文档，关于JS 运行机制的说明：
JS 执行是单线程的，它是基于事件循环的。事件循环大致分为以下几个步骤：
（1）所有同步任务都在主线程上执行，形成一个执行栈（execution context stack）。
（2）主线程之外，还存在一个"任务队列"（task queue）。只要异步任务有了运行结果，就在"任务队列"之中放置一个事件。
（3）一旦"执行栈"中的所有同步任务执行完毕，系统就会读取"任务队列"，看看里面有哪些事件。那些对应的异步任务，于是结束等待状态，进入执行栈，开始执行。
（4）主线程不断重复上面的第三步。
在主线程中执行修改数据这一同步任务，DOM的渲染事件就会被放到“任务队列”中，当“执行栈”中同步任务执行完毕，本次事件循环结束，系统才会读取并执行“任务队列”中的页面渲染事件。而Vue.nextTick中的回调函数则会在页面渲染后才执行。
例子如下：
<template> <div> <div ref="test">{{test}}</div> </div></template>// Some code...data () { return { test: 'begin' };},// Some code...this.test = 'end';console.log(this.$refs.test.innerText);//"begin"this.nextTick(() => { console.log(this.$refs.test.innerText) //"end"})Vue.nextTick实现原理
Vue.nextTick的源码vue项目/src/core/util/路径下的next-tick.js文件，文章最后也会贴出完整的源码
我们先来看看对于异步调用函数的实现方法：
let timerFuncif (typeof Promise !== 'undefined' && isNative(Promise)) { const p = Promise.resolve() timerFunc = () => { p.then(flushCallbacks) if (isIOS) setTimeout(noop) } isUsingMicroTask = true} else if (!isIE && typeof MutationObserver !== 'undefined' && ( isNative(MutationObserver) || MutationObserver.toString() === '[object MutationObserverConstructor]')) { 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)) { timerFunc = () => { setImmediate(flushCallbacks) }} else { timerFunc = () => { setTimeout(flushCallbacks, 0) }}这段代码首先的检测运行环境的支持情况，使用不同的异步方法。优先级依次是Promise、MutationObserver、setImmediate和setTimeout。这是根据运行效率来做优先级处理，有兴趣可以去了解一下这几种方法的差异。总的来说，Event Loop分为宏任务以及微任务，宏任务耗费的时间是大于微任务的，所以优先使用微任务。例如Promise属于微任务，而setTimeout就属于宏任务。最终timerFunc则是我们调用nextTick函数时要内部会调用的主要方法，那么flushCallbacks又是什么呢，我们在看看flushCallbacks函数：
function flushCallbacks () { pending = false const copies = callbacks.slice(0) callbacks.length = 0 for (let i = 0; i < copies.length; i++) { copies[i]() }}这个函数比较简单，就是依次调用callbacks数组里面的方法。还使用slice()方法复制callbacks数组并把callbacks数组清空，这里的callbacks数组就是存放主线程执行过程中的Vue.nextTick()函数所传的回调函数集合（主线程可能会多次使用Vue.nextTick()方法）。到这里就已经实现了根据环境选择异步方法，并在异步方法中依次调用传入Vue.nextTick()方法的回调函数。nextTick函数主要就是要将callback函数存在数组callbacks中，并调用timerFunc方法：
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 }) }}可以看到可以传入第二个参数作为回调函数的this。另外如果参数一的条件判断为false时则返回一个Promise对象。例如
Vue.nextTick(null, {value: 'test'}) .then((data) => { console.log(data.value) // 'test' })这里还使用了一个优化技巧，用pending来标记异步任务是否被调用，也就是说在同一个tick内只调用一次timerFunc函数，这样就不会开启多个异步任务。
完整的源码：
import { noop } from 'shared/util'import { handleError } from './error'import { isIE, isIOS, isNative } from './env'export let isUsingMicroTask = falseconst 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) } 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 }) }}