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
// 修改数据vm.msg = 'Hello';// DOM 还没有更新Vue.nextTick(function() {// DOM 更新了});// 作为一个 Promise 使用Vue.nextTick().then(function() {// DOM 更新了});
按照优先级检测
native promise
MutationObserver(非ie)
setImmediate
setTimeout
官方文档注释中,MutationObserver有更广的适配,但是ie11和UIWebView in iOS >= 9.3.3有严重的bug,所以native promise是第一优先级的
// 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.xMutationObserver.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 = 1const observer = new MutationObserver(flushCallbacks)const textNode = document.createTextNode(String(counter))observer.observe(textNode, {characterData: true})timerFunc = () => {counter = (counter + 1) % 2textNode.data = String(counter)}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)}}
总体流程就是,接收回调函数,将回调函数推入回调函数队列中。
同时,在接收第一个回调函数时,执行能力检测中对应的异步方法(异步方法中调用了回调函数队列)。
如何保证只在接收第一个回调函数时执行异步方法?
nextTick源码中使用了一个异步锁的概念,即接收第一个回调函数时,先关上锁,执行异步方法。此时,浏览器处于等待执行完同步代码就执行异步代码的情况。
打个比喻:相当于一群旅客准备上车,当第一个旅客上车的时候,车开始发动,准备出发,等到所有旅客都上车后,就可以正式开车了。
当然执行flushCallbacks函数时有个难以理解的点,即:为什么需要备份回调函数队列?执行的也是备份的回调函数队列?
因为,会出现这么一种情况:nextTick套用nextTick。如果flushCallbacks不做特殊处理,直接循环执行回调函数,会导致里面nextTick中的回调函数会进入回调队列。这就相当于,下一个班车的旅客上了上一个班车。
// 回调函数队列const callbacks = []// 异步锁let pending = false// 执行回调函数function flushCallbacks () {// 重置异步锁pending = false// 防止出现nextTick中包含nextTick时出现问题,在执行回调函数队列前,提前复制备份,清空回调函数队列const copies = callbacks.slice(0)callbacks.length = 0// 执行回调函数队列for (let i = 0; i < copies.length; i++) {copies[i]()}}...// 我们调用的nextTick函数export function nextTick (cb?: Function, ctx?: Object) {let _resolve// 将回调函数推入回调队列callbacks.push(() => {if (cb) {try {cb.call(ctx)} catch (e) {handleError(e, ctx, 'nextTick')}} else if (_resolve) {_resolve(ctx)}})// 如果异步锁未锁上,锁上异步锁,调用异步函数,准备等同步函数执行完后,就开始执行回调函数队列if (!pending) {pending = truetimerFunc()}// $flow-disable-line// 2.1.0新增,如果没有提供回调,并且支持Promise,返回一个Promiseif (!cb && typeof Promise !== 'undefined') {return new Promise(resolve => {_resolve = resolve})}}
MutationObserver 如何模拟 nextTick 这点,其实就是创建一个 TextNode 并监听内容变化,然后要 nextTick 的时候去改一下这个节点的文本内容:
const observer = new MutationObserver(flushCallbacks)const textNode = document.createTextNode(String(counter))observer.observe(textNode, {characterData: true})timerFunc = () => {counter = (counter + 1) % 2textNode.data = String(counter)}