用法
nextTick 是 vue 中的一个重要的API,先看下官方文档的介绍。
vue.nextTick( [callback, context] )
-
参数:
- {Function} [callback]
- {Object} [context]
-
用法:
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 (2.1.0 起新增,详见接下来的提示) Vue.nextTick() .then(function () { // DOM 更新了 })
可以看出,nextTick 实际上就是 DOM 更新后的一个回调。那DOM更新的时机是什么时候呢?
DOM 的更新时机
这要从 js 的运行机制说起。(硬广:js 的运行机制)。简单点说就是,js 代码的执行顺序是基于事件循环的,大致分为几个步骤:
- 1、同步任务都在主线程上执行,形成一个执行栈。
- 2、异步任务会被放入主线程之外的任务队列中。
- 3、主线程同步任务执行完后,会去任务队列中查找异步任务,将其放到执行栈中执行。
- 4、主线程不断重复上面三步,这也就是常说的事件循环。
当然异步任务分为宏任务和微任务。具体就不说了,可以看下硬广。主线程在同步任务执行完后,会首先从任务队列中查找并执行微任务,直到最后一个微任务执行完,第一次事件循环结束。
开始第二次事件循环,任务队列中第一个宏任务变成同步任务,被首先执行,然后执行微任务,不断这样循环执行,直到所有任务执行完毕。
而 DOM 的更新时机则是在微任务执行结束后。我们看个栗子:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.box {
400px;
height: 200px;
margin: 0 auto;
display: flex;
justify-content: center;
align-items: center;
border: 1px solid #259;
}
</style>
</head>
<body>
<div class="box">
<input class="input" type="text">
</div>
</body>
<script>
const box = document.querySelector('.box')
function sleep(time) {
const start = new Date().getTime();
while (new Date().getTime() - start < time) { }
}
// click监听事件
function onClick() {
const testElement = document.querySelector('.input') || ''
testElement.focus()
Promise.resolve().then(function () {
console.log('promise')
sleep(2000)
})
setTimeout(() => {
sleep(2000)
console.log('timeout')
})
}
box.addEventListener('click', onClick)
</script>
</html>
我们可以看到,DOM 的渲染是在微任务之后的,而且在下一次事件循环之前。
nextTick 源码解析
/* @flow */
/* globals MutationObserver */
// 空函数
import { noop } from 'shared/util'
// 处理错误的函数
import { handleError } from './error'
// 判断运行环境
import { isIE, isIOS, isNative } from './env'
// 是否使用微任务的标识符
export let isUsingMicroTask = false
// 存放回调函数的数组
const callbacks = []
// nexttick 执行状态
let pending = false
// 遍历回调函数
function flushCallbacks() {
pending = false
// 浅拷贝存放回调函数的数组
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)) {
// 如果浏览器支持原生 promise, 则直接使用 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)
}
// 将是否使用微任务改为 true
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)
// MutationObserver:如果浏览器支持 MutationObserver,则使用 MutationObserver
//(该 API 是个微任务,可以监听 DOM 元素是否变动,当所有 DOM 操作完成后,会触发相应的事件)
// 当 不支持 promise 时,优先使用该 API
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.
// setImmediate:宏任务,只支持 IE、edge 浏览器。
// 与 setTimeout 相比,优势在于它是立即执行,而 setTimeout 最小间隔是 4ms
timerFunc = () => {
setImmediate(flushCallbacks)
}
} else {
// Fallback to setTimeout.
timerFunc = () => {
setTimeout(flushCallbacks, 0)
}
}
// 封装的nextTick函数,cb:回调函数,ctx:this 指向
export function nextTick(cb?: Function, ctx?: Object) {
let _resolve
// 回调函数统一进入 callbacks 中进行处理
callbacks.push(() => {
if (cb) {
try {
cb.call(ctx)
} catch (e) {
handleError(e, ctx, 'nextTick')
}
} else if (_resolve) {
_resolve(ctx)
}
})
if (!pending) {
pending = true
timerFunc()
}
// 如果没有传入回调函数,且当前环境支持 promise ,则返回一个 promise 对象
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}