這個API的官方說明是:
在下次 DOM 更新循環結束之後執行延遲迴調。在修改數據之後立即使用這個方法,獲取更新後的 DOM。
這是怎麼做的呢?
Vue源碼裏有一個next-tick.js,源碼比較短,我們來仔細看一看:
/* @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[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.
// 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)
}
}
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
})
}
}
$nextTick實際上是把傳入的函數存到一個數組裏。然後根據環境情況,添加一個微任務或者宏任務來順序執行數組裏的函數。
代碼的註釋裏寫了,之前使用過一次宏任務,結果bug比較多。又換回了微任務。代碼來首先使用原生的Promise來添加微任務,如果沒有,就使用MutationObserver。如果兩個都沒有,就降級到宏任務。依次順序這樣的:
添加微任務
Promise
MutationObserver
添加宏任務
setImmediate
setTimeout
問題1
微任務和宏任務又啥區別呢?
我在網上找到下面這些圖片,對理解又很好的幫助:
藍色的代表宏任務,黃色的是微任務,灰色的是瀏覽器重新渲染UI界面。
那麼這兩個任務的區別就是,在每一次渲染界面之前,都會執行宏任務然後清空微任務隊列。爲了驗證這個過程,可以寫下面的代碼進行測試。
<template>
<p id="test">{{ i }}</p>
</template>
<script>
export default {
data () {
return {
i: 1
}
},
components: {
BookCard
},
mounted () {
const time = new Date().getTime()
Promise.resolve().then(() => {
console.log('promise ' + document.querySelector('#test').innerHTML)
while ((new Date().getTime() - time) < 1000 * 5) {
}
})
this.i = 2
}
}
</script>
上面代碼會輸出promise 2,卡住5秒之後,在瀏覽器裏才能看到數字1變成了2。這是因爲我們利用Promise在微任務裏卡了瀏覽器5秒,5秒以後瀏覽器纔會重新渲染界面。但是在渲染界面之前,dom節點已經更新了,這是因爲在微任務執行的時候,dom節點已經更新完畢了,這裏猜測Vue用於更新Dom的操作在這一輪循環的微任務最前面(也就是我們寫的Promise.resolve代碼之前,Vue應該就插入了一個用於更新Dom的微任務),但是這個時候瀏覽器還沒重新渲染頁面。
問題2
爲什麼添加了微任務和宏任務就能在Dom更新以後執行?
根據第一個問題的圖片,我們知道,如果在當前任務裏添加宏任務。那麼下一個循環週期,也就是重新渲染之後會執行(這個時候肯定是Dom更新以後執行的)。
那麼微任務呢,這應該是Vue保證的,在Vue生命週期或者事件函數裏添加微任務,微任務會在Dom更新後,瀏覽器重新渲染頁面前執行。所以猜測Vue每次會在執行用戶代碼之前先添加更新Dom的微任務。等我看看源碼再來和大家分享。
問題3
哪些函數能添加微任務,哪些函數能添加宏任務呢?
宏任務: setTimeout, setInterval, setImmediate, requestAnimationFrame, I/O
微任務: process.nextTick, Promises, Object.observe, MutationObserver
其中requestAnimationFrame,比較特殊
window.requestAnimationFrame()
告訴瀏覽器——你希望執行一個動畫,並且要求瀏覽器在下次重繪之前調用指定的回調函數更新動畫。該方法需要傳入一個回調函數作爲參數,該回調函數會在瀏覽器下一次重繪之前執行。這個函數實際測試,會在微任務之後,瀏覽器重新渲染界面之前執行。很多動畫庫都是利用了這個函數。
參考文章:
https://github.com/aooy/blog/issues/5
https://segmentfault.com/q/1010000017571945