背景
我們先來看一段Vue的實行代碼:
export default {
data () {
return {
msg: 0
}
},
mounted () {
this.msg = 1
this.msg = 2
this.msg = 3
},
watch: {
msg () {
console.log(this.msg)
}
}
}
這段劇本實行我們猜想1000m後會順次打印:1、2、3。然則實際效果中,只會輸出一次:3。為何會湧現如許的狀況?我們來一探終究。
queueWatcher
我們定義watch
監聽msg
,實際上會被Vue如許挪用vm.$watch(keyOrFn, handler, options)
。$watch
是我們初始化的時刻,為vm
綁定的一個函數,用於建立Watcher
對象。那末我們看看Watcher
中是怎樣處置懲罰handler
的:
this.deep = this.user = this.lazy = this.sync = false
...
update () {
if (this.lazy) {
this.dirty = true
} else if (this.sync) {
this.run()
} else {
queueWatcher(this)
}
}
...
初始設定this.deep = this.user = this.lazy = this.sync = false
,也就是當觸發update
更新的時刻,會去實行queueWatcher
要領:
const queue: Array<Watcher> = []
let has: { [key: number]: ?true } = {}
let waiting = false
let flushing = false
...
export function queueWatcher (watcher: Watcher) {
const id = watcher.id
if (has[id] == null) {
has[id] = true
if (!flushing) {
queue.push(watcher)
} else {
// if already flushing, splice the watcher based on its id
// if already past its id, it will be run next immediately.
let i = queue.length - 1
while (i > index && queue[i].id > watcher.id) {
i--
}
queue.splice(i + 1, 0, watcher)
}
// queue the flush
if (!waiting) {
waiting = true
nextTick(flushSchedulerQueue)
}
}
}
這裏面的nextTick(flushSchedulerQueue)
中的flushSchedulerQueue
函數實在就是watcher
的視圖更新:
function flushSchedulerQueue () {
flushing = true
let watcher, id
...
for (index = 0; index < queue.length; index++) {
watcher = queue[index]
id = watcher.id
has[id] = null
watcher.run()
...
}
}
別的,關於waiting
變量,這是很重要的一個標誌位,它保證flushSchedulerQueue
回調只允許被置入callbacks
一次。
接下來我們來看看nextTick
函數,在說nexTick
之前,須要你對Event Loop
、microTask
、macroTask
有肯定的相識,Vue nextTick 也是重要用到了這些基本道理。假如你還不相識,能夠參考我的這篇文章Event Loop 簡介
好了,下面我們來看一下他的完成:
export const nextTick = (function () {
const callbacks = []
let pending = false
let timerFunc
function nextTickHandler () {
pending = false
const copies = callbacks.slice(0)
callbacks.length = 0
for (let i = 0; i < copies.length; i++) {
copies[i]()
}
}
// An asynchronous deferring mechanism.
// In pre 2.4, we used to use microtasks (Promise/MutationObserver)
// but microtasks actually has too high a priority and fires in between
// supposedly sequential events (e.g. #4521, #6690) or even between
// bubbling of the same event (#6566). Technically setImmediate should be
// the ideal choice, but it's not available everywhere; and 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)) {
timerFunc = () => {
setImmediate(nextTickHandler)
}
} else if (typeof MessageChannel !== 'undefined' && (
isNative(MessageChannel) ||
// PhantomJS
MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
const channel = new MessageChannel()
const port = channel.port2
channel.port1.onmessage = nextTickHandler
timerFunc = () => {
port.postMessage(1)
}
} else
/* istanbul ignore next */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
// use microtask in non-DOM environments, e.g. Weex
const p = Promise.resolve()
timerFunc = () => {
p.then(nextTickHandler)
}
} else {
// fallback to setTimeout
timerFunc = () => {
setTimeout(nextTickHandler, 0)
}
}
return function queueNextTick (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, reject) => {
_resolve = resolve
})
}
}
})()
起首Vue經由過程callback
數組來模仿事宜行列,事宜隊里的事宜,經由過程nextTickHandler
要領來實行挪用,而何事舉行實行,是由timerFunc
來決議的。我們來看一下timeFunc
的定義:
if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
timerFunc = () => {
setImmediate(nextTickHandler)
}
} else if (typeof MessageChannel !== 'undefined' && (
isNative(MessageChannel) ||
// PhantomJS
MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
const channel = new MessageChannel()
const port = channel.port2
channel.port1.onmessage = nextTickHandler
timerFunc = () => {
port.postMessage(1)
}
} else
/* istanbul ignore next */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
// use microtask in non-DOM environments, e.g. Weex
const p = Promise.resolve()
timerFunc = () => {
p.then(nextTickHandler)
}
} else {
// fallback to setTimeout
timerFunc = () => {
setTimeout(nextTickHandler, 0)
}
}
能夠看出timerFunc
的定義優先遞次macroTask
–> microTask
,在沒有Dom
的環境中,運用microTask
,比方weex
setImmediate、MessageChannel VS setTimeout
我們是優先定義setImmediate
、MessageChannel
為何要優先用他們建立macroTask而不是setTimeout?
HTML5中劃定setTimeout的最小時候耽誤是4ms,也就是說抱負環境下異步回調最快也是4ms才觸發。Vue運用這麼多函數來模仿異步使命,其目標只要一個,就是讓回調異步且儘早挪用。而MessageChannel 和 setImmediate 的耽誤顯著是小於setTimeout的。
解決題目
有了這些基本,我們再看一遍上面提到的題目。由於Vue
的事宜機制是經由過程事宜行列來調理實行,會等主歷程實行餘暇后舉行調理,所以先回去守候一切的歷程實行完成今後再去一次更新。如許的機能上風很顯著,比方:
如今有如許的一種狀況,mounted的時刻test的值會被++輪迴實行1000次。 每次++時,都邑依據相應式觸發setter->Dep->Watcher->update->run
。 假如這時刻沒有異步更新視圖,那末每次++都邑直接操縱DOM更新視圖,這是異常斲喪機能的。 所以Vue完成了一個queue
行列,鄙人一個Tick(或者是當前Tick的微使命階段)的時刻會一致實行queue
中Watcher
的run。同時,具有雷同id的Watcher不會被反覆加入到該queue中去,所以不會實行1000次Watcher的run。終究更新視圖只會直接將test對應的DOM的0變成1000。 保證更新視圖操縱DOM的行動是在當前棧實行完今後下一個Tick(或者是當前Tick的微使命階段)的時刻挪用,大大優化了機能。
風趣的題目
var vm = new Vue({
el: '#example',
data: {
msg: 'begin',
},
mounted () {
this.msg = 'end'
console.log('1')
setTimeout(() => { // macroTask
console.log('3')
}, 0)
Promise.resolve().then(function () { //microTask
console.log('promise!')
})
this.$nextTick(function () {
console.log('2')
})
}
})
這個的實行遞次想必人人都曉得前後打印:1、promise、2、3。
- 由於起首觸發了
this.msg = 'end'
,致使觸發了watcher
的update
,從而將更新操縱callback push進入vue的事宜行列。 -
this.$nextTick
也為事宜行列push進入了新的一個callback函數,他們都是經由過程setImmediate
–>MessageChannel
–>Promise
–>setTimeout
來定義timeFunc
。而Promise.resolve().then
則是microTask,所以會先去打印promise。 - 在支撐
MessageChannel
和setImmediate
的狀況下,他們的實行遞次是優先於setTimeout
的(在IE11/Edge中,setImmediate耽誤能夠在1ms之內,而setTimeout有最低4ms的耽誤,所以setImmediate比setTimeout(0)更早實行回調函數。其次由於事宜行列里,優先收入callback數組)所以會打印2,接着打印3 - 然則在不支撐
MessageChannel
和setImmediate
的狀況下,又會經由過程Promise
定義timeFunc
,也是老版本Vue 2.4 之前的版本會優先實行promise
。這類狀況會致使遞次成為了:1、2、promise、3。由於this.msg一定先會觸發dom更新函數,dom更新函數會先被callback收納進入異步時候行列,其次才定義Promise.resolve().then(function () { console.log('promise!')})
如許的microTask,接着定義$nextTick
又會被callback收納。我們曉得行列滿足先進先出的準繩,所以優先去實行callback收納的對象。
跋文
假如你對Vue源碼感興趣,能夠來這裏:
參考文章: