vue源码nextTick使用及原理解析

<template>
 <div class="box">{{msg}}</div>
</template>
export default {
 name: 'index',
 data () {
  return {
   msg: 'hello'
  }
 },
 mounted () {
  this.msg = 'world'
  let box = document.getElementsByClassName('box')[0]
  console.log(box.innerHTML) // hello
 }
}

this.msg = 'world'
let box = document.getElementsByClassName('box')[0]
this.$nextTick(() => {
 console.log(box.innerHTML) // world
})

Object.defineProperty(obj, key, {
 enumerable: true,
 configurable: true,
 // 数据修改后触发set函数 经过一系列操作 完成dom更新
 set: function reactiveSetter (newVal) {
  const value = getter ? getter.call(obj) : val
  if (getter && !setter) return
  if (setter) {
   setter.call(obj, newVal)
  } else {
   val = newVal
  }
  childOb = !shallow && observe(newVal)
  dep.notify() // 执行dep notify方法
 }
})

export default class Dep {
 constructor () {
  this.id = uid++
  this.subs = []
 }
 notify () {
  const subs = this.subs.slice()
  for (let i = 0, l = subs.length; i < l; i++) {
   // 实际上遍历执行了subs数组中元素的update方法
   subs[i].update()
  }
 }
}

update () {
 /* istanbul ignore else */
 if (this.lazy) {
  this.dirty = true
 } else if (this.sync) {
  this.run()
 } else {
  queueWatcher(this) //执行queueWatcher
 }
}

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) {
   // 通过waiting 保证nextTick只执行一次
   waiting = true
   // 最终queueWatcher 方法会把flushSchedulerQueue 传入到nextTick中执行
   nextTick(flushSchedulerQueue)
  }
 }
}

function flushSchedulerQueue () {
 currentFlushTimestamp = getNow()
 flushing = true
 let watcher, id
 ...
 for (index = 0; index < queue.length; index++) {
  watcher = queue[index]
  if (watcher.before) {
   watcher.before()
  }
  id = watcher.id
  has[id] = null
  // 遍历执行渲染watcher的run方法 完成视图更新
  watcher.run()
 }
 // 重置waiting变量 
 resetSchedulerState()
 ...
}

export function nextTick (cb?: Function, ctx?: Object) {
 let _resolve
 // push进callbacks数组
 callbacks.push(() => {
   cb.call(ctx)
 })
 if (!pending) {
  pending = true
  // 执行timerFunc方法
  timerFunc()
 }
}

let timerFunc
// 判断是否原生支持Promise
if (typeof Promise !== 'undefined' && isNative(Promise)) {
 const p = Promise.resolve()
 timerFunc = () => {
  // 如果原生支持Promise 用Promise执行flushCallbacks
  p.then(flushCallbacks)
  if (isIOS) setTimeout(noop)
 }
 isUsingMicroTask = true
// 判断是否原生支持MutationObserver
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
 isNative(MutationObserver) ||
 // PhantomJS and iOS 7.x
 MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
 let counter = 1
 // 如果原生支持MutationObserver 用MutationObserver执行flushCallbacks
 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
// 判断是否原生支持setImmediate 
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
 timerFunc = () => {
 // 如果原生支持setImmediate 用setImmediate执行flushCallbacks
  setImmediate(flushCallbacks)
 }
// 都不支持的情况下使用setTimeout 0
} else {
 timerFunc = () => {
  // 使用setTimeout执行flushCallbacks
  setTimeout(flushCallbacks, 0)
 }
}

// flushCallbacks 最终执行nextTick 方法传进来的回调函数
function flushCallbacks () {
 pending = false
 const copies = callbacks.slice(0)
 callbacks.length = 0
 for (let i = 0; i < copies.length; i++) {
  copies[i]()
 }
}

<template>
 <div class="box">{{msg}}</div>
</template>

<script>
export default {
 name: 'index',
 data () {
  return {
   msg: 'hello'
  }
 },
 mounted () {
  this.msg = 'world'
  let box = document.getElementsByClassName('box')[0]
  setTimeout(() => {
   console.log(box.innerHTML) // world
  })
 }
}

<template>
 <div class="box">{{msg}}</div>
</template>

<script>
export default {
 name: 'index',
 data () {
  return {
   msg: 'hello'
  }
 },
 mounted () {
  this.$nextTick(() => {
   console.log(box.innerHTML) // hello
  })
  this.msg = 'world'
  let box = document.getElementsByClassName('box')[0]
 }
}