VuenextTick延迟回调获取更新后DOM机制详解

for (MacroTask of macroTaskQueue) { 
    // 1. 执行一个宏任务
    handleMacroTask();
    // 2. 执行所有的微任务
    for (microTask of microTaskQueue) { 
        handleMicroTask(microTask);
    }
 }

<template>
  <div>
    <div>{{count}}</div>
    <div @click="handleClick">click</div>
  </div>
</template>
export default {
    data () {
        return {
            number: 0
        };
    },
    methods: {
        handleClick () {
            for(let i = 0; i < 10000; i++) {
                this.count++;
            }
        }
    }
}

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isiOS, isNative } from './env'
export let isUsingMicroTask = false
// 首先定义一个 callbacks 数组用来存储 nextTick，在下一个 tick 处理这些回调函数之前，
// 所有的 cb 都会被存在这个 callbacks 数组中
const callbacks = []
// pending 是一个标记位，代表一个等待的状态
let pending = false
// 最后执行 flushCallbacks() 方法，遍历callbacks数组，依次执行里边的每个函数
function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}
let timerFunc

if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  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)) {
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  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
    })
  }
}

created(){
  vm.$nextTick(() => {  
      //不使用this.$nextTick()方法操作DOM会报错
      this.$refs.test.innerhtml="created中操作了DOM"
  });
}

<template>
  <div class="test">
    <p ref='msg' id="msg">{{msg}}</p>
  </div>
</template>
<script>
export default {
  name: 'Test',
  data () {
    return {
      msg:"hello world",
    }
  },
  methods: {
    changeMsg() {
      this.msg = "hello Vue"  // vue数据改变，改变了DOM里的innerText
      let msgEle = this.$refs.msg.innerText  //后续js对dom的操作
      console.log(msgEle)  // hello world
      // 输出可以看到data里的数据修改后DOM并没有立即更新，后续的DOM不是最新的
      this.$nextTick(() => {
        console.log(this.$refs.msg.innerText) // hello Vue
      })
      this.$nextTick().then(() => {
        console.log(this.$refs.msg.innerText) // hello Vue
      })
    },
    changeMsg2() {
      this.$nextTick(() => {
        console.log(this.$refs.msg.innerText) // 1.hello world 
      })
      this.msg = "hello Vue" // 2.
      console.log(this.$refs.msg.innerText) // hello world
      this.$nextTick().then(() => {
        console.log(this.$refs.msg.innerText) // hello Vue
      })
      // nextTick中先添加的先执行，执行1后，才会执行2（Vue操作Dom的异步）
    }
  }
}
</script>