vue2 nextTick&源码

等待下一次 DOM 更新刷新的工具方法。

使用场景

vue中nextTick的使用场景

Vue.component("example", {
  template: "<span>{{ message }}</span>",
  data: function() {
    return {
      message: "未更新"
    };
  },
  methods: {
    updateMessage: function() {
      this.message = "已更新";
      console.log(this.$el.textContent); // => '未更新'
      this.$nextTick(function() {
        console.log(this.$el.textContent); // => '已更新'
      });
    }
  }
});

1. nextTick 入参是一个回调函数，这个回调函数就是一个任务
2. 每次接受任务nextTick不会立即执行，而是把它push到callbacks这个异步队列里
3. 检查pending的值，如果为false，意味着“现在还没有一个异步更新队列被派发出去”， 就调用timerFunc，把当前维护的异步队列派发出去
4. 然后调用flushCallbacks异步执行器函数，处理异步任务

源码

export let isUsingMicroTask = false//是否在使用微任务

const callbacks = [] //异步更新队列
let pending = false
//任务执行器
function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0 //清空队列
  for (let i = 0; i < copies.length; i++) {//逐个执行异步任务
    copies[i]()
  }
}
let timerFunc//派发异步任务的函数
//根据不同浏览器，选择不同api来派发异步任务
//当前环境支持 promise
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
  //当前环境支持MutationObserver
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  // (#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
  //当前环境支持setImmediate
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  //当前环境支持 setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {// 全局变量callbacks
    if (cb) {
      try {
        cb.call(ctx)// 这里调用回调
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true// 只执行一次timerFunc函数
    timerFunc()
  }
  // $flow-disable-line
  //处理入参不是回调的情况
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}