nextTick和异步更新视图

加深vue渲染页面原理记录

1. 栗子

先来了解一个例子关于vue中数据和DOM更新机制以及vm.nextTick作用。

template

<template>
    <div>
        <div ref="msg">{{msg}}</div>
        <button @click="changeMsg">点击</button>
    </div>
</template>

script

export default {
    data() {
        return {
            msg:"default",
        }
    },
    methods: {
        changeMsg() {
            //把 nextTick放在前面结果也是一样。
            // this.$nextTick(() => {
            //     console.log("我是nextTick:" + this.$refs.msg.textContent)//改变最新的值
            // })
            this.msg =  new Date().getTime();
            console.log(this.msg)
            this.msg =  new Date().getTime();
            console.log(this.msg)
            this.msg =  new Date().getTime();
            console.log(this.msg)
            this.msg =  new Date().getTime();
            console.log(this.msg)// 改变最新的值
            console.log("我是外边：" + this.$refs.msg.textContent)//初始值
            setTimeout(() => {
                console.log("我是setTimeout:" + this.$refs.msg.textContent)//改变最新的值
            }, 0)
            this.$nextTick(() => {
                console.log("我是nextTick:" + this.$refs.msg.textContent)//改变最新的值
            })
        }
    },
}

点击按钮 控制台结果图

nextTick.jpg

通过图上可知：DOM更新是异步的，我们只能通过异步的手法让获取DOM值是最新改变的值。并且nextTick和传统的setTimeout有异曲同工之处。

2. 什么是异步更新队列

官方文档解释

Vue 在更新 DOM 时是异步执行的。只要侦听到数据变化，Vue 将开启一个队列，并缓冲在同一事件循环中发生的所有数据变更。如果同一个 watcher 被多次触发，只会被推入到队列中一次。这种在缓冲时去除重复数据对于避免不必要的计算和 DOM 操作是非常重要的。然后，在下一个的事件循环“tick”中，Vue 刷新队列并执行实际 (已去重的) 工作。Vue 在内部对异步队列尝试使用原生的 Promise.then、MutationObserver 和 setImmediate，如果执行环境不支持，则会采用 setTimeout(fn, 0) 代替。

例如，当你设置 vm.someData = 'new value'，该组件不会立即重新渲染。当刷新队列时，组件会在下一个事件循环“tick”中更新。多数情况我们不需要关心这个过程，但是如果你想基于更新后的 DOM 状态来做点什么，这就可能会有些棘手。虽然 Vue.js 通常鼓励开发人员使用“数据驱动”的方式思考，避免直接接触 DOM，但是有时我们必须要这么做。为了在数据变化之后等待 Vue 完成更新 DOM，可以在数据变化之后立即使用 Vue.nextTick(callback)。这样回调函数将在 DOM 更新完成后被调用

注意：
看了大堆文字说明还是懵逼的感觉，简单来说：这也和vue数据响应原理有关联的，异步更新队列意思，就当vue检测到data变化以后，由于watcher触发了，所以会被推入到队列中，这也是操作DOM的时候，是不会立即更新DOM的，所以用nextTick(),可以立即更新DOM,获取最新的DOM状态。
要想理解官方文档的说明，我们必须先了解js中的事件循环机制和事件队列的概念可以借鉴
Promise 解析和事件循环机制

应用场景：
在Vue生命周期的created()钩子函数进行的DOM操作一定要放在Vue.nextTick()的回调函数中。

3. Vue 2.6.10 源码浅析

/* @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.
  // Techinically 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
    })
  }
}

参考文献

Vue官网-异步更新队列
浅谈vue之 this.$nextTrick()