Vue nextTick的源码理解，使用场景

Vue.nextTick( [callback, context] )

• 参数：
  {Function} [callback]
  {Object} [context]
• 用法：
  在下次 DOM 更新循环结束之后执行延迟回调。在修改数据之后立即使用这个方法，获取更新后的 DOM。

使用场景

• 在created里想要操纵dom时，因为在created的时候dom还没有渲染完成，直接操作dom是拿不到的，需要在nextTick获取。 在mounted()中进行任何DOM操作都不会有问题, 因为在mounted()执行时所有的DOM挂载和渲染都已完成

<div ref='myRef'>value</div>
* * *
created () {
  // 直接获取会报错，因为拿不到dom："TypeError: Cannot set property 'innerText' of undefined"
  console.log(this.$refs.myRef) // undefined
  this.$refs.myRef.innerText = "设值" //报错
}

所以在created钩子中进行的DOM操作，不使用$nextTick()会报错：

// 正确使用方式
created () {
 this.$nextTick(() => {
   this.$refs.myRef.innerText = "设值"
 })
}

• 在使用第三方插件时 ，这些插件需要dom动态变化后应用，这时候就需要使用$nextTick()来重新应用插件的方法，比如使用swiper

this.nextTick(function(){
   var swiper = new Swiper('.swiper-container', {
      pagination: '.swiper-pagination',
      nextButton: '.swiper-button-next',
      prevButton: '.swiper-button-prev',
      autoplay: 2500
  })
})

源码解析

Vue.nextTick中的回调函数会在页面渲染后才执行，因此会需要用到异步调用。
这里的异步主要使用了宏任务和微任务（可以看我这篇文章）
下面这段代码判断了当前运行环境支持的异步方法，优先级依次是Promise、MutationObserver、setImmediate和setTimeout，如果有兴趣可以去了解一下，简单的理解就是timerFunc就是一个异步执行函数

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]'
)) {
  // 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)
  }
}

上面代码里我们看到timerFunc这个异步函数里面执行的是flushCallbacks，这个函数的作用就是依次执行了callbacks数组中的方法，callbacks数组放着所有调用Vue.nextTick传过来的callback。

const callbacks = []

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

最后nextTick函数就是将传过来的callback放到callbacks数组里面，去调用timerFunc函数

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
    })
  }
}

总结

根据当前执行环境使用异步方法

1. Promise
2. MutationObserver
3. setImmediate
4. setTimeout

在下次 DOM 更新循环结束之后执行延迟回调

完整源码

Vue.nextTick源码路径/node_modules/vue/src/core/util/next-tick.js

/* @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]()
  }
}

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]'
)) {
  // 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
    })
  }
}