Vue nextTick的源碼理解，使用場景

Vue.nextTick( [callback, context] )

• 參數：
  {Function} [callback]
  {Object} [context]
• 用法：
  在下次 DOM 更新循環結束以後執行延遲迴調。在修改數據以後當即使用這個方法，獲取更新後的 DOM。

使用場景

• 在created裏想要操縱dom時，由於在created的時候dom尚未渲染完成，直接操做dom是拿不到的，須要在nextTick獲取

<div ref='myRef'>value</div>
* * *
~~created () {
  ~~// 直接獲取會報錯，由於拿不到dom："TypeError: Cannot set property 'innerText' of undefined"
  ~~console.log(this.$refs.myRef) // undefined
  ~~this.$refs.myRef.innerText = "設值"
}~~

因此在created鉤子中進行的DOM操做，不使用$nextTick()會報錯：

// 正確使用方式
created () {
 this.$nextTick(() => {
   this.$refs.myRef.innerText = "設值"
 })
}

• 在使用第三方插件時 ，這些插件須要dom動態變化後應用，這時候就須要使用$nextTick()來從新應用插件的方法，好比使用swiper

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

源碼解析

Vue.nextTick中的回調函數會在頁面渲染後才執行，所以會須要用到異步調用。
下面這段代碼判斷了當前運行環境支持的異步方法，優先級依次是Promise、MutationObserver、setImmediate和setTimeout，若是有興趣能夠去了解一下，簡單的理解就是timerFunc就是一個異步執行函數

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)
  }
}

上面代碼裏咱們看到timerFunc這個異步函數裏面執行的是flushCallbacks，這個函數的做用就是依次執行了callbacks數組中的方法，callbacks數組放着全部調用Vue.nextTick傳過來的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]()
  }
}

最後nextTick函數就是將傳過來的callback放到callbacks數組裏面，去調用timerFunc函數

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
    })
  }
}

完整源碼

Vue.nextTick源碼路徑/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
    })
  }
}