Vue源碼之旅-3- nextTick()實現原理

參考文檔的 異步更新隊列, 從官方文檔的說明， 我們可以看出， Vue 的更新是組件級的，這麼做的目的就是爲了提升性能， 它內部怎麼做的也比較好理解， 其實就是利用了 微任務 和 宏任務

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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 () { // 多個nextTick中傳遞的回調函數依次執行
  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.
  // Technically 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) { // flushQueue 會使用nextTick 保證當前視圖渲染完成
  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
    })
  }
}

這裏可以看出，先定義了一個 callbacks 的數組，然後在nextTick() 這個方法中把每次傳入的callback函數都放到這個數組裏，接着在內部判斷選擇哪種異步的方式，
1.使用原生的 Promise.then、
2. 使用MutationObserver 和 setImmediate，
3. 如果以上都不支持，則會採用 setTimeout(fn, 0) 代替。
最後循環這個callbacks 數組， 依次執行內部的函數，進行批處理

總結：

1. 當調用nextTick 傳入一個 cb 回調
2. 將回調函數存入數組中
3. 調用 timerFunc() 方法 , 會進行判斷採用哪種異步回調方式
   1. 嘗試採用Promise 回調,
   2 .嘗試採用MuationObserver回調，
   3. 嘗試採用 setImmediate回調，
   4. 最後採用setTimeout回調
   最後執行 flushCallbacks() 這個方法，便利callbacks 數組，依次執行裏邊的每個函數