小鹅通技术分享

Vue NextTick解析

2019-07-13

nextTick

官方文档给的描述：

将回调延迟到下次 DOM 更新循环之后执行。在修改数据之后立即使用它，然后等待 DOM 更新。它跟全局方法 Vue.nextTick 一样，不同的是回调的 this 自动绑定到调用它的实例上。 

// 修改数据
vm.msg = 'Hello'
// DOM 还没有更新
Vue.nextTick(function () {
  // DOM 更新了
})

// 作为一个 Promise 使用 (2.1.0 起新增，详见接下来的提示)
Vue.nextTick()
     .then(function () {
    // DOM 更新了
})

2.1.0 起新增：如果没有提供回调且在支持 Promise 的环境中，则返回一个 Promise。请注意 Vue 不自带 Promise 的 polyfill，所以如果你的目标浏览器不原生支持 Promise (IE：你们都看我干嘛)，你得自己提供 polyfill。

回调用于DOM更新后，一开始看到一脸懵逼，Vue不是响应式的吗？更新数据视图就同步更新吗？ 下面给个简单例子感受下

可以看出并不是数据更新视图就同步更新。要想了解其中原理，先要了解JS运行机制，Event Loop 宏任务（macro task）以及微任务（ micro task），主线程的执行过程就是一个 tick，而所有的异步结果都是通过 “任务队列” 来调度。 消息队列中存放的是一个个的任务（task）。 规范中规定 task 分为两大类，分别是 macro task 和 micro task，并且每个 macro task 结束后，都要清空所有的 micro task。下面是一个例子是他们的执行顺序，具体相关描述请自行百度。

for (macroTask of macroTaskQueue) {
    // 1. 处理当前MacroTask
    handleMacroTask();
      
    // 2. 处理所有microTask
    for (microTask of microTaskQueue) {
        handleMicroTask(microTask);
    }
}

在浏览器环境中，常见的 macro task 有 setTimeout、MessageChannel、postMessage、setImmediate；常见的 micro task 有 MutationObsever 和 Promise.then。

NextTick 在Vue 是一个单独的文件在 vue/src/core/util/next-tick.js 然而当我去准备去粘代码时发现，代码改了改了😱😱😱😱既改之则安之那就先看看2.5版本，在看2.6改了啥。

2.5nextTick

那就先看看2.5版本吧，代码就100来行就直接放上来了，逻辑很清晰

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIOS, isNative } from './env'

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 both microtasks and (macro) tasks.
// In < 2.4 we used microtasks everywhere, but there are some scenarios where
// microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690) or even between bubbling of the same
// event (#6566). However, using (macro) tasks everywhere also has subtle problems
// when state is changed right before repaint (e.g. #6813, out-in transitions).
// Here we use microtask by default, but expose a way to force (macro) task when
// needed (e.g. in event handlers attached by v-on).
let microTimerFunc
let macroTimerFunc
let useMacroTask = false

// Determine (macro) task defer implementation.
// Technically setImmediate should be the ideal choice, but it's only available
// in IE. The only polyfill that consistently queues the callback after all DOM
// events triggered in the same loop is by using MessageChannel.
/* istanbul ignore if */
if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  macroTimerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else if (typeof MessageChannel !== 'undefined' && (
  isNative(MessageChannel) ||
  // PhantomJS
  MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
  const channel = new MessageChannel()
  const port = channel.port2
  channel.port1.onmessage = flushCallbacks
  macroTimerFunc = () => {
    port.postMessage(1)
  }
} else {
  /* istanbul ignore next */
  macroTimerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

// Determine microtask defer implementation.
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  microTimerFunc = () => {
    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)
  }
} else {
  // fallback to macro
  microTimerFunc = macroTimerFunc
}

/**
 * Wrap a function so that if any code inside triggers state change,
 * the changes are queued using a (macro) task instead of a microtask.
 */
export function withMacroTask (fn: Function): Function {
  return fn._withTask || (fn._withTask = function () {
    useMacroTask = true
    const res = fn.apply(null, arguments)
    useMacroTask = false
    return res
  })
}

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
    if (useMacroTask) {
      macroTimerFunc()
    } else {
      microTimerFunc()
    }
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

首先声明了 microTimerFunc 和 macroTimerFunc 2 个变量，它们分别对应的是 micro task 的函数和 macro task 的函数。2.5版本会优先使用 microTimerFunc ，如果不兼容则会降级到macroTimerFunc。

microTimerFunc 的实现：原生的 Promise；

macroTimerFunc 的实现：setImmediate || MessageChannel || setTimeout。

next-tick对外暴露了2个函数，其中一个就是我们经常用的 nextTick他会把我们传进来的cb都放进 callbacks数组，最后根据useMacroTask选择调用执行方式， 最后在下个tick执行 flushCallbacks遍历callbacks执行所有回调函数。
另一个暴露出来的函数 withMacroTask他上面注释也很清楚，就是一个包装函数，确保执行的时候使用的是 macroTimerFunc 方式执行。

2.6nextTick

先不管变了什么上来先把代码粘上来，代码数量变化不大，也是100来行

/* @flow */
/* globals MutationObserver */

// noop 空函数，可用作函数占位符
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 */

// timerFunc 优先使用原生 Promise
// 原本 MutationObserver 支持更广，但在 iOS >= 9.3.3 的 UIWebView 中，触摸事件处理程序中触发会产生严重错误
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.
    // IOS 的 UIWebView，Promise.then 回调被推入 microtask 队列但是队列可能不会如期执行。
    // 因此，添加一个空计时器“强制”执行 microtask 队列。
    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)
  // 当原生 Promise 不可用时，timerFunc 使用原生 MutationObserver
  // 如 PhantomJS，iOS7，Android 4.4
  // issue #6466 MutationObserver 在 IE11 并不可靠，所以这里排除了 IE
  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
    // cb 回调函数会经统一处理压入 callbacks 数组
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  // 执行异步延迟函数 timerFunc
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  // 当 nextTick 没有传入函数参数的时候，返回一个 Promise 化的调用
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

发现microTimerFunc 和 macroTimerFunc都不见了被 timerFunc取代

// 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).

大概是这个意思吧：

2.6优先使用microtask 作为包装函数。
2.5版本使用macro结合micro，重绘之前状态改变时会有小问题。
此外，在事件处理程序中使用 macrotask 会导致一些无法规避的奇怪行为，去看了看 Issues

先看看2.6正常效果： 在看看2.5版本。监听了resize事件，当窗口大小变化，达到1000时改变状态，同时通过媒体查询改变了样式。拖动窗口变化可以看到明显的闪烁。 (貌似曾经我也遇到过这个问题，当时一直不知道为啥造成，看了这个终于知道了)

第二个问题，一般可以概括为由于使用 macroTask 处理 DOM 操作，会使得有些时候触发和执行之间间隔太大， 例如在移动端，单击的 handler 和音频播放功能不在同一 tick 里。

差异比较

2.6优先级顺序：Promise > MutationObserver > setImmediate >SetTiemout

2.5优先级顺序：Promise > setImmediate > MessageChannel > setTimeout 但在v-on事件处理程序中触发的的NextTick会优先使用 macrotask.

🎉 💯🎉 💯🎉 💯🎉 💯🎉 💯🎉 💯🎉 💯🎉 💯🎉 💯

使用须知 →