Vue 2.0 next-tick 实现

next-tick实现

next-tick 算是 Vue 内部的一个核心的方法，它提供了一种异步执行任务的机制。它具体的源码在 src/core/util/next-tick.js 内部。

/* @flow */
/* globals MessageChannel */

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')   // 处理当 cb 执行过程中出现的报错
      }
    } 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
    })
  }
}

在 next-tick 内部分别定义了 microTimerFunc 和 macroTimerFunc，用以存储当前宿主环境所支持的 mircoTask 和 marcoTask。在 Vue 当中使用的 marcoTask 包含了 setImmediate/messageChannel/setTimeout，mircoTask 包含了 Promise。

Vue 在全局环境下提供了 Vue.nextTick 方法，在实例上提供了 $nextTick 方法以供调用。就拿全局对象上提供的 Vue.nextTick 方法来说，首先将传入的 cb 缓存至 callbacks 内部。然后根据 useMacroTask 来决定使用 macroTimerFunc 还是 microTimerFunc。在 nextTick 内部并没有直接执行传入的 cb，而是缓存至 callbacks 内部，在下一帧遍历 callbacks 内部缓存的所有 cb，这个时候这些 cb 都是同步去执行的。

特别是使用 microTimerFunc 的情况下，我们可以看到首先在函数外部定义一个被 resolved 的 promise。然后在函数体内部将 flushCallbacks 方法至于promise.then当中：

const p = Promise.resolve()
  microTimerFunc = () => {
    p.then(flushCallbacks)
  }

通过代码我们得知 flushCallbacks 方法内部是通过一个 for 循环去遍历执行 callbacks 内部缓存的所有的回调函数。这样就会将这些 callbacks 放到同一帧当中去执行。

CommanderXL / Biu-blog

Vue 2.0 next-tick 实现 #21

next-tick实现