$nextTick 宏任务 微任务 macrotasks microtasks

Posted 2021-01-18 mengfangui

1、nextTick调用方法

首先看nextTick的调用方法：

https://cn.vuejs.org/v2/api/#Vue-nextTick

// 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
  })

即：既可以支持回调函数，也可以支持then方法（即Promise）。

 

2、vue nextTick源码分析

https://github.com/vuejs/vue/blob/dev/src/core/util/next-tick.js

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

 

更核心的代码：

if (!pending) {
    pending = true
    if (useMacroTask) {
      macroTimerFunc()
    } else {
      microTimerFunc()
    }
  }

即：nextTick既可以是宏任务，又可以是微任务！

 

接着看微任务的定义：

// 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
}

即：vue环境支持Promis的话，使用Promise。否则microTimerFunc 被定义为宏任务macroTimerFunc。

 

接着看macroTimerFunc的定义：

// 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)
  }
}

优先使用setImmediate，其次是MessageChannel，最后是setTimeout。以上三个都属于宏任务。

 

$nextTick属于宏任务还是微任务，你会了吗？