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Vue的$nextTick

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先说结论

$nextTick 会尽最大可能较快的在下一次异步任务中执行。

源码追溯

Vue 的 $nextTick 和定义在源码目录的src/core/util/next-tick.js 里面

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// src/core/instance/index.js
// line: 21
renderMixin(Vue)

// renderMixin定义在
// src/core/instance/render.js 里面

export function renderMixin (Vue: Class<Component>) {
  ...
  Vue.prototype.$nextTick = function (fn: Function) {
    return nextTick(fn, this)
  }
	...
}

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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
    })
  }
}

callbacks 为定义的一个全局数组，用来存放下一次 tick 需要执行的方法，每次执行完成后 callbacks 会清空。

当执行环境支持 promise 时 $nextTick 也支持 promise 化的调用方式。

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this.$nextTick().then(() => {
  // do something
})

关键的代码在这 4 行

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if (!pending) {
  pending = true
  timerFunc()
}

主要功能实现依托于 timerFunc 这是一个根据不同平台有不同的实现。

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function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

let timerFunc
// 如果支持promise就用promise
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)
  }

// 如果不支持promise看看支不支持 MutationObserver
// 关于 MutationObserver, 可以参考https://developer.mozilla.org/zh-CN/docs/Web/API/MutationObserver
} 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)
  }

// 如果也不支持MutationObserver看看支不支持setImmediate
} 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)
  }
// 如果都不支持至少保证$nextTick为异步的
} else {
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

timerFunc 会依次从 Promise > MutationObserver > setImmediate > setTimeout 这几个中选择一个平台支持的异步来实现。

做这么多，其实就是想让 $nextTick 在下一个异步阶段尽可能的考前执行。

总结

$nextTick 会尽最大可能较快的在下一次异步任务中执行。