「面试」- Vue nextTick实现原理

// 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
  })
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// 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)
  }
}
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