text: /* @flow */
  /* globals MutationObserver */
  
  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
    // 浅拷贝了 callbacks
    const copies = callbacks.slice(0)
    // 清空 callbacks
    callbacks.length = 0
    for (let i = 0; i < copies.length; i++) {
      // 执行
      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
  
  // 兼容性处理生成 timerFunc 
  // 先后尝试了 Promise MutationObserver setImmediate setTimeout
  // 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)) {
    // 直接 promise 就直接用 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]'
  )) {
    // 支持 MutationObserver
    // 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)) {
    // 支持 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)
    }
  }
  
  export function nextTick (cb?: Function, ctx?: Object) {
    let _resolve
    // push 进去需要执行的 callbacks
    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
      })
    }
  }
  