text: /* @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
    })
  }
  
  /**
   * 下一个macro或者micro task再执行队列里面的任务
   * 做的操作就是任务放入队列，并开启一个macro或者micro task，
   * 设置一个标志位标记已经开启了一个macro或者micro task，如果在有任务进来，
   * 就不要在开启一个macro或者micro task，只有当这个macro或者micro task执行结束，才能
   * 重新开启一个新的macro或者micro task，
   * @param {*} cb
   * @param {*} ctx
   */
  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
      })
    }
  }
  