// 辅助函数
import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'
// 是否使用微任务
export let isUsingMicroTask = false
// 回调函数队列
const callbacks = []
// 回调函数队列的执行状态
let pending = false
// 触发队列执行
function flushCallbacks () {
  // 改为执行状态
  pending = false
  // 这边就是对回调队列进行遍历执行
  // 注意这边会对回调函数进行浅复制
  // 因为在回调函数也会往callbacks中添加新值
  // 但是我们将它们归到下一个异步中执行
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}
// 这边的源码注释比较多不舍得删了
// 英文较差（如我）的可以借助翻译工具看看
// 大致就是解释了为什么nextTick异步实现选择微任务来实现以及改版历史
// 最先的时候是是否微任务来实现的但是后面发现了out-in transitions出现的bug
// 后面改为宏任务（MessageChannel ）与微任务结合的方式但是出现了更多的小问题
// 比较经典的就是我们知道有些事件需要用户手动点击触发 比如播放video
// 但是将这个播放事件作为宏任务放到下个异步中可能就不再执行了
// 所以后面又改为全部优先使用微任务实现的方式
// 当然微任务由于触发优先级很高也会导致某些问题 比较典型的#6566 就是由于微任务的优先级比事件冒泡还高导致的
// 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
// 下面就是异步函数timerFunc的实现
// 注释也比较清楚了分析了使用不同实现的优劣
// 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 */
// 优先使用微任务promise.then
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    // 在then回调中调用flushCallbacks 实现了异步调用flushCallbacks 下面的其它调用也是如此
    p.then(flushCallbacks)
    // 修复IOS的兼容性BUG
    // 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)
  }
  // 表明使用微任务
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  // 使用MutationObserver来实现异步微任务
  // MutationObserver的使用这边就不分析了
  // 大家可以看看MDN教程 https://developer.mozilla.org/zh-CN/docs/Web/API/MutationObserver/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
// 降级使用宏任务setImmediate
} 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)
  }
// 最后没办法降级使用宏任务setTimeout
} else {
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}
// 暴露的nextTick实现
export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  // 其实调用nextTick就是往回调队列中添加回调cb
  // 在flushCallbacks中进行遍历执行
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    // 这边有个条件分支
    // 如果没有cb参数则执行_resolve 而_resolve实际是Promise实例的resolve函数
    // 所以我们可以在代码中调用nextTick().then()这样的写法
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  // 如果不是正在执行态则立即执行timerFunc
  // 调用timerFunc实现 timerFunc（微任务/宏任务） -> flushCallbacks -> cb.call(ctx) 的完整流程
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}
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// true
nextTick().then(() => {})
// error
nextTick(() => {}).then(() => {})
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this.showXXEl = true;
this.$refs.XX; // undefined
this.nextTick(()  => {
  this.$refs.XX; // DOMElement
})
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