$nextTick 源码解析

/* @flow */
/* globals MutationObserver */

// 引入 noop、 handleError 错误处理、 isIE、 isIOS、 isNative 方法
import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'

// 是否正在使用微任务
export let isUsingMicroTask = false

// 需要处理的事件队列
const callbacks = []
// 设置一个标记，如果已经有 timerFunc 被推送到任务队列中去则不再推送
let pending = false

// 执行事件队列中的事件
function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}
// 这里我们有使用微任务的异步延迟包装器
// Here we have async deferring wrappers using microtasks.
// 在 2.5 版本我们使用宏任务和微任务相结合
// In 2.5 we used (macro) tasks (in combination with microtasks).
// 然而，当状态在重新绘制之前更改时，它有一些微妙的问题
// However, it has subtle problems when state is changed right before repaint 
// (e.g. #6813, out-in transitions).
// 另外，在事件处理程序中使用宏任务会导致一些无法回避的奇怪的行为
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// 所以我们现在重新使用微任务
// 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).

// 设置一个函数指针，将该指针添加到任务队列，待主线程任务执行完毕后，
// 再将任务队列中的 timerFunc 函数添加到执行栈中执行
let timerFunc

// 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 */

// 执行的优先顺序为 promise.then => MutationObserver => setImmediate => setTimeout
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)
  }
}
// 将回调函数 cb 包装成一个箭头函数 push 到事件队列 callbacks 中
export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  // 向事件队列中添加箭头函数作为参数
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  /* pending 在事件队列执行的时候（flushCallbacks调用）才设为 false
    为了确保 timerFunc 函数把 flushCallbacks 添加到队列一次
    （flushCallbacks 被添加到队里至尚未执行这段时间内）
  */
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

  this.$nextTick(cb);

this. message='更新完成1'//DOM更新一次
this. message='更新完成2'//DOM更新两次
this. message='更新完成3'//DOM更新三次

this.message = '更改数据'
this.age = 23
this.name = 'hello'

 /**
   * Subscriber interface.
   * Will be called when a dependency changes.
   */
  update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }
/**
   * Add a dependency to this directive.
   */
  addDep (dep: Dep) {
    const id = dep.id
    if (!this.newDepIds.has(id)) {
      this.newDepIds.add(id)
      this.newDeps.push(dep)
      if (!this.depIds.has(id)) {
        dep.addSub(this)
      }
    }
  }

export function queueWatcher (watcher: Watcher) {
  const id = watcher.id
  if (has[id] == null) {
    has[id] = true
    if (!flushing) {
      queue.push(watcher)
    } else {
      // if already flushing, splice the watcher based on its id
      // if already past its id, it will be run next immediately.
      let i = queue.length - 1
      while (i > index && queue[i].id > watcher.id) {
        i--
      }
      queue.splice(i + 1, 0, watcher)
    }
    // queue the flush
    if (!waiting) {
      waiting = true

      if (process.env.NODE_ENV !== 'production' && !config.async) {
        flushSchedulerQueue()
        return
      }
      nextTick(flushSchedulerQueue)
    }
  }
}