vue源碼學習之異步更新隊列

異步更新隊列

update()

src/core/observer/watchers.js
在執行dep.notify()之後，watcher就會執行update()方法。

  update () { // 通知更新
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this) // watcher進隊
    }
  }

queueWatcher()

執行watcher入隊操作，會進行去重處理。

export function queueWatcher (watcher: Watcher) { // watcher進隊
  const id = watcher.id
  if (has[id] == null) { // 去重（排除相同的watcher，只執行一次就行）
    has[id] = true // 設置爲true
    if (!flushing) { // 默認是false
      queue.push(watcher) // 隊列中添加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) { // 默認false
      waiting = true

      if (process.env.NODE_ENV !== 'production' && !config.async) {
        flushSchedulerQueue()
        return
      }
      // 啓動異步任務
      nextTick(flushSchedulerQueue)
    }
  }
}

nextTick()
src/core/util/next-tick.js

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try { // 做錯誤處理
        cb.call(ctx) // 執行刷新隊列函數
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    // 異步的啓動工作
    timerFunc()  
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

timerFunc()
src/core/util/next-tick.js

let timerFunc;
/*規則
 *1、首選微任務執行異步操作，promise，mutationobserver
 *2、次選setImmediate,最後選擇setTimeout;
 */
if (typeof Promise !== 'undefined' && isNative(Promise)) { // 選promise
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && ( 
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) { // 選mutationObserver
  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)) {  // 選setImmediate
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else { // 最後才選setTimeout
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
} 

跟瀏覽器的刷新機制有關，採用微任務執行刷新隊列函數時，瀏覽器不會進行刷新，就可以實現批量更新數據後進行刷新。

flushSchedulerQueue()

function flushSchedulerQueue () {
  currentFlushTimestamp = getNow()
  flushing = true
  let watcher, id
  queue.sort((a, b) => a.id - b.id) // 排序

  // do not cache length because more watchers might be pushed
  // as we run existing watchers
  for (index = 0; index < queue.length; index++) {
    watcher = queue[index]
    if (watcher.before) {
      watcher.before()
    }
    id = watcher.id
    has[id] = null
    // 最重要的就是執行了run()方法
    watcher.run()
    // in dev build, check and stop circular updates.
    if (process.env.NODE_ENV !== 'production' && has[id] != null) {
      circular[id] = (circular[id] || 0) + 1
      if (circular[id] > MAX_UPDATE_COUNT) {
        warn(
          'You may have an infinite update loop ' + (
            watcher.user
              ? `in watcher with expression "${watcher.expression}"`
              : `in a component render function.`
          ),
          watcher.vm
        )
        break
      }
    }
  }

  // keep copies of post queues before resetting state
  const activatedQueue = activatedChildren.slice()
  const updatedQueue = queue.slice()

  resetSchedulerState()

  // call component updated and activated hooks
  callActivatedHooks(activatedQueue)
  callUpdatedHooks(updatedQueue)

  // devtool hook
  /* istanbul ignore if */
  if (devtools && config.devtools) {
    devtools.emit('flush')
  }
}