const effect = (instance.effect = new ReactiveEffect(
  componentUpdateFn,
  () => queueJob(update), // updata: () => effect.run() 返回值 componentUpdateFn
  // 將effect添加到組件的scope.effects中
  instance.scope // track it in component's effect scope
))

// packages/runtime-core/src/scheduler.ts
export function queueJob(job: SchedulerJob) {
  if (
    !queue.length ||
    !queue.includes( // queue中是否已經(jīng)存在相同job
      job,
      isFlushing && job.allowRecurse ? flushIndex + 1 : flushIndex
    )
  ) {
    if (job.id == null) {
      // 向queue添加job queue是一個數(shù)組
      queue.push(job)
    } else {
      queue.splice(findInsertionIndex(job.id), 0, job)
    }
    // 執(zhí)行queueFlush
    queueFlush()
  }
}

function queueFlush() {
  // isFlushing和isflushPending初始值都是false
  // 說明當(dāng)前沒有flush任務(wù)在執(zhí)行，也沒有flush任務(wù)在等待執(zhí)行
  if (!isFlushing && !isFlushPending) {
    // 初次執(zhí)行queueFlush將isFlushPending設(shè)置為true 表示有flush任務(wù)在等待執(zhí)行
    isFlushPending = true
    // resolvedPromise是promise.resolve()
    // flushJobs被放到微任務(wù)隊列中 等待所有同步scheduler執(zhí)行完畢后執(zhí)行
    // 這樣就可以保證flushJobs在一次組件更新中只執(zhí)行一次
    // 更新currentFlushPromise 以供nextTick使用
    currentFlushPromise = resolvedPromise.then(flushJobs)
  }
}

function flushJobs(seen?: CountMap) {
  // 狀態(tài)改為有flush正在執(zhí)行
  isFlushPending = false
  isFlushing = true
  if (__DEV__) {
    seen = seen || new Map()
  }
  // Sort queue before flush.
  // This ensures that:
  // 1. Components are updated from parent to child. (because parent is always
  //    created before the child so its render effect will have smaller
  //    priority number)
  // 2. If a component is unmounted during a parent component's update,
  //    its update can be skipped.
  // 組件更新的順序是從父到子 因為父組件總是在子組件之前創(chuàng)建 所以它的渲染效果將具有更小的優(yōu)先級
  // 如果一個組件在父組件更新期間被卸載 則可以跳過它的更新
  queue.sort(comparator)
  ...
  // 先執(zhí)行queue中的job 然后執(zhí)行pendingPostFlushCbs中的job
  // 這里可以實現(xiàn)watch中的 postFlush
  try {
    for (flushIndex = 0; flushIndex < queue.length; flushIndex++) {
      const job = queue[flushIndex]
      if (job && job.active !== false) {
        if (__DEV__ && check(job)) {
          continue
        }
        // console.log(`running:`, job.id)
        // 執(zhí)行job
        callWithErrorHandling(job, null, ErrorCodes.SCHEDULER)
      }
    }
  } finally {
    // job執(zhí)行完畢后清空隊列
    flushIndex = 0
    queue.length = 0
    // 執(zhí)行flushPostFlushCbs 此時組件已經(jīng)更新完畢
    flushPostFlushCbs(seen)
    isFlushing = false
    currentFlushPromise = null
    // some postFlushCb queued jobs!
    // keep flushing until it drains.
    if (queue.length || pendingPostFlushCbs.length) {
      flushJobs(seen)
    }
  }
}

export function nextTick<T = void>(
  this: T,
  fn?: (this: T) => void
): Promise<void> {
  const p = currentFlushPromise || resolvedPromise
  // nextTick回調(diào)函數(shù)放到currentFlushPromise的微任務(wù)隊列中
  return fn ? p.then(this ? fn.bind(this) : fn) : p
}