React源码解析之completeUnitOfWork

//完成当前节点的 work，然后移动到兄弟节点，重复该操作，当没有更多兄弟节点时，返回至父节点
function completeUnitOfWork(unitOfWork: Fiber): Fiber | null {
  // Attempt to complete the current unit of work, then move to the next
  // sibling. If there are no more siblings, return to the parent fiber.

  //从下至上，移动到该节点的兄弟节点，如果一直往上没有兄弟节点，就返回父节点
  //可想而知，最终会到达 root 节点
  workInProgress = unitOfWork;
  do {
    // The current, flushed, state of this fiber is the alternate. Ideally
    // nothing should rely on this, but relying on it here means that we don't
    // need an additional field on the work in progress.

    //获取当前节点
    const current = workInProgress.alternate;
    //获取父节点
    const returnFiber = workInProgress.return;

    // Check if the work completed or if something threw.
    //判断节点的操作是否完成，还是有异常丢出
    //Incomplete表示捕获到该节点抛出的 error

    //&是表示位的与运算，把左右两边的数字转化为二进制，然后每一位分别进行比较，如果相等就为1，不相等即为0

    //如果该节点没有异常抛出的话，即可正常执行
    if ((workInProgress.effectTag & Incomplete) === NoEffect) {
      //dev 环境，可不看
      setCurrentDebugFiberInDEV(workInProgress);

      let next;
      //如果不能使用分析器的 timer 的话，直接执行completeWork，
      //否则执行分析器timer，并执行completeWork
      if (
        !enableProfilerTimer ||
        (workInProgress.mode & ProfileMode) === NoMode
      ) {
        //完成该节点的更新
        next = completeWork(current, workInProgress, renderExpirationTime);
      } else {
        //启动分析器的定时器，并赋成当前时间
        startProfilerTimer(workInProgress);
        //完成该节点的更新
        next = completeWork(current, workInProgress, renderExpirationTime);
        // Update render duration assuming we didn't error.
        //在没有报错的前提下，更新渲染持续时间

        //记录分析器的timer的运行时间间隔，并停止timer
        stopProfilerTimerIfRunningAndRecordDelta(workInProgress, false);
      }
      //停止 work 计时，可不看
      stopWorkTimer(workInProgress);
      //dev 环境，可不看
      resetCurrentDebugFiberInDEV();
      //更新该节点的 work 时长和子节点的 expirationTime
      resetChildExpirationTime(workInProgress);
      //如果next存在，则表示产生了新 work
      if (next !== null) {
        // Completing this fiber spawned new work. Work on that next.
        //返回 next，以便执行新 work
        return next;
      }
      //如果父节点存在，并且其 Effect 链没有被赋值的话
      if (
        returnFiber !== null &&
        // Do not append effects to parents if a sibling failed to complete
        (returnFiber.effectTag & Incomplete) === NoEffect
      ) {
        // Append all the effects of the subtree and this fiber onto the effect
        // list of the parent. The completion order of the children affects the
        // side-effect order.
        //子节点的完成顺序会影响副作用的顺序

        //如果父节点没有挂载firstEffect的话，将当前节点的firstEffect赋值给父节点的firstEffect
        if (returnFiber.firstEffect === null) {
          returnFiber.firstEffect = workInProgress.firstEffect;
        }
        //同上，根据当前节点的lastEffect，初始化父节点的lastEffect
        if (workInProgress.lastEffect !== null) {
          //如果父节点的lastEffect有值的话，将nextEffect赋值
          //目的是串联Effect链
          if (returnFiber.lastEffect !== null) {
            returnFiber.lastEffect.nextEffect = workInProgress.firstEffect;
          }
          returnFiber.lastEffect = workInProgress.lastEffect;
        }

        // If this fiber had side-effects, we append it AFTER the children's
        // side-effects. We can perform certain side-effects earlier if needed,
        // by doing multiple passes over the effect list. We don't want to
        // schedule our own side-effect on our own list because if end up
        // reusing children we'll schedule this effect onto itself since we're
        // at the end.
        //获取副作用标记
        const effectTag = workInProgress.effectTag;

        // Skip both NoWork and PerformedWork tags when creating the effect
        // list. PerformedWork effect is read by React DevTools but shouldn't be
        // committed.
        //如果该副作用标记大于PerformedWork
        if (effectTag > PerformedWork) {
          //当父节点的lastEffect不为空的时候，将当前节点挂载到父节点的副作用链的最后
          if (returnFiber.lastEffect !== null) {
            returnFiber.lastEffect.nextEffect = workInProgress;
          } else {
            //否则，将当前节点挂载在父节点的副作用链的头-firstEffect上
            returnFiber.firstEffect = workInProgress;
          }
          //无论父节点的lastEffect是否为空，都将当前节点挂载在父节点的副作用链的lastEffect上
          returnFiber.lastEffect = workInProgress;
        }
      }
    }
    //如果该 fiber 节点未能完成 work 的话(报错)
    else {
      // This fiber did not complete because something threw. Pop values off
      // the stack without entering the complete phase. If this is a boundary,
      // capture values if possible.
      //节点未能完成更新，捕获其中的错误
      const next = unwindWork(workInProgress, renderExpirationTime);

      // Because this fiber did not complete, don't reset its expiration time.
      //由于该 fiber 未能完成，所以不必重置它的 expirationTime
      if (
        enableProfilerTimer &&
        (workInProgress.mode & ProfileMode) !== NoMode
      ) {
        // Record the render duration for the fiber that errored.
        //记录分析器的timer的运行时间间隔，并停止timer
        stopProfilerTimerIfRunningAndRecordDelta(workInProgress, false);

        // Include the time spent working on failed children before continuing.
        //虽然报错了，但仍然会累计 work 时长
        let actualDuration = workInProgress.actualDuration;
        let child = workInProgress.child;
        while (child !== null) {
          actualDuration += child.actualDuration;
          child = child.sibling;
        }
        workInProgress.actualDuration = actualDuration;
      }
      //如果next存在，则表示产生了新 work
      if (next !== null) {
        // If completing this work spawned new work, do that next. We'll come
        // back here again.
        // Since we're restarting, remove anything that is not a host effect
        // from the effect tag.
        // TODO: The name stopFailedWorkTimer is misleading because Suspense
        // also captures and restarts.
        //停止失败的 work 计时，可不看
        stopFailedWorkTimer(workInProgress);
        //更新其 effectTag，标记是 restart 的
        next.effectTag &= HostEffectMask;
        //返回 next，以便执行新 work
        return next;
      }
      //停止 work 计时，可不看
      stopWorkTimer(workInProgress);
      //如果父节点存在的话，重置它的 Effect 链，标记为「未完成」
      if (returnFiber !== null) {
        // Mark the parent fiber as incomplete and clear its effect list.
        returnFiber.firstEffect = returnFiber.lastEffect = null;
        returnFiber.effectTag |= Incomplete;
      }
    }
    //获取兄弟节点
    const siblingFiber = workInProgress.sibling;
    if (siblingFiber !== null) {
      // If there is more work to do in this returnFiber, do that next.
      return siblingFiber;
    }
    // Otherwise, return to the parent
    //如果能执行到这一步的话，说明 siblingFiber 为 null，
    //那么就返回至父节点
    workInProgress = returnFiber;
  } while (workInProgress !== null);

  // We've reached the root.
  //当执行到这里的时候，说明遍历到了 root 节点，已完成遍历
  //更新workInProgressRootExitStatus的状态为「已完成」
  if (workInProgressRootExitStatus === RootIncomplete) {
    workInProgressRootExitStatus = RootCompleted;
  }
  return null;
}

//启动分析器的timer，并赋成当前时间
function startProfilerTimer(fiber: Fiber): void {
  //如果不能启动分析器的timer的话，就 return
  if (!enableProfilerTimer) {
    return;
  }
  //分析器的开始时间
  profilerStartTime = now();
  //如果 fiber 节点的实际开始时间 < 0 的话，则赋成当前时间
  if (((fiber.actualStartTime: any): number) < 0) {
    fiber.actualStartTime = now();
  }
}

//记录分析器的timer的work 时间，并停止timer
function stopProfilerTimerIfRunningAndRecordDelta(
  fiber: Fiber,
  overrideBaseTime: boolean,
): void {
  //如果不能启动分析器的定时器的话，就 return
  if (!enableProfilerTimer) {
    return;
  }
  //如果分析器的开始时间>=0的话
  if (profilerStartTime >= 0) {
    //获取运行的时间间隔
    const elapsedTime = now() - profilerStartTime;
    //累计实际 work 时间间隔
    fiber.actualDuration += elapsedTime;
    if (overrideBaseTime) {
      //记录时间间隔
      fiber.selfBaseDuration = elapsedTime;
    }
    //上述操作完成后，将分析器的timer的开始时间重置为-1
    profilerStartTime = -1;
  }
}

//更新该节点的 work 时长和获取优先级最高的子节点的 expirationTime
function resetChildExpirationTime(completedWork: Fiber) {
  //如果当前渲染的节点需要更新，但是子节点不需要更新的话，则 return
  if (
    renderExpirationTime !== Never &&
    completedWork.childExpirationTime === Never
  ) {
    // The children of this component are hidden. Don't bubble their
    // expiration times.
    return;
  }

  let newChildExpirationTime = NoWork;

  // Bubble up the earliest expiration time.
  if (enableProfilerTimer && (completedWork.mode & ProfileMode) !== NoMode) {
    // In profiling mode, resetChildExpirationTime is also used to reset
    // profiler durations.
    //获取当前节点的实际 work 时长
    let actualDuration = completedWork.actualDuration;
    //获取 fiber 树的 work 时长
    let treeBaseDuration = completedWork.selfBaseDuration;

    // When a fiber is cloned, its actualDuration is reset to 0. This value will
    // only be updated if work is done on the fiber (i.e. it doesn't bailout).
    // When work is done, it should bubble to the parent's actualDuration. If
    // the fiber has not been cloned though, (meaning no work was done), then
    // this value will reflect the amount of time spent working on a previous
    // render. In that case it should not bubble. We determine whether it was
    // cloned by comparing the child pointer.
    // 当一个 fiber 节点被克隆后，它的实际 work 时长被重置为 0.
    // 这个值只会在 fiber 自身上的 work 完成时被更新(顺利执行的话)
    // 当 fiber 自身 work 完成后，将自身的实际 work 时长冒泡赋给父节点的实际 work 时长
    // 如果 fiber 没有被克隆，即 work 未被完成的话，actualDuration 反映的是上次渲染的实际 work 时长
    // 如果是这种情况的话，不应该冒泡赋给父节点
    // React 通过比较 子指针 来判断 fiber 是否被克隆

    // 关于 alternate 的作用，请看：https://juejin.im/post/5d5aa4695188257573635a0d
    // 是否将 work 时间冒泡至父节点的依据是：
    // (1) 该 fiber 节点是否是第一次渲染
    // (2) 该 fiber 节点的子节点有更新
    const shouldBubbleActualDurations =
      completedWork.alternate === null ||
      completedWork.child !== completedWork.alternate.child;

    //获取当前节点的第一个子节点
    let child = completedWork.child;

    //当该子节点存在时，通过newChildExpirationTime来获取子节点、子子节点两者中优先级最高的那个expirationTime
    while (child !== null) {
      //获取该子节点的 expirationTime
      const childUpdateExpirationTime = child.expirationTime;
      //获取该子节点的 child 的 expirationTime
      const childChildExpirationTime = child.childExpirationTime;
      //如果子节点的优先级大于NoWork的话，则将newChild的 expirationTime 赋值为该子节点的 expirationTime
      if (childUpdateExpirationTime > newChildExpirationTime) {
        newChildExpirationTime = childUpdateExpirationTime;
      }
      //子节点的 child 同上
      if (childChildExpirationTime > newChildExpirationTime) {
        newChildExpirationTime = childChildExpirationTime;
      }

      if (shouldBubbleActualDurations) {
        //累计子节点的 work 时长
        actualDuration += child.actualDuration;
      }
      //累计 fiber 树的 work 时长
      treeBaseDuration += child.treeBaseDuration;
      //移动到兄弟节点，重复上述过程
      child = child.sibling;
    }
    //更新 fiber 的 work 时长
    completedWork.actualDuration = actualDuration;
    //更新 fiber 树的 work 时长
    completedWork.treeBaseDuration = treeBaseDuration;
  }
  //逻辑同上，不再赘述
  else {
    let child = completedWork.child;
    while (child !== null) {
      const childUpdateExpirationTime = child.expirationTime;
      const childChildExpirationTime = child.childExpirationTime;
      if (childUpdateExpirationTime > newChildExpirationTime) {
        newChildExpirationTime = childUpdateExpirationTime;
      }
      if (childChildExpirationTime > newChildExpirationTime) {
        newChildExpirationTime = childChildExpirationTime;
      }
      child = child.sibling;
    }
  }

  completedWork.childExpirationTime = newChildExpirationTime;
}