React源码解析————Render阶段（三）

• 2021SC@SDUSC
• CompleteWork
• • 流程预览
  • HostComponent
  • completeUnitOfWork
• 小结

2021SC@SDUSC

2021SC@SDUSC

CompleteWork

流程预览

类似beginWork，completeWork也是针对不同fiber.tag调用不同的处理逻辑。

function completeWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const newProps = workInProgress.pendingProps;

  switch (workInProgress.tag) {
    case IndeterminateComponent:
    case LazyComponent:
    case SimpleMemoComponent:
    case FunctionComponent:
    case ForwardRef:
    case Fragment:
    case Mode:
    case Profiler:
    case ContextConsumer:
    case MemoComponent:
      return null;
    case ClassComponent: {
      // ...省略
      return null;
    }
    case HostRoot: {
      // ...省略
      updateHostContainer(workInProgress);
      return null;
    }
    case HostComponent: {
      // ...省略
      return null;
    }
  // ...省略

我们重点关注页面渲染所必须的HostComponent（即原生DOM组件对应的Fiber节点），其他类型Fiber的处理留在具体功能实现时讲解。

HostComponent

和beginWork一样，我们根据current === null ?判断是mount还是update。
同时针对HostComponent，判断update时我们还需要考虑workInProgress.stateNode != null ?（即该Fiber节点是否存在对应的DOM节点）

    case HostComponent: {
      popHostContext(workInProgress);
      const rootContainerInstance = getRootHostContainer();
      const type = workInProgress.type;
      if (current !== null && workInProgress.stateNode != null) {
        updateHostComponent(
          current,
          workInProgress,
          type,
          newProps,
          rootContainerInstance,
        );

        if (current.ref !== workInProgress.ref) {
          markRef(workInProgress);
        }
      } else {
        if (!newProps) {
          invariant(
            workInProgress.stateNode !== null,
            'We must have new props for new mounts. This error is likely ' +
              'caused by a bug in React. Please file an issue.',
          );
          // This can happen when we abort work.
          bubbleProperties(workInProgress);
          return null;
        }

        const currentHostContext = getHostContext();
        // TODO: Move createInstance to beginWork and keep it on a context
        // "stack" as the parent. Then append children as we go in beginWork
        // or completeWork depending on whether we want to add them top->down or
        // bottom->up. Top->down is faster in IE11.
        const wasHydrated = popHydrationState(workInProgress);
        if (wasHydrated) {
          // TODO: Move this and createInstance step into the beginPhase
          // to consolidate.
          if (
            prepareToHydrateHostInstance(
              workInProgress,
              rootContainerInstance,
              currentHostContext,
            )
          ) {
            // If changes to the hydrated node need to be applied at the
            // commit-phase we mark this as such.
            markUpdate(workInProgress);
          }
        } else {
          const instance = createInstance(
            type,
            newProps,
            rootContainerInstance,
            currentHostContext,
            workInProgress,
          );

          appendAllChildren(instance, workInProgress, false, false);

          workInProgress.stateNode = instance;

          // Certain renderers require commit-time effects for initial mount.
          // (eg DOM renderer supports auto-focus for certain elements).
          // Make sure such renderers get scheduled for later work.
          if (
            finalizeInitialChildren(
              instance,
              type,
              newProps,
              rootContainerInstance,
              currentHostContext,
            )
          ) {
            markUpdate(workInProgress);
          }
        }

        if (workInProgress.ref !== null) {
          // If there is a ref on a host node we need to schedule a callback
          markRef(workInProgress);
        }
      }
      bubbleProperties(workInProgress);
      return null;
    }

当update时，Fiber节点已经存在对应DOM节点，所以不需要生成DOM节点。需要做的主要是处理props，比如：
1.onClick、onChange等回调函数的注册
2.处理style prop
3.处理DANGEROUSLY_SET_INNER_HTML prop
4.处理children prop
我们去掉一些当前不需要关注的功能（比如ref）。可以看到最主要的逻辑是调用updateHostComponent方法。

if (current !== null && workInProgress.stateNode != null) {
  // update的情况
  updateHostComponent(
    current,
    workInProgress,
    type,
    newProps,
    rootContainerInstance,
  );
}

我们可以从这里看到updateHostComponent方法定义。

在updateHostComponent内部，被处理完的props会被赋值给workInProgress.updateQueue，并最终会在commit阶段被渲染在页面上。

  updateHostComponent = function(
    current: Fiber,
    workInProgress: Fiber,
    type: Type,
    newProps: Props,
    rootContainerInstance: Container,
  ) {
    // If we have an alternate, that means this is an update and we need to
    // schedule a side-effect to do the updates.
    const oldProps = current.memoizedProps;
    if (oldProps === newProps) {
      // In mutation mode, this is sufficient for a bailout because
      // we won't touch this node even if children changed.
      return;
    }

    // If we get updated because one of our children updated, we don't
    // have newProps so we'll have to reuse them.
    // TODO: Split the update API as separate for the props vs. children.
    // Even better would be if children weren't special cased at all tho.
    const instance: Instance = workInProgress.stateNode;
    const currentHostContext = getHostContext();
    // TODO: Experiencing an error where oldProps is null. Suggests a host
    // component is hitting the resume path. Figure out why. Possibly
    // related to `hidden`.
    const updatePayload = prepareUpdate(
      instance,
      type,
      oldProps,
      newProps,
      rootContainerInstance,
      currentHostContext,
    );
    // TODO: Type this specific to this type of component.
    workInProgress.updateQueue = (updatePayload: any);
    // If the update payload indicates that there is a change or if there
    // is a new ref we mark this as an update. All the work is done in commitWork.
    if (updatePayload) {
      markUpdate(workInProgress);
    }
  };

其中updatePayload为数组形式，他的奇数索引的值为变化的prop key，偶数索引的值为变化的prop value。
当mount时，同样，我们省略了不相关的逻辑。可以看到，mount时的主要逻辑包括三个：
1.为Fiber节点生成对应的DOM节点
2.将子孙DOM节点插入刚生成的DOM节点中
3.与update逻辑中的updateHostComponent类似的处理props的过程

// mount的情况

// ...省略服务端渲染相关逻辑

const currentHostContext = getHostContext();
// 为fiber创建对应DOM节点
const instance = createInstance(
    type,
    newProps,
    rootContainerInstance,
    currentHostContext,
    workInProgress,
  );
// 将子孙DOM节点插入刚生成的DOM节点中
appendAllChildren(instance, workInProgress, false, false);
// DOM节点赋值给fiber.stateNode
workInProgress.stateNode = instance;

// 与update逻辑中的updateHostComponent类似的处理props的过程
if (
  finalizeInitialChildren(
    instance,
    type,
    newProps,
    rootContainerInstance,
    currentHostContext,
  )
) {
  markUpdate(workInProgress);
}

还记得我们讲到：mount时只会在rootFiber存在Placement effectTag。那么commit阶段是如何通过一次插入DOM操作（对应一个Placement effectTag）将整棵DOM树插入页面的呢？
原因就在于completeWork中的appendAllChildren方法。
由于completeWork属于“归”阶段调用的函数，每次调用appendAllChildren时都会将已生成的子孙DOM节点插入当前生成的DOM节点下。那么当“归”到rootFiber时，我们已经有一个构建好的离屏DOM树。

completeUnitOfWork

至此render阶段的绝大部分工作就完成了。

但是还有一个问题：作为DOM操作的依据，commit阶段需要找到所有有effectTag的Fiber节点并依次执行effectTag对应操作。难道需要在commit阶段再遍历一次Fiber树寻找effectTag !== null的Fiber节点么？
这显然是很低效的。

为了解决这个问题，在completeWork的上层函数completeUnitOfWork中，每个执行完completeWork且存在effectTag的Fiber节点会被保存在一条被称为effectList的单向链表中。

effectList中第一个Fiber节点保存在fiber.firstEffect，最后一个元素保存在fiber.lastEffect。

//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;
}

该代码的作用是完成当前节点的work，并赋值Effect链，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，最终返回至root节点
整体上看是一个大的while循环：
从当前节点开始，遍历到兄弟节点，当无兄弟节点时，返回至父节点，
再从父节点开始，遍历到兄弟节点，当无兄弟节点时，返回至父父节点，
可想而知，最终会返回至rootFiber节点
EffectList的赋值：
假设Span1有更新，Span2也有更新

那么父节点DIV的firstEffect和lastEffect在Span1执行completeUnitOfWork()后，会是下面这个样子：

workInProgress1即表示Span1对应的fiber对象

当轮到Span2执行completeUnitOfWork()后，又会变成下面这个样子：

也就是说：Effect链是帮助父节点简单判断子节点是否有更新及更新顺序的。

小结

至此，render阶段全部工作完成。在performSyncWorkOnRoot函数中fiberRootNode被传递给commitRoot方法，开启commit阶段工作流程。

流程图来源于网络，作者看到请和我联系。