React 16.8.0 初次渲染 源码分析（三）

书接上文，从 scheduleWork 开始

scheduleWork 主要初始化 FiberRoot Time相关字段

function scheduleWork(fiber: Fiber, expirationTime: ExpirationTime) {
  // fiber:FiberNode, expirationTime: Sync
  const root = scheduleWorkToRoot(fiber, expirationTime); // 返回FiberRoot
  if (root === null) {
    return;
  }

  if (
    !isWorking &&
    nextRenderExpirationTime !== NoWork &&
    expirationTime > nextRenderExpirationTime
  ) {
    // This is an interruption. (Used for performance tracking.)
    interruptedBy = fiber;
    resetStack();
  }
  markPendingPriorityLevel(root, expirationTime);
  if (
    // If we're in the render phase, we don't need to schedule this root
    // for an update, because we'll do it before we exit...
    !isWorking ||
    isCommitting ||
    // ...unless this is a different root than the one we're rendering.
    nextRoot !== root
  ) {
    const rootExpirationTime = root.expirationTime;
    requestWork(root, rootExpirationTime);
  }
  if (nestedUpdateCount > NESTED_UPDATE_LIMIT) {
    // Reset this back to zero so subsequent updates don't throw.
    nestedUpdateCount = 0;
    invariant(
      false,
      'Maximum update depth exceeded. This can happen when a ' +
        'component repeatedly calls setState inside ' +
        'componentWillUpdate or componentDidUpdate. React limits ' +
        'the number of nested updates to prevent infinite loops.',
    );
  }
}

function scheduleWorkToRoot(fiber: Fiber, expirationTime): FiberRoot | null {
  // fiber:FiberNode, expirationTime: Sync
  recordScheduleUpdate();

  // Update the source fiber's expiration time
  if (fiber.expirationTime < expirationTime) {
    fiber.expirationTime = expirationTime;
  }
  let alternate = fiber.alternate;
  if (alternate !== null && alternate.expirationTime < expirationTime) {
    alternate.expirationTime = expirationTime;
  }
  // Walk the parent path to the root and update the child expiration time.
  let node = fiber.return;
  let root = null;
  if (node === null && fiber.tag === HostRoot) {
    root = fiber.stateNode;
  } else {
    while (node !== null) {
      alternate = node.alternate;
      if (node.childExpirationTime < expirationTime) {
        node.childExpirationTime = expirationTime;
        if (
          alternate !== null &&
          alternate.childExpirationTime < expirationTime
        ) {
          alternate.childExpirationTime = expirationTime;
        }
      } else if (
        alternate !== null &&
        alternate.childExpirationTime < expirationTime
      ) {
        alternate.childExpirationTime = expirationTime;
      }
      if (node.return === null && node.tag === HostRoot) {
        root = node.stateNode;
        break;
      }
      node = node.return;
    }
  }

  if (enableSchedulerTracing) {
    if (root !== null) {
      const interactions = __interactionsRef.current;
      if (interactions.size > 0) {
        const pendingInteractionMap = root.pendingInteractionMap;
        const pendingInteractions = pendingInteractionMap.get(expirationTime);
        if (pendingInteractions != null) {
          interactions.forEach(interaction => {
            if (!pendingInteractions.has(interaction)) {
              // Update the pending async work count for previously unscheduled interaction.
              interaction.__count++;
            }

            pendingInteractions.add(interaction);
          });
        } else {
          pendingInteractionMap.set(expirationTime, new Set(interactions));

          // Update the pending async work count for the current interactions.
          interactions.forEach(interaction => {
            interaction.__count++;
          });
        }

        const subscriber = __subscriberRef.current;
        if (subscriber !== null) {
          const threadID = computeThreadID(
            expirationTime,
            root.interactionThreadID,
          );
          subscriber.onWorkScheduled(interactions, threadID);
        }
      }
    }
  }
  return root;
}

export function recordScheduleUpdate(): void {
  if (enableUserTimingAPI) {
    if (isCommitting) {
      hasScheduledUpdateInCurrentCommit = true;
    }
    if (
      currentPhase !== null &&
      currentPhase !== 'componentWillMount' &&
      currentPhase !== 'componentWillReceiveProps'
    ) {
      hasScheduledUpdateInCurrentPhase = true;
    }
  }
}

• packages/react-reconciler/src/ReactFiberScheduler.js

firstScheduledRoot = lastScheduledRoot = root;

// requestWork is called by the scheduler whenever a root receives an update.
// It's up to the renderer to call renderRoot at some point in the future.
function requestWork(root: FiberRoot, expirationTime: ExpirationTime) {
  //root: FiberRoot, expirationTime: Sync
  addRootToSchedule(root, expirationTime);
  if (isRendering) {
    // Prevent reentrancy. Remaining work will be scheduled at the end of
    // the currently rendering batch.
    return;
  }

  if (isBatchingUpdates) {
    // Flush work at the end of the batch.
    if (isUnbatchingUpdates) {
      // ...unless we're inside unbatchedUpdates, in which case we should
      // flush it now.
      nextFlushedRoot = root;
      nextFlushedExpirationTime = Sync;
      performWorkOnRoot(root, Sync, false);
    }
    return;
  }

  // TODO: Get rid of Sync and use current time?
  if (expirationTime === Sync) {
    performSyncWork();
  } else {
    scheduleCallbackWithExpirationTime(root, expirationTime);
  }
}

function performSyncWork() {
  performWork(Sync, false);
}

function performWork(minExpirationTime: ExpirationTime, isYieldy: boolean) {
  // minExpirationTime: Sync, isYieldy: false
  // Keep working on roots until there's no more work, or until there's a higher
  // priority event.
  findHighestPriorityRoot();

  if (isYieldy) {
    recomputeCurrentRendererTime();
    currentSchedulerTime = currentRendererTime;

    if (enableUserTimingAPI) {
      const didExpire = nextFlushedExpirationTime > currentRendererTime;
      const timeout = expirationTimeToMs(nextFlushedExpirationTime);
      stopRequestCallbackTimer(didExpire, timeout);
    }

    while (
      nextFlushedRoot !== null &&
      nextFlushedExpirationTime !== NoWork &&
      minExpirationTime <= nextFlushedExpirationTime &&
      !(didYield && currentRendererTime > nextFlushedExpirationTime)
    ) {
      performWorkOnRoot(
        nextFlushedRoot,
        nextFlushedExpirationTime,
        currentRendererTime > nextFlushedExpirationTime,
      );
      findHighestPriorityRoot();
      recomputeCurrentRendererTime();
      currentSchedulerTime = currentRendererTime;
    }
  } else {
    while (
      nextFlushedRoot !== null &&
      nextFlushedExpirationTime !== NoWork &&
      minExpirationTime <= nextFlushedExpirationTime
    ) {
      performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime, false);
      findHighestPriorityRoot();
    }
  }

  // We're done flushing work. Either we ran out of time in this callback,
  // or there's no more work left with sufficient priority.

  // If we're inside a callback, set this to false since we just completed it.
  if (isYieldy) {
    callbackExpirationTime = NoWork;
    callbackID = null;
  }
  // If there's work left over, schedule a new callback.
  if (nextFlushedExpirationTime !== NoWork) {
    scheduleCallbackWithExpirationTime(
      ((nextFlushedRoot: any): FiberRoot),
      nextFlushedExpirationTime,
    );
  }

  // Clean-up.
  finishRendering();
}

function findHighestPriorityRoot() {
  let highestPriorityWork = NoWork;
  let highestPriorityRoot = null;
  if (lastScheduledRoot !== null) {
    let previousScheduledRoot = lastScheduledRoot;
    let root = firstScheduledRoot;
    while (root !== null) {
      const remainingExpirationTime = root.expirationTime;
      if (remainingExpirationTime === NoWork) {
        // This root no longer has work. Remove it from the scheduler.

        // TODO: This check is redudant, but Flow is confused by the branch
        // below where we set lastScheduledRoot to null, even though we break
        // from the loop right after.
        invariant(
          previousScheduledRoot !== null && lastScheduledRoot !== null,
          'Should have a previous and last root. This error is likely ' +
            'caused by a bug in React. Please file an issue.',
        );
        if (root === root.nextScheduledRoot) {
          // This is the only root in the list.
          root.nextScheduledRoot = null;
          firstScheduledRoot = lastScheduledRoot = null;
          break;
        } else if (root === firstScheduledRoot) {
          // This is the first root in the list.
          const next = root.nextScheduledRoot;
          firstScheduledRoot = next;
          lastScheduledRoot.nextScheduledRoot = next;
          root.nextScheduledRoot = null;
        } else if (root === lastScheduledRoot) {
          // This is the last root in the list.
          lastScheduledRoot = previousScheduledRoot;
          lastScheduledRoot.nextScheduledRoot = firstScheduledRoot;
          root.nextScheduledRoot = null;
          break;
        } else {
          previousScheduledRoot.nextScheduledRoot = root.nextScheduledRoot;
          root.nextScheduledRoot = null;
        }
        root = previousScheduledRoot.nextScheduledRoot;
      } else {
        if (remainingExpirationTime > highestPriorityWork) {
          // Update the priority, if it's higher
          highestPriorityWork = remainingExpirationTime;
          highestPriorityRoot = root;
        }
        if (root === lastScheduledRoot) {
          break;
        }
        if (highestPriorityWork === Sync) {
          // Sync is highest priority by definition so
          // we can stop searching.
          break;
        }
        previousScheduledRoot = root;
        root = root.nextScheduledRoot;
      }
    }
  }

  nextFlushedRoot = highestPriorityRoot;
  nextFlushedExpirationTime = highestPriorityWork;
}

  nextFlushedRoot = highestPriorityRoot; // FiberRoot

  nextFlushedExpirationTime = highestPriorityWork; // Sync

 

performWork方法循环 查找需要优先执行Work的Root，然后执行 performWorkOnRoot

没有需要执行Work的Root时，或者 执行Work的时间 大于 下一次渲染的过期时间 退出循环。

function performWorkOnRoot(
  root: FiberRoot, // FiberRoot
  expirationTime: ExpirationTime, // Sync
  isYieldy: boolean, // false
) {
  invariant(
    !isRendering,
    'performWorkOnRoot was called recursively. This error is likely caused ' +
      'by a bug in React. Please file an issue.',
  );

  isRendering = true;

  // Check if this is async work or sync/expired work.
  if (!isYieldy) {
    // Flush work without yielding.
    // TODO: Non-yieldy work does not necessarily imply expired work. A renderer
    // may want to perform some work without yielding, but also without
    // requiring the root to complete (by triggering placeholders).

    // root.finishedWork = null
    let finishedWork = root.finishedWork;
    if (finishedWork !== null) {
      // This root is already complete. We can commit it.
      completeRoot(root, finishedWork, expirationTime);
    } else {
      root.finishedWork = null;
      // If this root previously suspended, clear its existing timeout, since
      // we're about to try rendering again.
      
      // root.timeoutHandle = -1
      const timeoutHandle = root.timeoutHandle;
      if (timeoutHandle !== noTimeout) {
        root.timeoutHandle = noTimeout;
        // $FlowFixMe Complains noTimeout is not a TimeoutID, despite the check above
        cancelTimeout(timeoutHandle);
      }
      // 渲染 Root
      renderRoot(root, isYieldy);
      finishedWork = root.finishedWork;
      if (finishedWork !== null) {
        // We've completed the root. Commit it.
        completeRoot(root, finishedWork, expirationTime);
      }
    }
  } else {
    // Flush async work.
    let finishedWork = root.finishedWork;
    if (finishedWork !== null) {
      // This root is already complete. We can commit it.
      completeRoot(root, finishedWork, expirationTime);
    } else {
      root.finishedWork = null;
      // If this root previously suspended, clear its existing timeout, since
      // we're about to try rendering again.
      const timeoutHandle = root.timeoutHandle;
      if (timeoutHandle !== noTimeout) {
        root.timeoutHandle = noTimeout;
        // $FlowFixMe Complains noTimeout is not a TimeoutID, despite the check above
        cancelTimeout(timeoutHandle);
      }
      renderRoot(root, isYieldy);
      finishedWork = root.finishedWork;
      if (finishedWork !== null) {
        // We've completed the root. Check the if we should yield one more time
        // before committing.
        if (!shouldYieldToRenderer()) {
          // Still time left. Commit the root.
          completeRoot(root, finishedWork, expirationTime);
        } else {
          // There's no time left. Mark this root as complete. We'll come
          // back and commit it later.
          root.finishedWork = finishedWork;
        }
      }
    }
  }

  isRendering = false;
}

renderRoot 是一个300行的恐怖方法。

重置了Stack

nextRoot = null;
nextRenderExpirationTime = NoWork;
nextLatestAbsoluteTimeoutMs = -1;
nextRenderDidError = false;
nextUnitOfWork = null;

创建当前循环的WIP

resetStack();
nextRoot = root;
nextRenderExpirationTime = expirationTime;
nextUnitOfWork = createWorkInProgress(
    nextRoot.current,
    null,
    nextRenderExpirationTime,
);

 执行一次 workLoop

function renderRoot(root: FiberRoot, isYieldy: boolean): void {
  invariant(
    !isWorking,
    'renderRoot was called recursively. This error is likely caused ' +
      'by a bug in React. Please file an issue.',
  );

  flushPassiveEffects();

  isWorking = true;
  const previousDispatcher = ReactCurrentDispatcher.current;
  ReactCurrentDispatcher.current = ContextOnlyDispatcher;

  const expirationTime = root.nextExpirationTimeToWorkOn;

  // Check if we're starting from a fresh stack, or if we're resuming from
  // previously yielded work.
  if (
    expirationTime !== nextRenderExpirationTime ||
    root !== nextRoot ||
    nextUnitOfWork === null
  ) {
    // Reset the stack and start working from the root.
    resetStack();
    nextRoot = root;
    nextRenderExpirationTime = expirationTime;
    nextUnitOfWork = createWorkInProgress(
      nextRoot.current,
      null,
      nextRenderExpirationTime,
    );
    root.pendingCommitExpirationTime = NoWork;

    if (enableSchedulerTracing) {
      // Determine which interactions this batch of work currently includes,
      // So that we can accurately attribute time spent working on it,
      // And so that cascading work triggered during the render phase will be associated with it.
      const interactions: Set<Interaction> = new Set();
      root.pendingInteractionMap.forEach(
        (scheduledInteractions, scheduledExpirationTime) => {
          if (scheduledExpirationTime >= expirationTime) {
            scheduledInteractions.forEach(interaction =>
              interactions.add(interaction),
            );
          }
        },
      );

      // Store the current set of interactions on the FiberRoot for a few reasons:
      // We can re-use it in hot functions like renderRoot() without having to recalculate it.
      // We will also use it in commitWork() to pass to any Profiler onRender() hooks.
      // This also provides DevTools with a way to access it when the onCommitRoot() hook is called.
      root.memoizedInteractions = interactions;

      if (interactions.size > 0) {
        const subscriber = __subscriberRef.current;
        if (subscriber !== null) {
          const threadID = computeThreadID(
            expirationTime,
            root.interactionThreadID,
          );
          try {
            subscriber.onWorkStarted(interactions, threadID);
          } catch (error) {
            // Work thrown by an interaction tracing subscriber should be rethrown,
            // But only once it's safe (to avoid leaving the scheduler in an invalid state).
            // Store the error for now and we'll re-throw in finishRendering().
            if (!hasUnhandledError) {
              hasUnhandledError = true;
              unhandledError = error;
            }
          }
        }
      }
    }
  }

  let prevInteractions: Set<Interaction> = (null: any);
  if (enableSchedulerTracing) {
    // We're about to start new traced work.
    // Restore pending interactions so cascading work triggered during the render phase will be accounted for.
    prevInteractions = __interactionsRef.current;
    __interactionsRef.current = root.memoizedInteractions;
  }

  let didFatal = false;

  startWorkLoopTimer(nextUnitOfWork);

  do {
    try {
      workLoop(isYieldy);
    } catch (thrownValue) {
      resetContextDependences();
      resetHooks();

      // Reset in case completion throws.
      // This is only used in DEV and when replaying is on.
      let mayReplay;
      if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
        mayReplay = mayReplayFailedUnitOfWork;
        mayReplayFailedUnitOfWork = true;
      }

      if (nextUnitOfWork === null) {
        // This is a fatal error.
        didFatal = true;
        onUncaughtError(thrownValue);
      } else {
        if (enableProfilerTimer && nextUnitOfWork.mode & ProfileMode) {
          // Record the time spent rendering before an error was thrown.
          // This avoids inaccurate Profiler durations in the case of a suspended render.
          stopProfilerTimerIfRunningAndRecordDelta(nextUnitOfWork, true);
        }

        if (__DEV__) {
          // Reset global debug state
          // We assume this is defined in DEV
          (resetCurrentlyProcessingQueue: any)();
        }

        if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
          if (mayReplay) {
            const failedUnitOfWork: Fiber = nextUnitOfWork;
            replayUnitOfWork(failedUnitOfWork, thrownValue, isYieldy);
          }
        }

        // TODO: we already know this isn't true in some cases.
        // At least this shows a nicer error message until we figure out the cause.
        // https://github.com/facebook/react/issues/12449#issuecomment-386727431
        invariant(
          nextUnitOfWork !== null,
          'Failed to replay rendering after an error. This ' +
            'is likely caused by a bug in React. Please file an issue ' +
            'with a reproducing case to help us find it.',
        );

        const sourceFiber: Fiber = nextUnitOfWork;
        let returnFiber = sourceFiber.return;
        if (returnFiber === null) {
          // This is the root. The root could capture its own errors. However,
          // we don't know if it errors before or after we pushed the host
          // context. This information is needed to avoid a stack mismatch.
          // Because we're not sure, treat this as a fatal error. We could track
          // which phase it fails in, but doesn't seem worth it. At least
          // for now.
          didFatal = true;
          onUncaughtError(thrownValue);
        } else {
          throwException(
            root,
            returnFiber,
            sourceFiber,
            thrownValue,
            nextRenderExpirationTime,
          );
          nextUnitOfWork = completeUnitOfWork(sourceFiber);
          continue;
        }
      }
    }
    break;
  } while (true);

  if (enableSchedulerTracing) {
    // Traced work is done for now; restore the previous interactions.
    __interactionsRef.current = prevInteractions;
  }

  // We're done performing work. Time to clean up.
  isWorking = false;
  ReactCurrentDispatcher.current = previousDispatcher;
  resetContextDependences();
  resetHooks();

  // Yield back to main thread.
  if (didFatal) {
    const didCompleteRoot = false;
    stopWorkLoopTimer(interruptedBy, didCompleteRoot);
    interruptedBy = null;
    // There was a fatal error.
    if (__DEV__) {
      resetStackAfterFatalErrorInDev();
    }
    // `nextRoot` points to the in-progress root. A non-null value indicates
    // that we're in the middle of an async render. Set it to null to indicate
    // there's no more work to be done in the current batch.
    nextRoot = null;
    onFatal(root);
    return;
  }

  if (nextUnitOfWork !== null) {
    // There's still remaining async work in this tree, but we ran out of time
    // in the current frame. Yield back to the renderer. Unless we're
    // interrupted by a higher priority update, we'll continue later from where
    // we left off.
    const didCompleteRoot = false;
    stopWorkLoopTimer(interruptedBy, didCompleteRoot);
    interruptedBy = null;
    onYield(root);
    return;
  }

  // We completed the whole tree.
  const didCompleteRoot = true;
  stopWorkLoopTimer(interruptedBy, didCompleteRoot);
  const rootWorkInProgress = root.current.alternate;
  invariant(
    rootWorkInProgress !== null,
    'Finished root should have a work-in-progress. This error is likely ' +
      'caused by a bug in React. Please file an issue.',
  );

  // `nextRoot` points to the in-progress root. A non-null value indicates
  // that we're in the middle of an async render. Set it to null to indicate
  // there's no more work to be done in the current batch.
  nextRoot = null;
  interruptedBy = null;

  if (nextRenderDidError) {
    // There was an error
    if (hasLowerPriorityWork(root, expirationTime)) {
      // There's lower priority work. If so, it may have the effect of fixing
      // the exception that was just thrown. Exit without committing. This is
      // similar to a suspend, but without a timeout because we're not waiting
      // for a promise to resolve. React will restart at the lower
      // priority level.
      markSuspendedPriorityLevel(root, expirationTime);
      const suspendedExpirationTime = expirationTime;
      const rootExpirationTime = root.expirationTime;
      onSuspend(
        root,
        rootWorkInProgress,
        suspendedExpirationTime,
        rootExpirationTime,
        -1, // Indicates no timeout
      );
      return;
    } else if (
      // There's no lower priority work, but we're rendering asynchronously.
      // Synchronsouly attempt to render the same level one more time. This is
      // similar to a suspend, but without a timeout because we're not waiting
      // for a promise to resolve.
      !root.didError &&
      isYieldy
    ) {
      root.didError = true;
      const suspendedExpirationTime = (root.nextExpirationTimeToWorkOn = expirationTime);
      const rootExpirationTime = (root.expirationTime = Sync);
      onSuspend(
        root,
        rootWorkInProgress,
        suspendedExpirationTime,
        rootExpirationTime,
        -1, // Indicates no timeout
      );
      return;
    }
  }

  if (isYieldy && nextLatestAbsoluteTimeoutMs !== -1) {
    // The tree was suspended.
    const suspendedExpirationTime = expirationTime;
    markSuspendedPriorityLevel(root, suspendedExpirationTime);

    // Find the earliest uncommitted expiration time in the tree, including
    // work that is suspended. The timeout threshold cannot be longer than
    // the overall expiration.
    const earliestExpirationTime = findEarliestOutstandingPriorityLevel(
      root,
      expirationTime,
    );
    const earliestExpirationTimeMs = expirationTimeToMs(earliestExpirationTime);
    if (earliestExpirationTimeMs < nextLatestAbsoluteTimeoutMs) {
      nextLatestAbsoluteTimeoutMs = earliestExpirationTimeMs;
    }

    // Subtract the current time from the absolute timeout to get the number
    // of milliseconds until the timeout. In other words, convert an absolute
    // timestamp to a relative time. This is the value that is passed
    // to `setTimeout`.
    const currentTimeMs = expirationTimeToMs(requestCurrentTime());
    let msUntilTimeout = nextLatestAbsoluteTimeoutMs - currentTimeMs;
    msUntilTimeout = msUntilTimeout < 0 ? 0 : msUntilTimeout;

    // TODO: Account for the Just Noticeable Difference

    const rootExpirationTime = root.expirationTime;
    onSuspend(
      root,
      rootWorkInProgress,
      suspendedExpirationTime,
      rootExpirationTime,
      msUntilTimeout,
    );
    return;
  }

  // Ready to commit.
  onComplete(root, rootWorkInProgress, expirationTime);
}

 beginWork 根据 WIP.tag 做处理，先处理子组件，再处理兄弟组件，每次重新赋值 nextUnitOfWork 直到所有单元Unit都执行完毕

应该着重看下这个方法，了解React对不同类型组件做了哪些什么处理。

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
): Fiber | null {
  const updateExpirationTime = workInProgress.expirationTime;

  if (current !== null) {
    const oldProps = current.memoizedProps;
    const newProps = workInProgress.pendingProps;

    if (oldProps !== newProps || hasLegacyContextChanged()) {
      // If props or context changed, mark the fiber as having performed work.
      // This may be unset if the props are determined to be equal later (memo).
      didReceiveUpdate = true;
    } else if (updateExpirationTime < renderExpirationTime) {
      didReceiveUpdate = false;
      // This fiber does not have any pending work. Bailout without entering
      // the begin phase. There's still some bookkeeping we that needs to be done
      // in this optimized path, mostly pushing stuff onto the stack.
      switch (workInProgress.tag) {
        case HostRoot:
          pushHostRootContext(workInProgress);
          resetHydrationState();
          break;
        case HostComponent:
          pushHostContext(workInProgress);
          break;
        case ClassComponent: {
          const Component = workInProgress.type;
          if (isLegacyContextProvider(Component)) {
            pushLegacyContextProvider(workInProgress);
          }
          break;
        }
        case HostPortal:
          pushHostContainer(
            workInProgress,
            workInProgress.stateNode.containerInfo,
          );
          break;
        case ContextProvider: {
          const newValue = workInProgress.memoizedProps.value;
          pushProvider(workInProgress, newValue);
          break;
        }
        case Profiler:
          if (enableProfilerTimer) {
            workInProgress.effectTag |= Update;
          }
          break;
        case SuspenseComponent: {
          const state: SuspenseState | null = workInProgress.memoizedState;
          const didTimeout = state !== null;
          if (didTimeout) {
            // If this boundary is currently timed out, we need to decide
            // whether to retry the primary children, or to skip over it and
            // go straight to the fallback. Check the priority of the primary
            // child fragment.
            const primaryChildFragment: Fiber = (workInProgress.child: any);
            const primaryChildExpirationTime =
              primaryChildFragment.childExpirationTime;
            if (
              primaryChildExpirationTime !== NoWork &&
              primaryChildExpirationTime >= renderExpirationTime
            ) {
              // The primary children have pending work. Use the normal path
              // to attempt to render the primary children again.
              return updateSuspenseComponent(
                current,
                workInProgress,
                renderExpirationTime,
              );
            } else {
              // The primary children do not have pending work with sufficient
              // priority. Bailout.
              const child = bailoutOnAlreadyFinishedWork(
                current,
                workInProgress,
                renderExpirationTime,
              );
              if (child !== null) {
                // The fallback children have pending work. Skip over the
                // primary children and work on the fallback.
                return child.sibling;
              } else {
                return null;
              }
            }
          }
          break;
        }
      }
      return bailoutOnAlreadyFinishedWork(
        current,
        workInProgress,
        renderExpirationTime,
      );
    }
  } else {
    didReceiveUpdate = false;
  }

  // Before entering the begin phase, clear the expiration time.
  workInProgress.expirationTime = NoWork;

  switch (workInProgress.tag) {
    case IndeterminateComponent: {
      const elementType = workInProgress.elementType;
      return mountIndeterminateComponent(
        current,
        workInProgress,
        elementType,
        renderExpirationTime,
      );
    }
    case LazyComponent: {
      const elementType = workInProgress.elementType;
      return mountLazyComponent(
        current,
        workInProgress,
        elementType,
        updateExpirationTime,
        renderExpirationTime,
      );
    }
    case FunctionComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);
      return updateFunctionComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderExpirationTime,
      );
    }
    case ClassComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);
      return updateClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderExpirationTime,
      );
    }
    case HostRoot:
      return updateHostRoot(current, workInProgress, renderExpirationTime);
    case HostComponent:
      return updateHostComponent(current, workInProgress, renderExpirationTime);
    case HostText:
      return updateHostText(current, workInProgress);
    case SuspenseComponent:
      return updateSuspenseComponent(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case HostPortal:
      return updatePortalComponent(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case ForwardRef: {
      const type = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === type
          ? unresolvedProps
          : resolveDefaultProps(type, unresolvedProps);
      return updateForwardRef(
        current,
        workInProgress,
        type,
        resolvedProps,
        renderExpirationTime,
      );
    }
    case Fragment:
      return updateFragment(current, workInProgress, renderExpirationTime);
    case Mode:
      return updateMode(current, workInProgress, renderExpirationTime);
    case Profiler:
      return updateProfiler(current, workInProgress, renderExpirationTime);
    case ContextProvider:
      return updateContextProvider(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case ContextConsumer:
      return updateContextConsumer(
        current,
        workInProgress,
        renderExpirationTime,
      );
    case MemoComponent: {
      const type = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      // Resolve outer props first, then resolve inner props.
      let resolvedProps = resolveDefaultProps(type, unresolvedProps);
      if (__DEV__) {
        if (workInProgress.type !== workInProgress.elementType) {
          const outerPropTypes = type.propTypes;
          if (outerPropTypes) {
            checkPropTypes(
              outerPropTypes,
              resolvedProps, // Resolved for outer only
              'prop',
              getComponentName(type),
              getCurrentFiberStackInDev,
            );
          }
        }
      }
      resolvedProps = resolveDefaultProps(type.type, resolvedProps);
      return updateMemoComponent(
        current,
        workInProgress,
        type,
        resolvedProps,
        updateExpirationTime,
        renderExpirationTime,
      );
    }
    case SimpleMemoComponent: {
      return updateSimpleMemoComponent(
        current,
        workInProgress,
        workInProgress.type,
        workInProgress.pendingProps,
        updateExpirationTime,
        renderExpirationTime,
      );
    }
    case IncompleteClassComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);
      return mountIncompleteClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderExpirationTime,
      );
    }
    default:
      invariant(
        false,
        'Unknown unit of work tag. This error is likely caused by a bug in ' +
          'React. Please file an issue.',
      );
  }
}