Function updateActivityComponent

packages/react-reconciler/src/ReactFiberBeginWork.js:1121–1212

(
  current: null | Fiber,
  workInProgress: Fiber,
  renderLanes: Lanes,
)

}

function updateActivityComponent(
  current: null | Fiber,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  const nextProps: ActivityProps = workInProgress.pendingProps;

  // Check if the first pass suspended.
  const didSuspend = (workInProgress.flags & DidCapture) !== NoFlags;
  workInProgress.flags &= ~DidCapture;

  if (current === null) {
    // Initial mount

    // Special path for hydration
    // If we're currently hydrating, try to hydrate this boundary.
    // Hidden Activity boundaries are not emitted on the server.
    if (getIsHydrating()) {
      if (nextProps.mode === 'hidden') {
        // SSR doesn't render hidden Activity so it shouldn't hydrate,
        // even at offscreen lane. Defer to a client rendered offscreen lane.
        const primaryChildFragment = mountActivityChildren(
          workInProgress,
          nextProps,
          renderLanes,
        );
        workInProgress.lanes = laneToLanes(OffscreenLane);
        return bailoutOffscreenComponent(null, primaryChildFragment);
      } else {
        // We must push the suspense handler context *before* attempting to
        // hydrate, to avoid a mismatch in case it errors.
        pushDehydratedActivitySuspenseHandler(workInProgress);
        const dehydrated: ActivityInstance =
          claimNextHydratableActivityInstance(workInProgress);
        return mountDehydratedActivityComponent(
          workInProgress,
          dehydrated,
          renderLanes,
        );
      }
    }

    return mountActivityChildren(workInProgress, nextProps, renderLanes);
  } else {
    // This is an update.

    // Special path for hydration
    const prevState: null | ActivityState = current.memoizedState;

    if (prevState !== null) {
      const dehydrated = prevState.dehydrated;
      return updateDehydratedActivityComponent(
        current,
        workInProgress,
        didSuspend,
        nextProps,
        dehydrated,
        prevState,