Function enqueueUpdate

packages/react-reconciler/src/ReactFiberClassUpdateQueue.js:223–274

(
  fiber: Fiber,
  update: Update<State>,
  lane: Lane,
)

}

export function enqueueUpdate<State>(
  fiber: Fiber,
  update: Update<State>,
  lane: Lane,
): FiberRoot | null {
  const updateQueue = fiber.updateQueue;
  if (updateQueue === null) {
    // Only occurs if the fiber has been unmounted.
    return null;
  }

  const sharedQueue: SharedQueue<State> = (updateQueue: any).shared;

  if (__DEV__) {
    if (
      currentlyProcessingQueue === sharedQueue &&
      !didWarnUpdateInsideUpdate
    ) {
      const componentName = getComponentNameFromFiber(fiber);
      console.error(
        'An update (setState, replaceState, or forceUpdate) was scheduled ' +
          'from inside an update function. Update functions should be pure, ' +
          'with zero side-effects. Consider using componentDidUpdate or a ' +
          'callback.\n\nPlease update the following component: %s',
        componentName,
      );
      didWarnUpdateInsideUpdate = true;
    }
  }

  if (isUnsafeClassRenderPhaseUpdate(fiber)) {
    // This is an unsafe render phase update. Add directly to the update
    // queue so we can process it immediately during the current render.
    const pending = sharedQueue.pending;
    if (pending === null) {
      // This is the first update. Create a circular list.
      update.next = update;
    } else {
      update.next = pending.next;
      pending.next = update;
    }
    sharedQueue.pending = update;

    // Update the childLanes even though we're most likely already rendering
    // this fiber. This is for backwards compatibility in the case where you
    // update a different component during render phase than the one that is
    // currently renderings (a pattern that is accompanied by a warning).
    return unsafe_markUpdateLaneFromFiberToRoot(fiber, lane);
  } else {
    return enqueueConcurrentClassUpdate(fiber, sharedQueue, update, lane);
  }
}

export function entangleTransitions(root: FiberRoot, fiber: Fiber, lane: Lane) {
  const updateQueue = fiber.updateQueue;