react next context readContext and ...

newContext.js

export function propagateContextChange(
  workInProgress: Fiber,
  context: ReactContext<mixed>,
  changedBits: number,
  renderExpirationTime: ExpirationTime,
): void {
  let fiber = workInProgress.child;
  if (fiber !== null) {
    // Set the return pointer of the child to the work-in-progress fiber.
    fiber.return = workInProgress;
  }
  while (fiber !== null) {
    let nextFiber;

    // Visit this fiber.
    let dependency = fiber.firstContextDependency;
    if (dependency !== null) {
      do {
        // Check if the context matches.
        if (
          dependency.context === context &&
          (dependency.observedBits & changedBits) !== 0
        ) {
          // Match! Schedule an update on this fiber.

          if (fiber.tag === ClassComponent) {
            // Schedule a force update on the work-in-progress.
            const update = createUpdate(renderExpirationTime);
            update.tag = ForceUpdate;
            // TODO: Because we don't have a work-in-progress, this will add the
            // update to the current fiber, too, which means it will persist even if
            // this render is thrown away. Since it's a race condition, not sure it's
            // worth fixing.
            enqueueUpdate(fiber, update);
          }

          if (
            fiber.expirationTime === NoWork ||
            fiber.expirationTime > renderExpirationTime
          ) {
            fiber.expirationTime = renderExpirationTime;
          }
          let alternate = fiber.alternate;
          if (
            alternate !== null &&
            (alternate.expirationTime === NoWork ||
              alternate.expirationTime > renderExpirationTime)
          ) {
            alternate.expirationTime = renderExpirationTime;
          }
          // Update the child expiration time of all the ancestors, including
          // the alternates.
          let node = fiber.return;
          while (node !== null) {
            alternate = node.alternate;
            if (
              node.childExpirationTime === NoWork ||
              node.childExpirationTime > renderExpirationTime
            ) {
              node.childExpirationTime = renderExpirationTime;
              if (
                alternate !== null &&
                (alternate.childExpirationTime === NoWork ||
                  alternate.childExpirationTime > renderExpirationTime)
              ) {
                alternate.childExpirationTime = renderExpirationTime;
              }
            } else if (
              alternate !== null &&
              (alternate.childExpirationTime === NoWork ||
                alternate.childExpirationTime > renderExpirationTime)
            ) {
              alternate.childExpirationTime = renderExpirationTime;
            } else {
              // Neither alternate was updated, which means the rest of the
              // ancestor path already has sufficient priority.
              break;
            }
            node = node.return;
          }
        }
        nextFiber = fiber.child;
        dependency = dependency.next;
      } while (dependency !== null);
    } else if (fiber.tag === ContextProvider) {
      // Don't scan deeper if this is a matching provider
      nextFiber = fiber.type === workInProgress.type ? null : fiber.child;
    } else {
      // Traverse down.
      nextFiber = fiber.child;
    }

    if (nextFiber !== null) {
      // Set the return pointer of the child to the work-in-progress fiber.
      nextFiber.return = fiber;
    } else {
      // No child. Traverse to next sibling.
      nextFiber = fiber;
      while (nextFiber !== null) {
        if (nextFiber === workInProgress) {
          // We're back to the root of this subtree. Exit.
          nextFiber = null;
          break;
        }
        let sibling = nextFiber.sibling;
        if (sibling !== null) {
          // Set the return pointer of the sibling to the work-in-progress fiber.
          sibling.return = nextFiber.return;
          nextFiber = sibling;
          break;
        }
        // No more siblings. Traverse up.
        nextFiber = nextFiber.return;
      }
    }
    fiber = nextFiber;
  }
}

export function prepareToReadContext(
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
): void {
  currentlyRenderingFiber = workInProgress;
  lastContextDependency = null;
  lastContextWithAllBitsObserved = null;

  // Reset the work-in-progress list
  workInProgress.firstContextDependency = null;
}

export function readContext<T>(
  context: ReactContext<T>,
  observedBits: void | number | boolean,
): T {
  if (lastContextWithAllBitsObserved === context) {
    // Nothing to do. We already observe everything in this context.
  } else if (observedBits === false || observedBits === 0) {
    // Do not observe any updates.
  } else {
    let resolvedObservedBits; // Avoid deopting on observable arguments or heterogeneous types.
    if (
      typeof observedBits !== 'number' ||
      observedBits === MAX_SIGNED_31_BIT_INT
    ) {
      // Observe all updates.
      lastContextWithAllBitsObserved = ((context: any): ReactContext<mixed>);
      resolvedObservedBits = MAX_SIGNED_31_BIT_INT;
    } else {
      resolvedObservedBits = observedBits;
    }

    let contextItem = {
      context: ((context: any): ReactContext<mixed>),
      observedBits: resolvedObservedBits,
      next: null,
    };

    if (lastContextDependency === null) {
      invariant(
        currentlyRenderingFiber !== null,
        'Context can only be read while React is ' +
          'rendering, e.g. inside the render method or getDerivedStateFromProps.',
      );
      // This is the first dependency in the list
      currentlyRenderingFiber.firstContextDependency = lastContextDependency = contextItem;
    } else {
      // Append a new context item.
      lastContextDependency = lastContextDependency.next = contextItem;
    }
  }
  return isPrimaryRenderer ? context._currentValue : context._currentValue2;
}