context的详细内容可以看这里

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 === maxSigned31BitInt
    ) {
      // Observe all updates.
      lastContextWithAllBitsObserved = ((context: any): ReactContext<mixed>);
      resolvedObservedBits = maxSigned31BitInt;
    } else {
      resolvedObservedBits = observedBits;
    }

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

    if (lastContextDependency === null) {
      invariant(
        currentlyRenderingFiber !== null,
        'Context.unstable_read(): 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;
}

TODO: changedBits和observedBits是干嘛的

最后调用children传入新的value得到children节点并reconcileChildren