React 深入学习：ReactFiberNewContext

path：packages/react-reconciler/src/ReactFiberNewContext.js

readContext

export function readContext<T>(
  context: ReactContext<T>,
  observedBits: void | number | boolean,
): T {
  if (__DEV__) {
    // do something
  }

  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.
    // 避免对可观察的参数或异构类型进行deopting。
    if (
      typeof observedBits !== 'number' ||
      observedBits === MAX_SIGNED_31_BIT_INT
    ) {
      // 观察所有更新
      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. ' +
          'In classes, you can read it in the render method or getDerivedStateFromProps. ' +
          'In function components, you can read it directly in the function body, but not ' +
          'inside Hooks like useReducer() or useMemo().',
      );

      // 如果这是该组件的第一个依赖项。创建一个新列表。
      lastContextDependency = contextItem;
      currentlyRenderingFiber.contextDependencies = {
        first: contextItem,
        expirationTime: NoWork,
      };
    } else {
      // 追加一个新的 context 项。
      lastContextDependency = lastContextDependency.next = contextItem;
    }
  }
  return isPrimaryRenderer ? context._currentValue : context._currentValue2;
}

readContext 函数接受两个参数：

• context：一个 ReactContext 对象
• observedBits

readContext 会将传入的参数 context 经过封装后添加到当前组件的 fiber 的 contextDependencies 链表的末尾。

最后，readContext 会返回当前 context 中存储的值。