React 上下文

export function createContext<T>(defaultValue: T): ReactContext<T> {
  // TODO: Second argument used to be an optional `calculateChangedBits`
  // function. Warn to reserve for future use?
  // TODO: 第二个参数以前是可选的 `calculateChangedBits` 函数。是否发出警告以保留未来使用？

  const context: ReactContext<T> = {
    $$typeof: REACT_CONTEXT_TYPE,
    // As a workaround to support multiple concurrent renderers, we categorize
    // some renderers as primary and others as secondary. We only expect
    // there to be two concurrent renderers at most: React Native (primary) and
    // Fabric (secondary); React DOM (primary) and React ART (secondary).
    // Secondary renderers store their context values on separate fields.
    //
    // 为了支持多个并发渲染器，作为一种变通方法，我们将一些渲染器分类为主要渲染器，另一些为次要渲染器。
    // 我们只期望最多存在两个并发渲染器：React Native（主要渲染器）和 Fabric（次要渲染器）；React
    // DOM（主要渲染器）和 React ART（次要渲染器）。次要渲染器将它们的上下文值存储在单独的字段中。
    _currentValue: defaultValue,
    _currentValue2: defaultValue,
    // Used to track how many concurrent renderers this context currently
    // supports within in a single renderer. Such as parallel server rendering.
    // 用于跟踪此上下文当前在单个渲染器中支持的并发渲染器数量。例如并行服务器渲染。
    _threadCount: 0,
    // These are circular
    // 这些是循环的
    Provider: null as any,
    Consumer: null as any,
  };

  context.Provider = context;
  context.Consumer = {
    $$typeof: REACT_CONSUMER_TYPE,
    _context: context,
  };
  if (__DEV__) {
    context._currentRenderer = null;
    context._currentRenderer2 = null;
  }

  return context;
}