ReactContext

export function createContext<T>(
    // 默认值
  defaultValue: T,
  // 判断新老变化
  calculateChangedBits: ?(a: T, b: T) => number,
): ReactContext<T> {
  // calculateChangedBits 类型校验
  // ...
  // 创建 context 实例，同时其实也是 Consumer
  const context: ReactContext<T> = {
    $$typeof: REACT_CONTEXT_TYPE,
    _calculateChangedBits: calculateChangedBits,
    // As a workaround to support multiple concurrent renderers, we categorize
    // some renderers as primary and others as secondary. We only expect
    // there to be two concurrent renderers at most: React Native (primary) and
    // Fabric (secondary); React DOM (primary) and React ART (secondary).
    // Secondary renderers store their context values on separate fields.
    _currentValue: defaultValue,
    _currentValue2: defaultValue,
    // These are circular
    Provider: (null: any),
    Consumer: (null: any),
  };
    // 创建 Provide 元素
  context.Provider = {
    $$typeof: REACT_PROVIDER_TYPE,
    _context: context,
  };
    // 开发时非法使用的错误抛出
    // ...
    // 将 Consumer 指向 context 自身
    context.Consumer = context;
  return context;
}