Function readContextForConsumer

packages/react-reconciler/src/ReactFiberNewContext.js:529–571

(
  consumer: Fiber | null,
  context: ReactContext<T>,
)

}

function readContextForConsumer<T>(
  consumer: Fiber | null,
  context: ReactContext<T>,
): T {
  const value = isPrimaryRenderer
    ? context._currentValue
    : context._currentValue2;

  const contextItem = {
    context: ((context: any): ReactContext<mixed>),
    memoizedValue: value,
    next: null,
  };

  if (lastContextDependency === null) {
    if (consumer === null) {
      throw new Error(
        '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().',
      );
    }

    // This is the first dependency for this component. Create a new list.
    lastContextDependency = contextItem;
    consumer.dependencies = __DEV__
      ? {
          lanes: NoLanes,
          firstContext: contextItem,
          _debugThenableState: null,
        }
      : {
          lanes: NoLanes,
          firstContext: contextItem,
        };
    consumer.flags |= NeedsPropagation;
  } else {
    // Append a new context item.
    lastContextDependency = lastContextDependency.next = contextItem;
  }
  return value;
}