为什么我们不能在具有 bloc 属性的 Consumer 内部使用 BlocProvider.of(blocContext)？什么是解决方案？

Question

构建方法内部：

  return BlocBuilder<UserCubit, UserState>(
    bloc: UserCubit("firstName", "lastName"),
    builder: (BuildContext blocContext, UserState state) {
                  print(BlocProvider.of<UserCubit>(blocContext).user.fullName);  // error here
});

错误：

BlocProvider.of() called with a context that does not contain a UserCubit.

显然不是这样...

问题是，如果 BlocBuilder/BlocConsumer bloc 属性不为空，我如何访问肘部。

Answer 1

基本上是因为小部件 BlocBuilder 没有在您的 BuildContext blocContext.

中注入提供的 bloc 参数

flutter_bloc documentation 说 BlocBuilder 像 StreamBuilder 一样工作，它不会在你的小部件树中注入任何东西它只会“听”你提供的 UserCubit 在 bloc 属性中。因为 UserCubit 没有注入到你的小部件树中，你将永远无法使用 BlocProvider.

访问它

BlocBuilder handles building a widget in response to new states. BlocBuilder is analogous to StreamBuilder but has simplified API to reduce the amount of boilerplate code needed as well as bloc-specific performance improvements.

当您使用 BlocBuilder 时，您将需要依赖 state 值来访问您的 bloc 或 cubit 的属性，并且您必须将 bloc/cubit 注入使用父 BlocProvider 的小部件树，这是一个示例：

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final userCubit = UserCubit("firstName", "lastName");

    // Using BlocProvider to inject your UserCubit
    return BlocProvider(
      create: (_) => userCubit,
      child: BlocBuilder<UserCubit, UserState>(
        // Still using the instance of userCubit as the injected instance is not
        // available in the widget tree at this point.
        bloc: userCubit,
        builder: (blocContext, state) {
          // Now that you are in your builder with an up-to-date blocContext you can
          // have access to your instance of UserCubit with BlocProvider.
          print(BlocProvider.of<UserCubit>(blocContext).user.fullName);
          return Text("State is $state");
        },
      ),
    );
  }
}

You can try the full example on DartPad

为什么我们不能在具有 bloc 属性的 Consumer 内部使用 BlocProvider.of(blocContext)？什么是解决方案？

why can't we use BlocProvider.of(blocContext) inside of Consumer with bloc attribute? and what is the solution?

flutter

bloc

state-management