flutter - BlocProvider.of() 使用不包含 Bloc 的上下文调用 - 即使它包含

Question

首先，我确实知道 BLoC 应该如何工作，它背后的想法和我知道 BlocProvider() 之间的区别和 BlocProvider.value()构造器。

为简单起见，我的应用程序有 3 个页面，其中包含一个像这样的小部件树:
App() => LoginPage() => HomePage() => UserTokensPage()
我要我的 LoginPage()有权访问 UserBloc因为我需要登录用户等。为此，我将 LoginPage() 包裹起来。 build 者在 App()像这样的小部件:

void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'My App',
      home: BlocProvider<UserBloc>(
        create: (context) => UserBloc(UserRepository()),
        child: LoginPage(),
      ),
    );
  }
}

这显然工作得很好。然后，如果用户登录成功，他将被导航到 HomePage .现在，我需要在我的 HomePage 上访问两个不同的区块。所以我使用 MultiBlocProvider通过现有 UserBloc进一步创建一个全新的名为 DataBloc .我这样做:

  @override
  Widget build(BuildContext context) {
    return BlocListener<UserBloc, UserState>(
      listener: (context, state) {
        if (state is UserAuthenticated) {
          Navigator.of(context).push(
            MaterialPageRoute<HomePage>(
              builder: (_) => MultiBlocProvider(
                providers: [
                  BlocProvider.value(
                    value: BlocProvider.of<UserBloc>(context),
                  ),
                  BlocProvider<DataBloc>(
                    create: (_) => DataBloc(DataRepository()),
                  ),
                ],
                child: HomePage(),
              ),
            ),
          );
        }
      },
[...]

这也有效。当来自 HomePage 时出现问题用户导航到 UserTokensPage .在 UserTokensPage我需要我现有的 UserBloc我想通过 BlocProvider.value()构造函数。我这样做:

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        centerTitle: false,
        title: Text('My App'),
        actions: <Widget>[
          CustomPopupButton(),
        ],
      ),

[...]

class CustomPopupButton extends StatelessWidget {
  const CustomPopupButton({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return PopupMenuButton<String>(
      icon: Icon(Icons.more_horiz),
      onSelected: (String choice) {
        switch (choice) {
          case PopupState.myTokens:
            {
              Navigator.of(context).push(
                MaterialPageRoute<UserTokensPage>(
                  builder: (_) => BlocProvider.value(
                    value: BlocProvider.of<UserBloc>(context),
                    child: UserTokensPage(),
                  ),
                ),
              );
            }
            break;
          case PopupState.signOut:
            {
              BlocProvider.of<UserBloc>(context).add(SignOut());
              Navigator.of(context).pop();
            }
        }
      },
[...]

当我按下按钮导航到 MyTokensPage我收到错误消息:

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown building Builder(dirty):
        BlocProvider.of() called with a context that does not contain a Bloc of type UserBloc.

        No ancestor could be found starting from the context that was passed to BlocProvider.of<UserBloc>().

        This can happen if:
        1. The context you used comes from a widget above the BlocProvider.
        2. You used MultiBlocProvider and didn't explicity provide the BlocProvider types.

        Good: BlocProvider<UserBloc>(create: (context) => UserBloc())
        Bad: BlocProvider(create: (context) => UserBloc()).

        The context used was: CustomPopupButton

我究竟做错了什么？是不是因为我提取了 PopupMenuButton不知何故丢失块的小部件？我不明白我做错了什么。

Answer 1

我修好了它。内App我创建的小部件 LoginPage和

home: BlocProvider<UserBloc>(
        create: (context) => UserBloc(UserRepository()),
        child: LoginPage(),

在 LoginPage我只是简单地包装 BlocBuilders一个到另一个

Widget build(BuildContext context) {
    return BlocListener<UserBloc, UserState>(
      listener: (context, state) {
        if (state is UserAuthenticated) {
          Navigator.of(context).push(
            MaterialPageRoute<HomePage>(
              builder: (_) => BlocProvider.value(
                value: BlocProvider.of<UserBloc>(context),
                child: BlocProvider<NewRelicBloc>(
                  create: (_) => NewRelicBloc(NewRelicRepository()),
                  child: HomePage(),
                ),
              ),
            ),
          );
        }
      },
[...]

PopupMenuButton将用户导航到 TokenPage和

              Navigator.of(context).push(
                MaterialPageRoute<UserTokensPage>(
                  builder: (_) => BlocProvider.value(
                    value: BlocProvider.of<UserBloc>(context),
                    child: UserTokensPage(),
                  ),
                ),
              );

这解决了我所有的问题。