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

首先，我确实知道 BLoC 是如何运作的，它背后的想法，我知道两者之间的区别BlocProvider() and BlocProvider.value()构造函数。

为简单起见，我的应用程序有 3 个页面，其中有一个小部件树，如下所示：

App() => LoginPage() => HomePage() => UserTokensPage()

我想要我的LoginPage()有权访问UserBloc因为我需要登录用户等。为此，我将LoginPage()建造者在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(),
      ),
    );
  }
}

这显然效果很好。那么，如果User登录成功，他被导航到HomePage。现在，我需要访问我的两个不同的区块HomePage所以我用MultiBlocProvider通过现有的UserBloc进一步创建一个名为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。问题发生在以下时间：HomePage user导航到UserTokensPage. At UserTokensPage我需要我已经存在的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不知何故失去块的小部件？我不明白我做错了什么。

您可以将需要在整个应用程序中访问的块包装在应用程序的入口点，如下所示

  runApp(
      MultiBlocProvider(
          providers: [
            BlocProvider<UserBloc>(
              create: (context) =>
                  UserBloc(UserRepository()),
            ),

          ],
          child: App()
      )
  );
}

您可以通过以下方式在应用程序的任何位置访问该块

BlocProvider.of<UserBloc>(context).add(event of user bloc());