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

Question

First of, I do know how BLoC suppose to work, the idea behind it and I know the difference between BlocProvider() and BlocProvider.value() constructors.

For simplicity, my application has 3 pages with a widget tree like this:

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

I want my LoginPage() to have access to UserBloc because i need to log in user etc. To do that, I wrap LoginPage() builder at App() widget like this:

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(),
      ),
    );
  }
}

That obviously works just fine. Then, if User logs in successfully, he is navigated to HomePage . Now, I need to have access to two different blocs at my HomePage so I use MultiBlocProvider to pass existing UserBloc further and create a brand new one named DataBloc . I do it like this:

  @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(),
              ),
            ),
          );
        }
      },
[...]

This also works. Problem happens when from HomePage user navigates to UserTokensPage . At UserTokensPage I need my already existing UserBloc that I want to pass with BlocProvider.value() constructor. I do it like this:

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();
            }
        }
      },
[...]

When I press button to navigate to MyTokensPage i get error with message:

════════ 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

What am I doing wrong? Is it because i have extracted PopupMenuButton widget that somehow loses blocs? I don't understand what I can be doing wrong.

Answer 1

I fixed it. Inside App widget i create LoginPage with

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

At LoginPage I simply wrap BlocBuilders one into another

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 navigates User to TokenPage with

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

And that solved all my problems.

Answer 2

You can just wrap the Blocs you need to access through out the app by wrapping it at the entry point of the app like this

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

          ],
          child: App()
      )
  );
}

and you can access this bloc at anywhere of your app by

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

Answer 3

Solution

Method A: Access UserBloc provider instance directly without passing it

I prefer this solution since it requires less code.

A.1 Wrap CustomPopupButton instance with provider Consumer so it rebuilds itself whenever UserBloc notifies listeners of value changes.

Change this:

actions: <Widget>[
    CustomPopupButton(),
],

To:

actions: <Widget>[
    Consumer<UserBloc>(builder: (BuildContext context, UserBloc userBloc, Widget child) {
      return CustomPopupButton(),
    });
],

A.2 Change Provider instance invocation inside the stateless widget to disable listening to value changes -- "listening" and resulting "rebuilds" are already done by Consumer .

A.2.1 Change this:

value: BlocProvider.of<UserBloc>(context),

To:

value: BlocProvider.of<UserBloc>(context, listen: false),

A.2.2 And change this:

BlocProvider.of<UserBloc>(context).add(SignOut());

To:

BlocProvider.of<UserBloc>(context, listen: false).add(SignOut());

Method B: pass UserBloc provider instance

Same thing as Method A, but:

• In A.1 you'd pass userBloc like this: return CustomPopupButton(userBloc: userBloc), .
• You'd declare final UserBloc userBloc; member property inside CustomPopupButton .
• In A.2 you'd do this: userBloc.add(SignOut()); instead of BlocProvider.of<UserBloc>(context, listen: false).add(SignOut());

Explanation

flutter_bloc is using Provider , to be aware what's going on it's better understand Provider. Please refer to my answer here to understand my answer to your question, and to understand Provider and listen flag better.

Answer 4

You need to either decompose your widget into two widgets (which I recommend for testability reasons) or use a Builder widget to get a child context.

class MyHomePage extends StatelessWidget { @override Widget build(BuildContext context) { return BlocProvider( create: (_) => TestCubit(), child: MyHomeView(), ); } } class MyHomeView extends StatelessWidget { @override Widget build(BuildContext context) { return Scaffold( body: Center( child: RaisedButton(onPressed: () => BlocProvider.of<TestCubit>(context)...) ), ); } }

source: solved by Felix Angelov, https://github.com/felangel/bloc/issues/2064

Answer 5

您不必使用 BlocProvider.value() 导航到另一个屏幕，您只需将 MaterialApp 包装到 BlocProvider 作为它的子项

Answer 6

Change name of context in builder whether in bottomSheet or materialPageRoute.

So that bloc can access parent context through context unless it's going to take context from builder (bottom sheet). This can lead to an error which you can't reach the instance of bloc .

showModalBottomSheet(
     context: context,
     builder: (context2) {  ===> change here to context2
     BlocProvider.value(
          value: BlocProvider.of<BlocA>(context),
          child: widgetA(),
                       ),
     }