Flutter BLoC: Navigator.pop in StreamBuilder in build() method

Question

I'm following BLoC pattern and subscribing to stream, and reacting to state changes in build method. When data is loaded I want to close the screen.

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Bloc'),
      ),
      body: SafeArea(
        child: StreamBuilder<UserState>(
          stream: _userBloc.user,
          initialData: UserInitState(),
          builder: (context, snapshot) {
            if (snapshot.data is UserInitState) {
              return _buildInit();
            }
            if (snapshot.data is UserDataState) {
              Navigator.pop(context, true);
              return Container();
            }
            if (snapshot.data is UserLoadingState) {
              return _buildLoading();
            }
          },
        ),
      ),
    );
  } 

When I do Navigator.pop(context, true); in build() method I get:

I/flutter ( 4360): ══╡ EXCEPTION CAUGHT BY ANIMATION LIBRARY ╞═════════════════════════════════════════════════════════
I/flutter ( 4360): The following assertion was thrown while notifying status listeners for AnimationController:
I/flutter ( 4360): setState() or markNeedsBuild() called during build.
I/flutter ( 4360): This Overlay widget cannot be marked as needing to build because the framework is already in the
I/flutter ( 4360): process of building widgets. A widget can be marked as needing to be built during the build phase
I/flutter ( 4360): only if one of its ancestors is currently building. This exception is allowed because the framework
I/flutter ( 4360): builds parent widgets before children, which means a dirty descendant will always be built.
I/flutter ( 4360): Otherwise, the framework might not visit this widget during this build phase.

What is the right way to handle such cases in BLoC pattern?

On of the solutions I come up with is to start listening to stream on initState(). In this case I need to broadcast() my stream because I have 2 subscribers.

Are there any better solutions for this?

Answer 1

I think I have got a solution for you. (Please check it)

Make your code look like :

Widget build(BuildContext context) {
    // other stuff

    if (snapshot.data is UserDataState) {
      myCallback(() {
        Navigator.pop(context, true);
      });
    }

    // other stuff
}
// after build method (but still in the same class,...) write below method

void myCallback(Function callback) {
    WidgetsBinding.instance.addPostFrameCallback((_) {
      callback();
    });
}

Hope it helps. Just try it and please report here to help others too!

Source (flutter_bloc Login medium article)

Description

Answer 2

bool hasPop = false;

  @override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: const Text('Bloc'),
    ),
    body: SafeArea(
      child: StreamBuilder<UserState>(
        stream: _userBloc.user,
        initialData: UserInitState(),
        builder: (context, snapshot) {
          if (snapshot.data is UserInitState) {
            return _buildInit();
          }
          if (snapshot.data is UserDataState) {
            if(!hasPop){
               hasPop = true;
               Navigator.pop(context, true);
             }
            
            return Container();
          }
          if (snapshot.data is UserLoadingState) {
            return _buildLoading();
          }
        },
      ),
    ),
  );
} ```