flutter bloc pattern navigation back results in bad state

Question

I have a problem/question regarding the bloc plattern with flutter. Currently, i am starting my app like this

class _MyAppState extends State<MyApp> {

@override
  Widget build(BuildContext context) {        
    return BlocProvider(
        bloc: MyBloc(),
        child: MaterialApp(
          title: "MyApp",
          home: MyHomePage(),    
          routes: {
            '/homePage': (context) => MyHomePage(),
            '/otherPage': (context) => OtherPage(),
            '/otherPage2': (context) => OtherPage2(),
            ...
          },
        ));

So that i can retrieve/access myBloc like

myBloc = BlocProvider.of(context) as MyBloc;

and the data represented by the state like

BlocBuilder<MyBlocEvent, MyObject>(
    bloc: myBloc,
    builder: (BuildContext context, MyObject myObject) {         
     ....
     var t = myObject.data;
     ....
     myBloc.onFirstEvent();
     ...
 }; 

wherever i need it.

MyBloc is implemented like this:

abstract clas MyBlocEvent {}
class FirstEvent extends MyBlocEvent {}
class SecondEvent extends MyBlocEvent {}

class MyBloc extends Bloc<MyBlocEvent , MyObject>

void onFirstEvent()
{
  dispatch(FirstEvent());
}

void onSecondEvent()
{
  dispatch(SecondEvent());
}

@override
Stream<MyObject> mapEventToState( MyObject state, MyBlocEvent event) async* {
if (event is FirstEvent) {
    state.data = "test1";
}
else if (event is SecondEvent) {
    state.otherData = 5;
}
    yield state;
}

The problem i now have, is that as soon as i change on of the state values and call

Navigator.pop(context)

to go back in the current stack, i can't change anything is the state anymore because the underlying stream seems to be closed. It fails with the message:

Another exception was thrown: Bad state: Cannot add new events after calling close"

Now this only happens after i call pop. If i only push new screens i can happily change the state data without any problems.

Am i doing something wrong regarding the Navigation here or is there something else i didn't catch regarding flutter or the bloc pattern itself?

Answer 1

Bad state: Cannot add new events after calling close

This error means that you are calling add on a StreamController after having called close:

 var controller = StreamController<int>();
 controller.close();
 controller.add(42); // Bad state: Cannot add new events after calling close

It is likely related to you calling close inside the dispose method the "wrong" widget.

A good rule of thumb is to never dispose/close an object outside of the widget that created it. This ensure that you cannot use an object already disposed of.