Simple Bloc Pattern and Network Connection check - flutter

Question

Thank you for checking out my question :). I appreciate it!
What I am trying to do
I am trying to check if the user has an internet connection or not. For this, I use the bloc pattern. I am just starting out and I don't know what to do with these errors.
What the error messages are
The relevant error-causing widget was:
TestScreen
Testscreen:file//filepath (etc,etc)
Another exception was thrown: Each child must be laid out exactly once.
Another exception was thrown: Updated layout information required for RenderErrorBox NEEDS-LAYOUT NEEDS-PAINT to calculate semantics.
Another exception was thrown: Bad state: Future already completed.
I am new and these errors are overwhelming. These errors tell me nothing. I hope you can help me out!
Code
Homescreen
class HomeScreen extends StatelessWidget {
const HomeScreen({Key? key}) : super(key: key);
static String routeName = '/home';
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Homepage'),
),
body: BlocProvider(
create: (context) => NetworkBloc()..add(ListenConnection()),
child: TestScreen(),
));
}
}
class TestScreen extends StatelessWidget {
const TestScreen({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return Center(
child: BlocBuilder<NetworkBloc, NetworkState>(
builder: (context, state) {
if (state is ConnectionFailure) return Text("No Internet Connection");
if (state is ConnectionSuccess)
return Text("You're Connected to Internet");
else
return Text("");
},
),
);
}
}
Network Block
class NetworkBloc extends Bloc<NetworkEvent, NetworkState> {
NetworkBloc() : super(ConnectionInitial());
late StreamSubscription _subscription;
#override
Stream<NetworkState> mapEventToState(NetworkEvent event) async* {
if (event is ListenConnection) {
_subscription = DataConnectionChecker().onStatusChange.listen((status) {
add(ConnectionChanged(status == DataConnectionStatus.disconnected
? ConnectionFailure()
: ConnectionSuccess()));
});
}
if (event is ConnectionChanged) yield event.connection;
}
#override
Future<void> close() {
_subscription.cancel();
return super.close();
}
}
Network Event
abstract class NetworkEvent {}
class ListenConnection extends NetworkEvent {}
class ConnectionChanged extends NetworkEvent {
NetworkState connection;
ConnectionChanged(this.connection);
}
Network State
abstract class NetworkState {}
class ConnectionInitial extends NetworkState {}
class ConnectionSuccess extends NetworkState {}
class ConnectionFailure extends NetworkState {}
I hope you can help me out. Thank you very much!

Answer 1

There are a lot of issues with your code so I'm not sure if I got all of them.
You should not call .add() if you don't have registered event handler
//This is how you should register an event handler
NetworkBloc() : super(ConnectionInitial()) {
on<ListenConnection>((event, emit) {
// TODO: implement event handler
});
}
You are overriding a function mapEventToState(), when it doesn't override any of inherited methods
//#override NO NEED OF OVERRIDE HERE
Stream<NetworkState> mapEventToState(NetworkEvent event) async* {
if (event is ListenConnection) {
_subscription = DataConnectionChecker().onStatusChange.listen((status) {
add(ConnectionChanged(status == DataConnectionStatus.disconnected
? ConnectionFailure()
: ConnectionSuccess()));
});
}
if (event is ConnectionChanged) yield event.connection;
}
And I'm not sure if this is intended, but your mapEventToState() also never gets called and without any emit() methods your UI will never rebuild.