Flutter BLOC state updating but always null value - flutter

Question

I update state bloc builder context like this context.read<SurveyBloc>().add(SurveyModeChanged(mode: 'draft')); in the bloc file state changing is triggered but value always null. the last 2days I struck with this someone please help to resolve this issue.
if (event is SurveyModeChanged) {
print('mode==>');
print(state.mode);
yield state.copyWith(mode: state.mode);
}
This is Survey screen file
class SurveyView extends StatefulWidget {
#override
State<StatefulWidget> createState() => _SurveyViewState();
}
class _SurveyViewState extends State<SurveyView> {
#override
Widget build(BuildContext context) {
final sessionCubit = context.read<SessionCubit>();
return BlocProvider(
create: (context) => SurveyBloc(
user: sessionCubit.selectedUser ?? sessionCubit.currentUser,
surveyId: '4aa842ff-2b7d-4364-9669-29c200a3fe9b',
dataRepository: context.read<DataRepository>(),
),
child: BlocListener<SurveyBloc, SurveyState>(
listener: (context, state) {},
child: Scaffold(
backgroundColor: Color(0xFFF2F2F7),
appBar: _appbar(),
body: stepFormContainer(context),
resizeToAvoidBottomInset: false,
),
),
);
}
Widget saveButton() {
return BlocBuilder<SurveyBloc, SurveyState>(builder: (context, state) {
return Padding(
padding: EdgeInsets.symmetric(horizontal: 10),
child: ElevatedButton.icon(
onPressed: () {
context
.read<SurveyBloc>()
.add(SurveyModeChanged(mode: 'draft'));
},
label: Text('Save')));
});
}
}
This is my Survey event code
abstract class SurveyEvent {}
class SurveyResultChanged extends SurveyEvent {
final String surveyResult;
SurveyResultChanged({this.surveyResult});
}
class SurveyModeChanged extends SurveyEvent {
final String mode;
SurveyModeChanged({this.mode});
}
class SurveyIdChanged extends SurveyEvent {
final String surveyId;
SurveyIdChanged({this.surveyId});
}
class SaveSurveyChanges extends SurveyEvent {}
Survey State dart
class SurveyState {
final User user;
final FormSubmissionStatus formSubmissionStatus;
final String surveyId;
final String mode;
final String surveyResult;
SurveyState(
{#required User user,
#required String surveyId,
String mode,
String surveyResult,
this.formSubmissionStatus = const InitialFormStatus()})
: this.user = user,
this.surveyId = surveyId,
this.mode = mode,
this.surveyResult = surveyResult;
SurveyState copyWith({
User user,
FormSubmissionStatus formSubmissionStatus,
String surveyId,
String mode,
String surveyResult,
}) {
return SurveyState(
user: user ?? this.user,
surveyId: surveyId ?? this.surveyId,
mode: mode ?? this.mode,
surveyResult: surveyResult ?? this.surveyResult,
formSubmissionStatus:
formSubmissionStatus ?? this.formSubmissionStatus);
}
}
SurveyBloc.dart
class SurveyBloc extends Bloc<SurveyEvent, SurveyState> {
final DataRepository dataRepository;
SurveyBloc({
#required User user,
#required String surveyId,
this.dataRepository,
}) : super(SurveyState(user: user, surveyId: surveyId));
#override
Stream<SurveyState> mapEventToState(SurveyEvent event) async* {
if (event is SurveyModeChanged) {
print('mode==>');
print(state.mode);
yield state.copyWith(mode: state.mode);
}
}
}

Answer 1

class SurveyBloc extends Bloc<SurveyEvent, SurveyState> {
final DataRepository dataRepository;
SurveyBloc({
#required User user,
#required String surveyId,
this.dataRepository,
}) : super(SurveyState(user: user, surveyId: surveyId));
#override
Stream<SurveyState> mapEventToState(SurveyEvent event) async* {
if (event is SurveyModeChanged) {
print('mode==>');
print(state.mode);
// This is where the problem occurs. You are emitting the state
// value again and again which is null. Change this:
yield state.copyWith(mode: state.mode);
// into this:
yield state.copyWith(mode: event.mode);
}
}
}