Why are child widget StreamBuilders not receiving errors? - dart

Question

I am struggling with RxDart (maybe just straight up Rx programming). I currently have a stateful widget that calls my bloc in it's didChangeDependencies(). That call goes out and gets data via http request and adds it to a stream. I'm using BehaviorSubject and this works fine. I have child widgets using StreamBuilders and they get data no problem. My issue comes in dealing with errors. If my http request fails, I hydrate the stream with addError('whatever error') but my child widget's StreamBuilder is not receiving that error. It doesn't get anything at all.
So I have a few questions.
Is that expected?
Should error handling not be done in StreamBuilder? Ideally, I want to show something in the UI if something goes wrong so not sure how else to do it.
I could make my child widget stateful and use stream.listen. I do receive the errors there but it seems like overkill to have that and the StreamBuilder.
Am I missing something fundamental here about streams and error handling?
Here is my bloc:
final _plans = BehaviorSubject<List<PlanModel>>();
Observable<List<PlanModel>> get plans => _plans.stream;
fetchPlans() async {
try {
final _plans = await _planRepository.getPlans();
_plans.add(_plans);
}
on AuthenticationException {
_plans.addError('authentication error');
}
on SocketException {
_plans.addError('no network connection');
}
catch(error) {
_plans.addError('fetch unsuccessful');
}
}
Simplified Parent Widget:
class PlanPage extends StatefulWidget {
#override
PlanPageState createState() {
return new PlanPageState();
}
}
class PlanPageState extends State<PlanPage> {
#override
void didChangeDependencies() async {
super.didChangeDependencies();
var planBloc = BaseProvider.of<PlanBloc>(context);
planBloc.fetchPlans();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar( title: const Text('Your Plan') ),
body: PlanWrapper()
);
}
}
Simplified Child Widget with StreamBuilder:
class PlanWrapper extends StatelessWidget {
#override
Widget build(BuildContext context) {
var planBloc = BaseProvider.of<PlanBloc>(context);
return StreamBuilder(
stream: planBloc.plans,
builder: (BuildContext context, AsyncSnapshot<List<PlanModel>> plans) {
if (plans.hasError) {
//ERROR NEVER COMES IN HERE
switch(plans.error) {
case 'authentication error':
return RyAuthErrorCard();
case 'no network connection':
return RyNetworkErrorCard();
default:
return RyGenericErrorCard(GeneralException().errorMessages()['message']);
}
}
if (plans.hasData && plans.data.isNotEmpty) {
return ListView(
physics: const AlwaysScrollableScrollPhysics(),
children: _buildPlanTiles(context, plans.data)
);
}
return Center(child: const CircularProgressIndicator());
}
);
}
}

Answer 1

There was an issue about this in the RxDart GitHub (https://github.com/ReactiveX/rxdart/issues/227). BehaviorSubjects were not replaying errors to new listeners.
It was fixed in version 0.21.0. "Breaking Change: BehaviorSubject will now emit an Error, if the last event was also an Error. Before, when an Error occurred before a listen, the subscriber would not be notified of that Error."