Triggering initial event in BLoC - flutter

Question

example_states:
abstract class ExampleState extends Equatable {
const ExampleState();
}
class LoadingState extends ExampleState {
//
}
class LoadedState extends ExampleState {
//
}
class FailedState extends ExampleState {
//
}
example_events:
abstract class ExampleEvent extends Equatable {
//
}
class SubscribeEvent extends ExampleEvent {
//
}
class UnsubscribeEvent extends ExampleEvent {
//
}
class FetchEvent extends ExampleEvent {
//
}
example_bloc:
class ExampleBloc extends Bloc<ExampleEvent, ExampleState> {
#override
ExampleState get initialState => LoadingState();
#override
Stream<ExampleState> mapEventToState(
ExampleEvent event,
) async* {
if (event is SubscribeEvent) {
//
} else if (event is UnsubscribeEvent) {
//
} else if (event is FetchEvent) {
yield LoadingState();
try {
// network calls
yield LoadedState();
} catch (_) {
yield FailedState();
}
}
}
}
example_screen:
class ExampleScreenState extends StatelessWidget {
// ignore: close_sinks
final blocA = ExampleBloc();
#override
Widget build(BuildContext context) {
return Scaffold(
body: BlocBuilder<ExampleBloc, ExampleState>(
bloc: blocA,
// ignore: missing_return
builder: (BuildContext context, state) {
if (state is LoadingState) {
blocA.add(Fetch());
return CircularProgressBar();
}
if (state is LoadedState) {
//...
}
if (state is FailedState) {
//...
}
},
),
);
}
}
As you can see in example_bloc, initial state is LoadingState() and in build it shows circular progress bar. I use Fetch() event to trigger next states. But I don't feel comfortable using it there. What I want to do is:
When app starts, it should show LoadingState and start networking calls, then when it's all completed, it should show LoadedState with networking call results and FailedState if something goes wrong. I want to achieve these without doing
if (state is LoadingState) {
blocA.add(Fetch());
return CircularProgressBar();
}

Answer 1

Your discomfort really has reason - no event should be fired from build() method (build() could be fired as many times as Flutter framework needs)
Our case is to fire initial event on Bloc creation
Possibilities overview
case with inserting Bloc with BlocProvider - this is preferred way
create: callback is fired only once when BlocProvider is mounted & BlocProvider would close() bloc when BlocProvider is unmounted
class ExampleScreenState extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
body: BlocProvider(
create: (context) => ExampleBloc()..add(Fetch()), // <-- first event,
child: BlocBuilder<ExampleBloc, ExampleState>(
builder: (BuildContext context, state) {
...
},
),
),
);
}
}
case when you create Bloc in State of Statefull widget
class _ExampleScreenStateState extends State<ExampleScreenState> {
ExampleBloc _exampleBloc;
#override
void initState() {
super.initState();
_exampleBloc = ExampleBloc();
_exampleBloc.add(Fetch());
// or use cascade notation
// _exampleBloc = ExampleBloc()..add(Fetch());
}
#override
void dispose() {
super.dispose();
_exampleBloc.close(); // do not forget to close, prefer use BlocProvider - it would handle it for you
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: BlocBuilder<ExampleBloc, ExampleState>(
bloc: _exampleBloc,
builder: (BuildContext context, state) {
...
},
),
);
}
}
add first event on Bloc instance creation - this way has drawbacks when testing because first event is implicit
class ExampleBloc extends Bloc<ExampleEvent, ExampleState> {
...
ExampleBloc() {
add(Fetch());
}
}
// insert it to widget tree with BlocProvider or create in State
BlocProvider( create: (_) => ExampleBloc(), ...
// or in State
class _ExampleScreenStateState extends State<ExampleScreenState> {
final _exampleBloc = ExampleBloc();
...
PS feel free to reach me in comments

Answer 2

Sergey Salnikov has a great answer. I think I can add another suggestion however.
In my main.dart file I am using a MultiBlocProvider to create all my blocs for use further down the tree. Like so
Widget build(BuildContext context) {
return MultiBlocProvider(
providers: <BlocProvider<dynamic>>[
BlocProvider<OneBloc>(create: (_) => OneBloc()),
BlocProvider<TwoBloc>(create: (_) => TwoBloc()),
],
child: MaterialApp( // Rest of your app )
Then when I need to call an event when I load a page, in this case I wanted to fetch some data depending on a list tile selected, and I needed more options than FutureBuilder can provide me, I simple used initState(); and called the bloc provider and added an event.
class _ExampleScreenState extends State<ExampleScreen> {
#override
void initState() {
super.initState();
BlocProvider.of<OneBloc>(context)
.add(FetchData);
}
It works because the bloc has already been provided from the root widget.

Answer 3

In simple terms:
Using BlocProvider, call it during creation.
BlocProvider(create: (context) => ExampleBloc()..add(Fetch()))
Using BlocState, use it as
class _ExampleScreenStateState extends State<ExampleScreenState> {
ExampleBloc _exampleBloc;
#override
void initState() {
super.initState();
_exampleBloc = ExampleBloc()..add(Fetch());
}
#override
void dispose() {
super.dispose();
_exampleBloc.close();
}