Triggering initial event in BLoC

问题

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();
}

回答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

来源：https://stackoverflow.com/questions/62648103/triggering-initial-event-in-bloc

标签

Flutter

dart

bloc

flutter-bloc