BlocBuilder Race Condition Test

main.dart

import 'package:flutter/material.dart';
import 'package:bloc/bloc.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

abstract class DataState {}

class InitialDataState extends DataState {}

class LoadedDataState extends DataState {
  final String data;

  LoadedDataState({@required this.data});
}

abstract class DataEvent {}

class FetchData extends DataEvent {}

class DataBloc extends Bloc<DataEvent, DataState> {
  @override
  DataState get initialState => InitialDataState();

  @override
  void onTransition(Transition<DataEvent, DataState> transition) {
    print(transition);
  }

  @override
  Stream<DataState> mapEventToState(
    DataState currentState,
    DataEvent event,
  ) async* {
    if (event is FetchData) {
      yield LoadedDataState(data: 'Cool Data');
    }
  }
}

DataBloc dataBloc = DataBloc();

void main() async {
  dataBloc.dispatch(FetchData());
  await Future.delayed(Duration(seconds: 1));
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('building HomePage');
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Demo'),
      ),
      body: BlocBuilder(
        bloc: dataBloc,
        builder: (BuildContext context, DataState state) {
          if (state is InitialDataState) {
            return Center(child: CircularProgressIndicator());
          }
          if (state is LoadedDataState) {
            return Center(child: Text(state.data));
          }
          return Container();
        },
      ),
    );
  }
}