bloc_example.dart

import 'package:bloc/bloc.dart';

class SomeState {
  final bool isLoading;
  final int data;

  SomeState({this.isLoading = false, this.data = 0});

  SomeState copyWith({bool isLoading, int data}) {
    return SomeState(
      isLoading: isLoading ?? this.isLoading,
      data: data ?? this.data,
    );
  }

  @override
  String toString() => 'SomeState { isLoading: $isLoading, data: $data }';
}

Future<int> slowComputation(int input) async {
  return Future.delayed(Duration(seconds: 5), () {
    return input + 1;
  });
}

class MyBloc extends Bloc<String, SomeState> {
  @override
  SomeState get initialState => SomeState();

  @override
  Stream<SomeState> mapEventToState(String event) async* {
    yield currentState.copyWith(isLoading: true);
    try {
      int result = await slowComputation(currentState.data);
      yield currentState.copyWith(data: result);
    }
    finally {
      yield currentState.copyWith(isLoading: false);
    }
  }
  
}

void main() {
  final bloc = MyBloc();
  bloc.dispatch('something');
  bloc.state.forEach((state) {
    print('State received: $state');
  });
}

Output:

$ dart lib/main.dart
lib/main.dart:1: Warning: Interpreting this as package URI, 'package:bloc_ex/main.dart'.
State received: SomeState { isLoading: false, data: 0 }
State received: SomeState { isLoading: true, data: 0 }
State received: SomeState { isLoading: true, data: 1 }
State received: SomeState { isLoading: false, data: 1 }

try this instead

@override
  Stream<SomeState> mapEventToState(String event) async* {
    yield* _event();
  }

  Stream<SomeState> _event() async* {
    yield currentState.copyWith(isLoading: true);
    try {
      int result = await slowComputation(currentState.data);
      yield currentState.copyWith(data: result);
    } finally {
      yield currentState.copyWith(isLoading: false);
    }
  }