An example of how to test bloc streams and ideas to make it simpler

bloc_test.dart

import 'dart:async';

import 'package:bloc/bloc.dart';
import 'package:mockito/mockito.dart';
import 'package:test/test.dart';

enum CounterEvent { increment, decrement }

class CounterBloc extends Bloc<CounterEvent, int> {
  @override
  int get initialState => 0;

  @override
  Stream<int> mapEventToState(CounterEvent event) async* {
    switch (event) {
      case CounterEvent.increment:
        yield state + 1;
        break;
      case CounterEvent.decrement:
        yield state - 1;
        break;
    }
  }
}

class MockBloc<State> extends Mock {
  Stream<State> get mockState;

  StreamSubscription<State> listen(
    void onData(State value), {
    Function onError,
    void onDone(),
    bool cancelOnError,
  }) {
    return mockState.listen(
      onData,
      onError: onError,
      onDone: onDone,
      cancelOnError: cancelOnError,
    );
  }
}

class RawMockCounterBloc extends Mock implements CounterBloc {}

class MockCounterBloc extends MockBloc<int> implements CounterBloc {}

void main() {
  group('CounterBloc', () {
    test('starts with 0', () {
      expect(CounterBloc().state, 0);
    });

    test('increments by 1', () {
      final bloc = CounterBloc()..add(CounterEvent.increment);

      expect(bloc, emitsInOrder([0, 1]));
    });

    test('decrements by 1', () {
      final bloc = CounterBloc()..add(CounterEvent.decrement);

      expect(bloc, emitsInOrder([0, -1]));
    });

    test('can mock the stream', () {
      final mockBloc = RawMockCounterBloc();

      when(mockBloc.listen(any,
              onError: anyNamed('onError'),
              onDone: anyNamed('onDone'),
              cancelOnError: anyNamed('cancelOnError')))
          .thenAnswer((Invocation invocation) {
        return Stream.fromIterable([1, 2, 3]).listen(
          invocation.positionalArguments.first,
          onError: invocation.namedArguments['onError'],
          onDone: invocation.namedArguments['onDone'],
          cancelOnError: invocation.namedArguments['cancelOnError'],
        );
      });

      expect(mockBloc, emitsInOrder([1, 2, 3]));
    });

    test('can mock the stream with a helper', () {
      final mockBloc = RawMockCounterBloc();

      whenListen(mockBloc, Stream.fromIterable([1, 2, 3]));

      expect(mockBloc, emitsInOrder([1, 2, 3]));
    });

    test('can mock the stream with an exteded class', () {
      final mockBloc = MockCounterBloc();
      when(mockBloc.mockState)
          .thenAnswer((_) => Stream.fromIterable([1, 2, 3]));

      expect(mockBloc, emitsInOrder([1, 2, 3]));
    });
  });
}

void whenListen<Event, State>(
  Bloc<Event, State> bloc,
  Stream<State> stream,
) {
  when(bloc.listen(any,
          onError: anyNamed('onError'),
          onDone: anyNamed('onDone'),
          cancelOnError: anyNamed('cancelOnError')))
      .thenAnswer((Invocation invocation) {
    return stream.listen(
      invocation.positionalArguments.first,
      onError: invocation.namedArguments['onError'],
      onDone: invocation.namedArguments['onDone'],
      cancelOnError: invocation.namedArguments['cancelOnError'],
    );
  });
}