Example using inversion of control with flutter bloc

flutter_bloc_ioc.dart

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

abstract class AbstractBloc<Event, State> implements Bloc<Event, State> {}

@immutable
class TestState {
  final int value;

  const TestState(this.value);
}

@immutable
abstract class TestEvent {}

class TestIncrement extends TestEvent {}

class TestBloc extends Bloc<TestEvent, TestState> implements AbstractBloc<TestEvent, TestState> {
  @override
  TestState get initialState => const TestState(0);

  @override
  Stream<TestState> mapEventToState(TestEvent event) async* {
    if (event is TestIncrement) {
      yield TestState(state.value + 1);
    }
  }
}

class TestScreen extends StatelessWidget {
  const TestScreen({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return BlocProvider<AbstractBloc<TestEvent, TestState>>(
      create: (context) => TestBloc(),
      child: Builder(
        builder: (context) {
          return Scaffold(
            appBar: AppBar(title: Text('Test Screen')),
            body: Center(
              child: Column(
                children: [
                  BlocBuilder<AbstractBloc<TestEvent, TestState>, TestState>(
                    builder: (context, state) => Text('Value ${state.value}'),
                  ),
                  TextButton(
                    child: Text('Increment'),
                    onPressed: () => context.bloc<AbstractBloc<TestEvent, TestState>>().add(TestIncrement()),
                  ),
                ],
              ),
            ),
          );
        },
      ),
    );
  }
}