What Is the BLoC Pattern? Separating UI and Logic in Flutter
The one-way Event → BLoC → State flow
BLoC (Business Logic Component) is a Flutter architecture pattern proposed by Google.
When UI generates an Event, BLoC processes the business logic and emits a new State. UI subscribes only to State to update the screen.
The flutter_bloc package is most widely used, offering two approaches: Cubit (simplified) and Bloc (event-based).
Implementation Steps
1
Start with Cubit: Define State → Write Cubit class → Connect UI with BlocBuilder
2
Switch to Bloc: Define Event class → Write on<Event>() handlers
3
Inject with BlocProvider, provide multiple Blocs with MultiBlocProvider
4
Unit test business logic with bloc_test package
Pros
- ✓ 100% unit testable business logic
- ✓ Easy debugging with event logs
Cons
- ✗ Lots of boilerplate code (Event, State, Bloc each needed)
- ✗ Excessive structure for small apps
Use Cases
Role separation in large team projects
Complex form validation logic separation