Flutter’s recommended architecture: why not BLoC?

Recently, Flutter team has published a architecure guide and suggested using the MVVM approach to organize the layers of your Flutter app, following an established pattern adopted in other mobile frameworks like native Kotlin and Swift. However, when we discuss app architecture and state management options, we can’t overlook the strengths of the BLoC approach. Why didn’t the Flutter team suggest using it?

Understanding the Basics

BLoC stands for Business Logic Component. It follows a reactive programming paradigm using Streams to manage state and events. The primary goal of BLoC is to enforce a unidirectional data flow that keeps the business logic entirely separate from the UI.

Key components of BLoC:

1. Events: User actions or external triggers.
2. States: Represent the UI’s current state.
3. Blocs: Map events to states using Streams.

BLoC’s unidirectional data flow (from https://bloclibrary.dev/architecture)

On the other hand, the MVVM (Model-View-ViewModel) pattern offers a simpler way to manage state. Here, the ViewModel acts as a middleman between the UI (View) and the data (Model). It notifies the UI of changes using the notifyListeners() method.

MVVM layered architecture (from https://docs.flutter.dev/app-architecture/concepts)

Did you notice any similarities between them? 😅 Both aim to manage state efficiently and update the UI reactively while both integrate seamlessly with Flutter widgets and both support async operations, though BLoC’s Streams are more naturally suited for complex workflows.

Key components of MVVM:

1. Model: represents the app’s data.
2. ViewModel: manages state and business logic.
3. View: displays the UI and observes changes.

Components of the MVVM approach (from https://docs.flutter.dev/app-architecture/guide)

Considering they’re mostly based on the same idea of separating concerns into a well-defined layered architecture, let’s focus on the differences between them.

Key Differences

1. Data Flow

• BLoC: enforces a unidirectional data flow. Events trigger state changes, and states are emitted to the UI.
• MVVM: uses bi-directional data flow, where state updates and UI interaction can occur more freely.

2. Boilerplate

• BLoC: requires more boilerplate, including the definition of Events, States, and Bloc classes.
• MVVM: minimal boilerplate with direct updates via notifyListeners() or ValueNotifier.value.

3. Performance

• BLoC: streams are highly efficient for handling complex, asynchronous operations.
• MVVM: lightweight and fast but can struggle in larger, more complex applications.

4. Learning Curve

• BLoC: steeper learning curve due to reactive programming concepts like Streams and StreamControllers.
• MVVM: easier to learn, especially for beginners.

5. Testability

• BLoC: offers excellent testability, allowing you to test state transitions independently.
• MVVM: testability is moderate and requires effort to decouple logic from UI.

6. Scalability

• BLoC: ideal for large, complex apps with shared state.
• MVVM: best for smaller or medium-sized apps due to potential tight coupling in larger apps.

Wrapping Up

Both BLoC and MVVM with ChangeNotifier or ValueNotifier enable cleaner, more maintainable Flutter code. The right choice depends on your project’s size, complexity, and the level of structure you prefer.

When to Choose BLoC

Choose BLoC if you’re building a large, complex application and want the peace of mind that comes with strict architectural rules. BLoC’s event-to-state pattern and immutability make it easier to test, debug, and keep track of your data flow. If your team values predictability and a well-defined approach that scales well, BLoC is a solid choice.

When to Choose MVVM with Notifiers

If you’re looking for something quick to implement and easy to understand, MVVM with might be your style. It’s great for smaller or medium-sized projects or when you’re just starting out and don’t need the full complexity of BLoC. You’ll spend less time setting things up and more time iterating rapidly, though you’ll need discipline to avoid creating chaos over time.

By understanding the nuances of each pattern, you can choose the one that best fits your team’s needs and your app’s complexity. Whichever you choose, you’ll be taking a big step toward writing cleaner, more efficient Flutter code.

Which approach do you prefer for your Flutter projects? Let me know in the comments!