Architecture Feature First

Flutter Architecture — Feature-First Skill

This skill defines how to design, structure, and implement Flutter applications using the recommended layered architecture with feature-first file organization.

It is state management agnostic: the business logic holder in the UI layer may be named ViewModel, Controller, Cubit, Bloc, Provider, or Notifier — depending on the chosen state management approach. The architectural rules apply equally to all of them.

When to Use

Use this skill when:

• Designing the folder/file structure of a new Flutter app or feature.
• Creating a new View, ViewModel, Repository, or Service.
• Deciding which layer owns a piece of logic.
• Wiring dependency injection between components.
• Adding a domain (logic) layer for complex business logic.

1. Layers

Separate every app into a UI Layer and a Data Layer. Add a Logic (Domain) Layer between them only for complex apps.

┌──────────────────────────────────────────────────────────────┐
│   UI Layer    │  Views + business logic holders              │
│               │  (ViewModel / Cubit / Controller / Provider) │
├──────────────────────────────────────────────────────────────┤
│  Logic Layer  │  Use Cases / Interactors  (optional)         │
├──────────────────────────────────────────────────────────────┤
│   Data Layer  │  Repositories + Services                     │
└──────────────────────────────────────────────────────────────┘

Rules:

• Only adjacent layers may communicate. The UI layer must never access a Service directly.
• The Logic layer is added only when business logic is too complex for the business logic holder or is reused across multiple screens.
• Data changes always happen in the Data layer (SSOT = Repository). No mutation in UI or Logic layers.
• Follow unidirectional data flow: state flows down (Data → UI), events flow up (UI → Data).

2. Feature-First File Structure

Organize code by feature, not by type. Group all layers belonging to one feature together in a single directory.

Each feature directory contains the files needed for that feature, named according to the chosen state management approach:

Example: an auth feature would have files like auth_viewmodel.dart (or auth_cubit.dart, auth_provider.dart depending on approach), login_screen.dart, and optionally login_usecase.dart.

3. Component Responsibilities

View

• Describes how to present data to the user; keep logic minimal and only UI-related.
• Passes events to the business logic holder in response to user interactions.
• Keep views focused on presentation; extract reusable widgets into separate components.
• Use StatelessWidget when possible; keep build methods simple.

Business Logic Holder (ViewModel / Cubit / Controller / Provider)

• Contains logic to convert app data into UI state and maintains current state needed by the view.
• Exposes callbacks (commands) to the View and retrieves/transforms data from repositories.
• Business logic must not live in UI widgets; only interact with repositories via the business logic holder.

Repository

• Single Source of Truth (SSOT) for a given type of model data.
• The only class allowed to mutate its data; all other classes read from it.
• Handles business logic such as caching, error handling, and refreshing data.
• Transforms raw data from services into domain models consumed by business logic holders.

Service

• Wraps API endpoints and exposes asynchronous response objects.
• Isolates data-loading and holds no state.

4. Domain Layer (Use Cases)

Introduce use cases/interactors only when:

• Logic is complex or does not fit cleanly in the UI or Data layers.
• Logic is reused across multiple business logic holders or merges data from multiple repositories.

Do not add a domain layer for simple CRUD apps.

5. Dependency Injection

Use dependency injection to provide components with their dependencies, enabling testability and flexibility.

• Supply repositories to business logic holders via constructors.
• Supply services to repositories via constructors.
• Design all components with well-defined interfaces so implementations can be swapped without changing consumers.

References

• Flutter app architecture guide
• Architecture case study (Compass app)
• Architecture recommendations