Architecture Expert

Layer Responsibilities

Presentation Layer

What it does:

• Displays UI
• Manages user interactions
• Holds screen-specific state
• Reacts to state changes

What it CANNOT do:

• Call repositories directly
• Contain business logic
• Make API/database calls
• Transform DTOs to domain models

Dependencies: Application layer only (use cases)

Application Layer

What it does:

• Implements use cases (business operations)
• Orchestrates data flow
• Calls repository interfaces
• Transforms domain models for presentation

What it CANNOT do:

• Know about UI
• Know about database/DTOs
• Call data sources directly

Dependencies: Domain layer only (models, repository interfaces)

Domain Layer

What it does:

• Defines core business entities (models)
• Defines repository contracts (interfaces)
• Contains business rules and validation
• Platform and framework agnostic

What it CANNOT do:

• Know about database/DTOs
• Know about UI
• Contain implementation details

Dependencies: None (pure code)

Data Layer

What it does:

• Implements repository interfaces from domain
• Communicates with database/external APIs
• Transforms DTOs ↔ domain models
• Handles caching and error recovery

What it CANNOT do:

• Contain business logic
• Be called directly by presentation layer

Dependencies: Domain layer (implements interfaces, uses models)

Data Flow

The complete data flow through all layers:

User Action (UI)
  ↓
Presentation Layer (Component/Provider)
  ↓
Application Layer (Use Case)
  ↓
Domain Layer (Repository Interface)
  ↓
Data Layer (Repository Implementation)
  ↓
Database / Backend
  ↓
Data Layer (DTO → Domain Model)
  ↓
Application Layer (Business Logic)
  ↓
Presentation Layer (Display)

State Management

Provider/State Types

Reference your framework's state management patterns here.

State Guidelines

• One state object per screen
• Immutable state
• Async data handled properly

Feature Module Structure

When creating a new feature (e.g., profile):

mkdir -p lib/features/profile/{presentation,application,domain,data}
mkdir -p lib/features/profile/presentation/{pages,widgets,providers}
mkdir -p lib/features/profile/application/{use_cases,services}
mkdir -p lib/features/profile/domain/{models,repositories}
mkdir -p lib/features/profile/data/{dto,data_sources,repository_impl}

Testing Strategy per Layer

Reference: docs/07_app_architecture.md testing section

Unit Tests

What to test:

• Domain models (business logic methods)
• Use cases (application layer)
• DTO transformations
• Repository implementations (mocked database)

Widget/Component Tests

What to test:

• Screen rendering
• User interactions
• State transitions
• Error states

Integration Tests

What to test:

• Database read flows (with emulator if available)
• Navigation paths
• End-to-end user journeys

Core Module (Shared Code)

Shared utilities live in lib/core/:

lib/core/
├── config/          # Configuration
├── constants/       # App-wide constants
├── domain/models/   # Shared domain models
├── error/           # Error types and handlers
├── logging/         # Logging utilities
├── network/         # Network utilities
├── services/        # Shared services
├── theme/           # Theming
├── utils/           # Helper functions
└── widgets/         # Shared widgets

Critical Architectural Rules

1. Never Skip Layers

Wrong: Presentation → Data Correct: Presentation → Application → Domain → Data

2. Dependencies Point Inward

• Presentation depends on Application
• Application depends on Domain
• Data depends on Domain (implements interfaces)
• Domain depends on nothing

3. DTOs Stay in Data Layer

Wrong: Pass DTOs to use cases or presentation Correct: Transform DTOs to domain models in data layer

4. Business Logic in Use Cases

Wrong: Business logic in widgets or repositories Correct: Business logic in application layer use cases

5. State Management for All State

Wrong: Local component state for shared data Correct: Proper state management providers

Platform Considerations

• Responsive layouts
• Test on all target platforms
• Avoid platform-specific code where possible
• Optimize for performance

Performance Best Practices

• Enable database persistence (offline cache)
• Avoid rebuilding expensive components
• Cache computed data in memory
• Use pagination for large lists

When to Use This Skill

Use this skill when:

• Creating a new feature module
• Understanding layer responsibilities
• Setting up state providers
• Writing tests for specific layers
• Refactoring code to follow architecture
• Reviewing code for architectural compliance

Quick Reference Checklist

When implementing a feature:

• Create four-layer directory structure
• Define domain models (pure code, no dependencies)
• Define repository interface in domain layer
• Implement repository in data layer with DTOs
• Create use case in application layer
• Set up state providers in presentation layer
• Build UI components consuming providers
• Write unit tests for use cases and models
• Write component tests for screens
• Verify no layers are skipped in data flow

Summary

GigLedger's architecture ensures:

• Clean separation of concerns
• Testability at every layer
• Scalability for future features
• Framework independence in domain layer
• Consistent patterns across all features

Remember: Always reference docs/07_app_architecture.md as the ultimate source of truth for architectural decisions.