23  lib Structure

Flutter production structure with inline explanations:

23.1 Recommended Production Structure

lib/
├── main.dart                           # App entry point - initializes dependency injection and runs app
├── app/
│   ├── app.dart                       # Main app widget with MaterialApp configuration
│   ├── routes/                        # Navigation routing - defines all app routes centrally
│   └── theme/                         # App-wide styling - colors, fonts, component themes
├── core/
│   ├── constants/                     # App constants - API URLs, string literals, magic numbers
│   ├── errors/                        # Error handling - custom exceptions, error models
│   ├── utils/                         # Utility functions - date formatters, validators, helpers
│   └── extensions/                    # Dart extensions - add methods to existing classes
├── features/                          # Feature-based organization - each major app function
│   ├── authentication/                # Example: Login, signup, password reset
│   │   ├── data/                     # External data sources - Firebase Auth, API calls
│   │   │   ├── datasources/          # Raw data access - REST clients, local storage
│   │   │   ├── models/               # Data transfer objects - JSON serialization
│   │   │   └── repositories/         # Data coordination - combines multiple datasources
│   │   ├── domain/                   # Business logic - pure Dart, no Flutter dependencies
│   │   │   ├── entities/             # Core business objects - User, Session models
│   │   │   ├── repositories/         # Abstract contracts - interfaces for data layer
│   │   │   └── usecases/             # Business rules - LoginUser, ValidateCredentials
│   │   └── presentation/             # UI components - everything user sees and interacts with
│   │       ├── pages/                # Full screens - LoginPage, SignupPage
│   │       ├── widgets/              # Reusable UI components - LoginForm, PasswordField
│   │       └── providers/            # State management - Riverpod, BLoC, or Provider
│   ├── dashboard/                    # Example: Main app dashboard
│   │   ├── data/                     # Patient data APIs, imaging service calls
│   │   ├── domain/                   # Dashboard business logic, data aggregation rules
│   │   └── presentation/             # Dashboard UI, charts, summary widgets
│   └── profile/                      # Example: User profile management
│       ├── data/                     # User data CRUD operations
│       ├── domain/                   # Profile validation, update rules
│       └── presentation/             # Profile forms, avatar widgets
├── shared/                           # Cross-feature components - used by multiple features
│   ├── widgets/                      # Common UI elements - buttons, dialogs, loading indicators
│   ├── services/                     # Global services - network client, local storage, analytics
│   └── models/                       # Shared data models - Response wrapper, pagination
└── config/                           # App configuration - environment-specific settings
    ├── environment/                  # Environment variables - dev, staging, production URLs
    └── dependency_injection/         # Service locator - registers all app dependencies

23.2 Architecture Flow Explanation

User Interaction
       ↓
Presentation Layer (UI)
       ↓
Domain Layer (Business Logic)
       ↓
Data Layer (External Sources)
       ↓
Backend Services (Firebase, APIs)

Data Flow: User taps button → Presentation calls UseCase → UseCase calls Repository → Repository calls DataSource → DataSource hits Firebase/API → Response flows back up the chain → UI updates

23.3 Key Benefits for Your Background

Python Similarity: The domain layer works like pure Python modules - no framework dependencies, just business logic. You can test these independently, similar to testing Python functions.

JavaScript Comparison: The presentation layer manages state like React components, but with Flutter’s reactive widget system. State flows down, events flow up.

Separation of Concerns: Each layer has one job, making debugging easier. If there’s a UI bug, check presentation. If business logic fails, check domain. If data doesn’t load, check data layer.

Testing Strategy: You can unit test domain logic without UI, integration test data layer without business logic, and widget test presentation without backend calls.

This structure scales well as your app grows and makes it easy to add new features like “medical_imaging” or “patient_reports” following the same three-layer pattern within the features directory.