Layered Architecture

Layered Architecture

Layered monorepo architecture for Flutter apps — four layers organized as independent Dart packages with strict unidirectional dependencies.

Core Standards

Apply these standards to ALL layered architecture work:

• Four layers — Data, Repository, Business Logic, Presentation — every feature spans exactly these four layers
• Unidirectional dependencies — Presentation → Business Logic → Repository → Data — never skip or invert a layer
• Data and Repository layers live in packages/ — each is an independent Dart package with its own pubspec.yaml
• Business Logic and Presentation live in lib/ — organized by feature within the app
• Data layer packages contain zero domain/business logic — they must be reusable in unrelated projects
• No inter-repository dependencies — repositories never import other repositories
• No Flutter SDK in data or repository packages — scaffold with the very_good_cli MCP server create dart_package tool
• One repository per domain — user_repository, weather_repository, auth_repository
• Path dependencies for local packages — never git: or pub version references for packages in the same repo
• Barrel exports at every package boundary — src/ is never imported directly by consumers
• Repositories accept data layer dependencies via constructor injection — never instantiate clients internally
• App bootstrap wires all layers — main_<flavor>.dart creates clients and repositories, provides them via RepositoryProvider

Cross-harness fallback. This skill scaffolds and tests packages via the Very Good CLI MCP server. On a host without this plugin's Bash hooks and without that MCP server connected, run the equivalent very_good create dart_package …, very_good packages get, and very_good test commands directly.

Architecture Overview

┌─────────────────────────────────────────────┐
│              Presentation                   │
│          (lib/<feature>/view/)              │
└──────────────────┬──────────────────────────┘
                   │ reads state / dispatches events
┌──────────────────▼──────────────────────────┐
│            Business Logic                   │
│        (lib/<feature>/bloc/)                │
└──────────────────┬──────────────────────────┘
                   │ calls repository methods
┌──────────────────▼──────────────────────────┐
│              Repository                     │
│      (packages/<name>_repository/)          │
└──────────────────┬──────────────────────────┘
                   │ calls data clients
┌──────────────────▼──────────────────────────┐
│               Data                          │
│     (packages/<name>_api_client/)           │
└─────────────────────────────────────────────┘

Monorepo Structure

my_app/
├── lib/
│   ├── app/
│   │   ├── app.dart                          # Barrel file
│   │   └── view/
│   │       └── app.dart                      # App widget with MultiRepositoryProvider
│   ├── login/                                # Feature: login
│   │   ├── login.dart                        # Barrel file
│   │   ├── bloc/
│   │   │   ├── login_bloc.dart
│   │   │   ├── login_event.dart
│   │   │   └── login_state.dart
│   │   └── view/
│   │       ├── login_page.dart               # Page provides Bloc
│   │       └── login_view.dart               # View consumes state
│   ├── profile/                              # Feature: profile
│   │   ├── profile.dart
│   │   ├── cubit/
│   │   │   ├── profile_cubit.dart
│   │   │   └── profile_state.dart
│   │   └── view/
│   │       ├── profile_page.dart
│   │       └── profile_view.dart
│   ├── main_development.dart                 # Flavor entrypoint
│   ├── main_staging.dart
│   └── main_production.dart
├── packages/
│   ├── auth_api_client/                      # Data layer: auth API
│   │   ├── lib/
│   │   │   ├── auth_api_client.dart          # Barrel file
│   │   │   └── src/
│   │   │       ├── auth_api_client.dart
│   │   │       └── models/
│   │   │           ├── models.dart
│   │   │           └── auth_response.dart
│   │   └── pubspec.yaml
│   ├── local_storage_client/                 # Data layer: local storage
│   │   ├── lib/
│   │   │   ├── local_storage_client.dart
│   │   │   └── src/
│   │   │       └── local_storage_client.dart
│   │   └── pubspec.yaml
│   ├── auth_repository/                      # Repository layer: auth
│   │   ├── lib/
│   │   │   ├── auth_repository.dart          # Barrel file
│   │   │   └── src/
│   │   │       ├── auth_repository.dart
│   │   │       └── models/
│   │   │           ├── models.dart
│   │   │           └── user.dart             # Domain model
│   │   └── pubspec.yaml
│   └── user_repository/                      # Repository layer: user
│       ├── lib/
│       │   ├── user_repository.dart
│       │   └── src/
│       │       ├── user_repository.dart
│       │       └── models/
│       │           ├── models.dart
│       │           └── user_profile.dart
│       └── pubspec.yaml
├── test/
│   └── ...                                   # Mirrors lib/ structure
└── pubspec.yaml                              # Root app pubspec

Data Layer

The data layer handles all external communication. Each data package wraps a single external source (REST API, local database, platform plugin) and exposes typed methods and response models.

Rules:

• Models represent the external data shape — match the API/storage schema exactly
• No Flutter imports — use the very_good_cli MCP server create dart_package tool
• Constructor-inject HTTP clients for testability
• Response models use fromJson / toJson factories
• Export everything through a barrel file — never expose src/

Pattern: Data Client Class

Constructor-inject the HTTP client for testability. Return typed response models — never raw JSON.

/// HTTP client for the User API.
class UserApiClient {
  // http.Client injected — tests pass a mock, production gets a real client
  UserApiClient({
    required String baseUrl,
    http.Client? httpClient,
  })  : _baseUrl = baseUrl,
        _httpClient = httpClient ?? http.Client();

  final String _baseUrl;
  final http.Client _httpClient;

  /// Every method returns a typed response model.
  Future<UserResponse> getUser(String userId) async {
    final response = await _httpClient.get(
      Uri.parse('$_baseUrl/users/$userId'),
    );
    if (response.statusCode != 200) {
      throw UserApiException(response.statusCode, response.body);
    }
    return UserResponse.fromJson(
      json.decode(response.body) as Map<String, dynamic>,
    );
  }
}

See worked-example.md for the complete user_api_client package with pubspec, barrel files, response models, and exception class.

Repository Layer

The repository layer orchestrates data sources and exposes domain models. Each repository composes one or more data clients, transforms response models into domain models, and provides a clean API for the business logic layer.

Rules:

• No inter-repository dependencies — repositories are isolated
• No Flutter SDK — the very_good_cli MCP server create dart_package tool
• Domain models live in the repository package — not in data packages
• Transform data models into domain models — never leak API response shapes upstream
• Accept all data clients via constructor injection

Pattern: Domain Model + Repository Transformation

Domain models extend Equatable and represent the app's internal data shape — distinct from the API response shape. The repository method transforms between them.

/// Domain model — lives in the repository package, NOT the data package.
/// Fields match the app's needs, not the API schema.
class User extends Equatable {
  const User({
    required this.id,
    required this.email,
    required this.displayName,
    this.avatarUrl,
  });

  final String id;
  final String email;
  final String displayName;
  final String? avatarUrl;

  @override
  List<Object?> get props => [id, email, displayName, avatarUrl];
}

/// Repository accepts data client via constructor — never creates its own.
class UserRepository {
  const UserRepository({
    required UserApiClient userApiClient,
  }) : _userApiClient = userApiClient;

  final UserApiClient _userApiClient;

  /// Transforms UserResponse (API shape) → User (domain shape).
  Future<User> getUser(String userId) async {
    final response = await _userApiClient.getUser(userId);
    return User(
      id: response.id,
      email: response.email,
      displayName: response.displayName,
      avatarUrl: response.avatarUrl,
    );
  }
}

See worked-example.md for the complete user_repository package with pubspec, barrel files, and error handling. See model-transformation.md for detailed transformation patterns between data and domain models.

Dependency Graph

Each layer's pubspec.yaml enforces the architecture through path dependencies.

Data Package (packages/user_api_client/pubspec.yaml)

dependencies:
  # External packages only — no local dependencies
  http: ^1.4.0
  json_annotation: ^4.9.0

Repository Package (packages/user_repository/pubspec.yaml)

dependencies:
  equatable: ^2.0.7
  # Path dependency on data layer package
  user_api_client:
    path: ../user_api_client

Root App (pubspec.yaml)

dependencies:
  flutter:
    sdk: flutter
  flutter_bloc: ^9.1.0
  # Repository packages only — data packages are transitive
  auth_repository:
    path: packages/auth_repository
  user_repository:
    path: packages/user_repository

The app never depends on data packages directly. Data packages are transitive dependencies through repositories. This enforces the layer boundary — business logic and presentation cannot bypass the repository layer.

Data Flow

Step-by-step walkthrough: user taps "Load Profile" button.

1. Presentation dispatches event — context.read<ProfileBloc>().add(ProfileLoadRequested(userId: '123'))
2. Business Logic calls repository — Bloc handler invokes _userRepository.getUser(event.userId) and emits state based on the result
3. Repository calls data client — UserRepository.getUser delegates to _userApiClient.getUser and transforms the response into a domain User
4. Data layer communicates with external source — UserApiClient.getUser makes the HTTP request and returns a typed UserResponse
5. Data flows back up — Presentation rebuilds via BlocBuilder based on the new state

// lib/profile/bloc/profile_bloc.dart
Future<void> _onLoadRequested(
  ProfileLoadRequested event,
  Emitter<ProfileState> emit,
) async {
  emit(const ProfileState.loading());
  try {
    final user = await _userRepository.getUser(event.userId);
    emit(ProfileState.success(user: user));
  } on UserNotFoundException {
    emit(const ProfileState.notFound());
  } catch (_) {
    emit(const ProfileState.failure());
  }
}

See data-flow.md for the full data flow walkthrough with code at each layer.

App Bootstrap

The app's main_<flavor>.dart creates all data clients and repositories, then passes them to the App widget. MultiRepositoryProvider makes repositories available to the entire widget tree.

lib/main_development.dart

import 'package:flutter/material.dart';
import 'package:my_app/app/app.dart';
import 'package:auth_api_client/auth_api_client.dart';
import 'package:local_storage_client/local_storage_client.dart';
import 'package:auth_repository/auth_repository.dart';
import 'package:user_api_client/user_api_client.dart';
import 'package:user_repository/user_repository.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();

  const baseUrl = 'https://api.dev.example.com';

  // Data layer
  final authApiClient = AuthApiClient(baseUrl: baseUrl);
  final userApiClient = UserApiClient(baseUrl: baseUrl);
  final localStorageClient = LocalStorageClient();

  // Repository layer
  final authRepository = AuthRepository(
    authApiClient: authApiClient,
    localStorageClient: localStorageClient,
  );
  final userRepository = UserRepository(
    userApiClient: userApiClient,
  );

  runApp(
    App(
      authRepository: authRepository,
      userRepository: userRepository,
    ),
  );
}

Flavors change only the configuration (base URLs, API keys) — the architecture stays identical across development, staging, and production. See worked-example.md for the App widget with MultiRepositoryProvider.

Anti-Patterns

Common Workflows

Adding a New Data Source

1. Scaffold the package with the very_good_cli MCP server create dart_package tool: <name>_api_client --output-directory packages
2. Add external dependencies to pubspec.yaml (e.g., http, json_annotation)
3. Create response models in lib/src/models/ with fromJson/toJson
4. Create barrel file lib/src/models/models.dart exporting all models
5. Implement the client class in lib/src/<name>_api_client.dart
6. Create the package barrel file lib/<name>_api_client.dart exporting src/ contents
7. Write unit tests in test/ mirroring lib/ structure — see the testing skill
8. Use very_good_cli MCP server tool test against the package directory — pass directory: 'packages/<name>_api_client' to scope the run

Adding a New Repository

1. Scaffold the package with the very_good_cli MCP server create dart_package tool: <name>_repository --output-directory packages
2. Add path dependencies to data layer packages in pubspec.yaml
3. Add equatable to dependencies for domain models
4. Create domain models in lib/src/models/ extending Equatable
5. Create barrel file lib/src/models/models.dart
6. Implement the repository class with constructor-injected data clients
7. Add transformation logic from response models to domain models
8. Create the package barrel file lib/<name>_repository.dart
9. Write unit tests with mocked data clients — see the testing skill

Connecting a Repository to a Feature

1. Add path dependency on the repository package to root pubspec.yaml
2. Create the repository in main_<flavor>.dart and pass it to App
3. Add RepositoryProvider.value in App's MultiRepositoryProvider
4. Create the Bloc/Cubit with the repository injected — see the bloc skill
5. Build the Page/View with BlocProvider and BlocBuilder — see the bloc skill

Additional Resources

• Complete worked example and pubspec reference
• Model transformation patterns — data model vs domain model conversion
• Package-level testing — testing data clients and repositories in isolation
• For Bloc/Cubit patterns and Page/View separation — see the bloc skill
• For project scaffolding use the very_good_cli MCP server create dart_package tool
• For testing data clients, repositories, and Blocs — see the testing skill