Clean Architecture for Swift/SwiftUI

Project Structure

root/
├── domain/                          # Pure Swift, no UI dependencies
│   ├── {BoundedContext}/            # e.g., Session/, Group/, Auth/
│   │   ├── Entities/
│   │   ├── UseCases/
│   │   ├── Ports/                   # Shared abstractions
│   │   └── Errors/
│   ├── Adapters/                    # Shared implementations
│   ├── DI/
│   ├── Constants/
│   └── Utilities/
├── {app}/                           # e.g., retime/, widget/
│   ├── Views/
│   ├── ViewModels/
│   ├── Ports/                       # UI-specific abstractions
│   ├── Adapters/                    # UI-specific implementations (RevenueCat, etc.)
│   └── {App}App.swift
├── {extension}/                     # Same structure as app
└── Secrets/

Bounded Context Structure

Each bounded context is self-contained:

domain/Session/
├── Entities/
│   └── Session.swift
├── UseCases/
│   ├── StartSessionUseCase.swift
│   └── StopSessionUseCase.swift
├── Ports/
│   └── SessionRepositoryProtocol.swift
└── Errors/
    └── SessionError.swift

Layer Rules

Domain Layer (Pure Swift)

• NO SwiftUI, UIKit, or any UI framework imports
• NO concrete infrastructure (UserDefaults, Network, etc.)
• ONLY Foundation and pure Swift
• Contains: Entities, UseCases, Ports (protocols), Errors

App/Extension Layer (SwiftUI)

• Imports domain module
• Contains: Views, ViewModels, UI-specific Ports/Adapters
• Adapters implement domain Ports

Dependency Direction

Views → ViewModels → UseCases → Ports ← Adapters
                                  ↑
                            (protocols)

Ports & Adapters Pattern

Port (Protocol in domain/)

// domain/Session/Ports/SessionRepositoryProtocol.swift
protocol SessionRepositoryProtocol {
    func save(_ session: Session) async throws
    func getCurrent() async throws -> Session?
    func delete(_ id: String) async throws
}

Adapter (Implementation)

// domain/Adapters/UserDefaultsSessionRepository.swift (shared)
// OR
// retime/Adapters/RevenueCatPaymentAdapter.swift (UI-specific)

class UserDefaultsSessionRepository: SessionRepositoryProtocol {
    private let monitor: MonitorProtocol
    
    init(monitor: MonitorProtocol) {
        self.monitor = monitor
    }
    
    func save(_ session: Session) async throws {
        // Implementation
    }
}

Placement Rules

Adapter Type	Location	Example
Shared across apps/extensions	domain/Adapters/	UserDefaultsRepository, DeviceActivityManager
UI-specific	{app}/Adapters/	RevenueCatAdapter, StoreKitAdapter

Error Handling

When to Use throws

• Single operation with clear success/failure
• Errors propagate up naturally
• Caller handles error immediately

func startSession(config: SessionConfig) async throws -> Session {
    guard config.isValid else {
        throw SessionError.invalidConfiguration
    }
    // ...
}

When to Use Result<T, E>

• Multiple possible outcomes beyond success/failure
• Caller needs to pattern match on specific cases
• Chaining operations with different error types

func validateAndStart(config: SessionConfig) -> Result<Session, SessionValidationError> {
    switch validate(config) {
    case .valid:
        return .success(createSession(config))
    case .invalidDuration(let reason):
        return .failure(.invalidDuration(reason))
    case .conflictingSession(let existing):
        return .failure(.conflict(existing))
    }
}

Error Definition

// domain/Session/Errors/SessionError.swift
enum SessionError: Error, LocalizedError {
    case notFound
    case alreadyActive
    case invalidConfiguration
    case storageFailed(underlying: Error)
    
    var errorDescription: String? {
        switch self {
        case .notFound: return "Session not found"
        case .alreadyActive: return "A session is already active"
        case .invalidConfiguration: return "Invalid session configuration"
        case .storageFailed(let error): return "Storage failed: \(error.localizedDescription)"
        }
    }
}

Dependency Injection

Container Registration

// domain/DI/ServiceRegistration.swift
extension DIContainer {
    public func configureServices() {
        // Infrastructure
        register(MonitorProtocol.self, implementation: AppleLogMonitor())
        register(TrackerProtocol.self, implementation: AppleLogTracker())
        
        // Repositories (resolve dependencies)
        let monitor = try! resolve(MonitorProtocol.self)
        register(
            SessionRepositoryProtocol.self,
            implementation: UserDefaultsSessionRepository(monitor: monitor)
        )
    }
}

Two Composition Roots

1. App Composition Root ({App}App.swift)

@main
struct retimeApp: App {
    @StateObject var sessionViewModel: SessionViewModel
    
    init() {
        // Configure DI
        let container = DIContainer.shared
        container.configureServices()
        
        // Resolve Ports
        let sessionRepository = try! container.resolve(SessionRepositoryProtocol.self)
        let shieldManager = try! container.resolve(ShieldManagerProtocol.self)
        let monitor = try! container.resolve(MonitorProtocol.self)
        
        // Inject Ports into ViewModels
        _sessionViewModel = StateObject(
            wrappedValue: SessionViewModel(
                sessionRepository: sessionRepository,
                shieldManager: shieldManager,
                monitor: monitor
            )
        )
    }
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(sessionViewModel)
        }
    }
}

2. Extension Composition Root ({Extension}.swift)

Extensions cannot use DIContainer (different process). Instantiate dependencies manually:

class DeviceActivityMonitorExtension: DeviceActivityMonitor {
    private let startSession: StartSessionUseCase
    private let stopSession: StopSessionUseCase
    
    override init() {
        // Manual instantiation (no DIContainer in extensions)
        let monitor = AppleLogMonitor()
        let tracker = AppleLogTracker()
        let sessionRepository = UserDefaultsSessionRepository(monitor: monitor, tracker: tracker)
        let shieldManager = ShieldManager(monitor: monitor, tracker: tracker)
        
        // Create UseCases
        self.startSession = StartSessionUseCase(
            sessionRepository: sessionRepository,
            shieldManager: shieldManager,
            monitor: monitor
        )
        self.stopSession = StopSessionUseCase(
            sessionRepository: sessionRepository,
            shieldManager: shieldManager,
            monitor: monitor
        )
        
        super.init()
    }
    
    override func intervalDidStart(for activity: DeviceActivityName) {
        super.intervalDidStart(for: activity)
        Task {
            _ = await startSession.execute(sessionId: extractSessionId(from: activity))
        }
    }
}

ViewModel Pattern

When to Use UseCase vs Port Direct

Situation	Use	Example
Business logic, orchestration, validation	UseCase	startSession.execute()
Simple read/write, no transformation	Port direct	repository.getAll()

Standard Pattern

ViewModels receive Ports via init, create UseCases internally when needed:

class SessionViewModel: ObservableObject {
    // State
    @Published var session: Session?
    @Published var isLoading = false
    @Published var error: SessionError?
    
    // Ports (injected)
    private let sessionRepository: SessionRepositoryProtocol
    private let shieldManager: ShieldManagerProtocol
    private let monitor: MonitorProtocol
    
    // UseCases (created internally)
    private let startSession: StartSessionUseCase
    private let stopSession: StopSessionUseCase
    
    init(
        sessionRepository: SessionRepositoryProtocol,
        shieldManager: ShieldManagerProtocol,
        monitor: MonitorProtocol
    ) {
        self.sessionRepository = sessionRepository
        self.shieldManager = shieldManager
        self.monitor = monitor
        
        // Create UseCases with injected Ports
        self.startSession = StartSessionUseCase(
            sessionRepository: sessionRepository,
            shieldManager: shieldManager,
            monitor: monitor
        )
        self.stopSession = StopSessionUseCase(
            sessionRepository: sessionRepository,
            shieldManager: shieldManager,
            monitor: monitor
        )
    }
    
    // Action using UseCase (business logic)
    func start(config: SessionConfig) {
        isLoading = true
        Task { @MainActor in
            do {
                session = try await startSession.execute(config: config)
            } catch let err as SessionError {
                error = err
            }
            isLoading = false
        }
    }
    
    // Action using Port direct (simple read)
    func loadCurrent() {
        Task { @MainActor in
            session = try? await sessionRepository.getCurrent()
        }
    }
}

Complex State Pattern (Forms)

class SessionFormViewModel: ObservableObject {
    struct State {
        var duration: TimeInterval = 0
        var selectedApps: Set<AppIdentifier> = []
        var breakInterval: TimeInterval?
        var isValid: Bool { duration > 0 && !selectedApps.isEmpty }
    }
    
    @Published var state = State()
}

Workflows

Create Feature

See references/file-templates.md for all templates.

1. Create Entity in domain/{Context}/Entities/
2. Create Error in domain/{Context}/Errors/
3. Create Port (if external dependency) in domain/{Context}/Ports/
4. Create Adapter in domain/Adapters/ or {app}/Adapters/
5. Register in DI in domain/DI/ServiceRegistration.swift
6. Create UseCase in domain/{Context}/UseCases/
7. Create ViewModel in {app}/ViewModels/ (receives Ports, creates UseCases)
8. Wire ViewModel in {App}App.swift composition root
9. Create View in {app}/Views/

Create UseCase

1. Identify bounded context
2. Define input/output types
3. Identify required Ports
4. Implement UseCase class
5. Wire in ViewModel

// domain/Session/UseCases/PauseSessionUseCase.swift
class PauseSessionUseCase {
    private let repository: SessionRepositoryProtocol
    private let shieldManager: ShieldManagerProtocol
    
    init(repository: SessionRepositoryProtocol, shieldManager: ShieldManagerProtocol) {
        self.repository = repository
        self.shieldManager = shieldManager
    }
    
    func execute(sessionId: String) async throws {
        guard var session = try await repository.getCurrent() else {
            throw SessionError.notFound
        }
        session.pause()
        try await repository.save(session)
        try await shieldManager.temporarilyDisable()
    }
}

Refactor to Bounded Context

1. Identify context boundaries - Group related entities, use cases, errors
2. Create context directory structure under domain/
3. Move entities first (no dependencies)
4. Move errors associated with entities
5. Move ports that serve this context
6. Move use cases last
7. Update imports across the codebase
8. Verify compilation after each move

Before

domain/
├── Entities/
│   ├── Session.swift
│   ├── Group.swift
│   └── User.swift
├── UseCases/
│   ├── StartSessionUseCase.swift
│   └── CreateGroupUseCase.swift

After

domain/
├── Session/
│   ├── Entities/Session.swift
│   ├── UseCases/StartSessionUseCase.swift
│   ├── Ports/SessionRepositoryProtocol.swift
│   └── Errors/SessionError.swift
├── Group/
│   ├── Entities/Group.swift
│   ├── UseCases/CreateGroupUseCase.swift
│   └── Errors/GroupError.swift

Code Review

See references/code-review-checklist.md for complete checklist.

File Templates

See references/file-templates.md for copy-paste templates.