Lag: domæne, data og præsentation

// Domain/UseCases/FetchUserUseCase.swift
struct FetchUserUseCase {
  let repo: UserRepository
  func execute(id: Int) async throws -> User {
    try await repo.getUser(id: id)
  }
}

// Domain/Repositories/UserRepository.swift
protocol UserRepository {
  func getUser(id: Int) async throws -> User
  func saveUser(_ user: User) async throws
}

// Data/Repositories/RemoteUserRepository.swift
struct RemoteUserRepository: UserRepository {
  func getUser(id: Int) async throws -> User {
    let data = try await URLSession.shared.data(from: userURL(id)).0
    return try JSONDecoder().decode(User.self, from: data)
  }
  func saveUser(_ user: User) async throws { /* PUT request */ }
}

struct UserDTO: Decodable {
  var user_id: Int
  var full_name: String
  func toDomain() -> User { User(id: user_id, name: full_name, email: "") }
}

// Presentation/ViewModels/ProfileViewModel.swift
@MainActor
final class ProfileViewModel: ObservableObject {
  @Published var user: User?
  private let useCase: FetchUserUseCase
  init(useCase: FetchUserUseCase) { self.useCase = useCase }
  func load(id: Int) async {
    user = try? await useCase.execute(id: id)
  }
}

// Outer → Inner dependency rule:
Presentation → Domain ← Data
// Domain is the stable core; outer layers are swappable

// MyApp/
// ├── Domain/
// │   ├── Entities/
// │   ├── UseCases/
// │   └── Repositories/  (protocols)
// ├── Data/
// │   └── Repositories/  (implementations)
// └── Presentation/
//     ├── Views/
//     └── ViewModels/

// Test Domain in isolation: no network, no DB
// Swap RemoteUserRepo with LocalUserRepo for offline mode
// Teams own layers independently

// WRONG: Domain entity with SwiftUI Color
struct Product {
  var color: Color  // UIKit/SwiftUI import in Domain!
}
// CORRECT: Use a plain string or enum
struct Product { var colorHex: String }