Clean Architecture — Fundamentals, Layers of Entities, Use Cases, and Gateways

// Entity — business object (Domain)
struct User: Equatable {
    let id: Int
    let name: String
    let email: String
}

// Repository Interface — data abstraction (Domain)
protocol UserRepository {
    func getUser(id: Int) async throws -> User
    func getUsers() async throws -> [User]
}

// Use Case — one scenario (Domain)
final class GetUserUseCase {
    private let repository: UserRepository

    init(repository: UserRepository) {
        self.repository = repository
    }

    func execute(id: Int) async throws -> User {
        return try await repository.getUser(id: id)
    }
}

// DTO — model for network (Data)
data class UserDto(
    @SerializedName("id") val id: Int,
    @SerializedName("first_name") val firstName: String,
    @SerializedName("last_name") val lastName: String,
    @SerializedName("email") val email: String
)

// RepositoryImpl — implementation (Data)
class UserRepositoryImpl(
    private val remoteDataSource: UserRemoteDataSource,
    private val localDataSource: UserLocalDataSource
) : UserRepository {

    override suspend fun getUser(id: Int): User {
        // Trying to get from cache
        localDataSource.getUser(id)?.let { return it.toDomain() }
        // If not — load from network
        val dto = remoteDataSource.fetchUser(id)
        val user = dto.toDomain()
        localDataSource.saveUser(user)
        return user
    }

    override suspend fun getUsers(): List<User> {
        return remoteDataSource.fetchAllUsers().map { it.toDomain() }
    }
}

// Mapper — DTO ↔ Domain conversion
fun UserDto.toDomain() = User(
    id = id,
    name = "$firstName $lastName",
    email = email
)

// Use Case (Domain) — pure business logic
class GetUserUseCase(
    private val repository: UserRepository
) {
    suspend operator fun invoke(id: Int): User {
        return repository.getUser(id)
    }
}

// ViewModel (Presentation) — only presentation
class UserViewModel(
    private val getUserUseCase: GetUserUseCase
) : ViewModel() {

    private val _state = MutableStateFlow<UserScreenState>(UserScreenState.Loading)
    val state: StateFlow<UserScreenState> = _state.asStateFlow()

    fun loadUser(id: Int) {
        viewModelScope.launch {
            _state.value = UserScreenState.Loading
            val user = getUserUseCase(id)
            val displayItem = UserDisplayItem(
                name = user.name,
                email = user.email,
                initials = user.name.split(" ").joinToString("") { it.first().toString() }
            )
            _state.value = UserScreenState.Success(displayItem)
        }
    }
}

data class UserDisplayItem(
    val name: String,
    val email: String,
    val initials: String
)

sealed interface UserScreenState {
    data object Loading : UserScreenState
    data class Success(val displayItem: UserDisplayItem) : UserScreenState
    data class Error(val message: String) : UserScreenState
}

// Data Layer: Remote DataSource (iOS)
final class UserRemoteDataSource {
    private let apiClient: APIClient

    func fetchUser(id: Int) async throws -> UserDTO {
        return try await apiClient.get("/users/\(id)")
    }
}

// Repository Implementation (Data)
final class UserRepositoryImpl: UserRepository {
    private let remote: UserRemoteDataSource
    private let local: UserLocalDataSource

    func getUser(id: Int) async throws -> User {
        if let cached = try await local.getUser(id) {
            return cached
        }
        let dto = try await remote.fetchUser(id)
        let user = dto.toDomain()
        try await local.saveUser(user)
        return user
    }
}

// Presentation: ViewModel + SwiftUI View
@MainActor
final class UserViewModel: ObservableObject {
    @Published private(set) var state: UserScreenState = .loading
    private let getUserUseCase: GetUserUseCase

    func loadUser(id: Int) {
        Task {
            state = .loading
            if let user = try? await getUserUseCase.execute(id: id) {
                state = .success(user)
            } else {
                state = .error("Failed to load")
            }
        }
    }
}