android-clean-architecture

# Android Clean Architecture Clean Architecture patterns for Android and KMP projects. Covers module boundaries, dependency inversion, UseCase/Repository patterns, and data layer design with Room, SQLDelight, and Ktor. ## When to Activate - Structuring Android or KMP project modules - Implementing UseCases, Repositories, or DataSources - Designing data flow between layers (domain, data, presentation) - Setting up dependency injection with Koin or Hilt - Working with Room, SQLDelight, or Ktor in a layered architecture ## Module Structure ### Recommended Layout ``` project/ ├── app/ # Android entry point, DI wiring, Application class ├── core/ # Shared utilities, base classes, error types ├── domain/ # UseCases, domain models, repository interfaces (pure Kotlin) ├── data/ # Repository implementations, DataSources, DB, network ├── presentation/ # Screens, ViewModels, UI models, navigation ├── design-system/ # Reusable Compose components, theme, typography └── feature/ # Feature modules (optional, for larger projects) ├── auth/ ├── settings/ └── profile/ ``` ### Dependency Rules ``` app → presentation, domain, data, core presentation → domain, design-system, core data → domain, core domain → core (or no dependencies) core → (nothing) ``` **Critical**: `domain` must NEVER depend on `data`, `presentation`, or any framework. It contains pure Kotlin only. ## Domain Layer ### UseCase Pattern Each UseCase represents one business operation. Use `operator fun invoke` for clean call sites: ```kotlin class GetItemsByCategoryUseCase( private val repository: ItemRepository ) { suspend operator fun invoke(category: String): Result<List<Item>> { return repository.getItemsByCategory(category) } } // Flow-based UseCase for reactive streams class ObserveUserProgressUseCase( private val repository: UserRepository ) { operator fun invoke(userId: String): Flow<UserProgress> { return repository.observeProgress(userId) } } ``` ### Domain Models Domain models are plain Kotlin data classes — no framework annotations: ```kotlin data class Item( val id: String, val title: String, val description: String, val tags: List<String>, val status: Status, val category: String ) enum class Status { DRAFT, ACTIVE, ARCHIVED } ``` ### Repository Interfaces Defined in domain, implemented in data: ```kotlin interface ItemRepository { suspend fun getItemsByCategory(category: String): Result<List<Item>> suspend fun saveItem(item: Item): Result<Unit> fun observeItems(): Flow<List<Item>> } ``` ## Data Layer ### Repository Implementation Coordinates between local and remote data sources: ```kotlin class ItemRepositoryImpl( private val localDataSource: ItemLocalDataSource, private val remoteDataSource: ItemRemoteDataSource ) : ItemRepository { override suspend fun getItemsByCategory(category: String): Result<List<Item>> { return runCatching { val remote = remoteDataSource.fetchItems(category) localDataSource.insertItems(remote.map { it.toEntity() }) localDataSource.getItemsByCategory(category).map { it.toDomain() } } } override suspend fun saveItem(item: Item): Result<Unit> { return runCatching { localDataSource.insertItems(listOf(item.toEntity())) } } override fun observeItems(): Flow<List<Item>> { return localDataSource.observeAll().map { entities -> entities.map { it.toDomain() } } } } ``` ### Mapper Pattern Keep mappers as extension functions near the data models: ```kotlin // In data layer fun ItemEntity.toDomain() = Item( id = id, title = title, description = description, tags = tags.split("|"), status = Status.valueOf(status), category = category ) fun ItemDto.toEntity() = ItemEntity( id = id, title = title, description = description, tags = tags.joinToString("|"), status = status, category = category ) ``` ### Room Database (Android) ```kotlin @Entity(tableName = "items") data class ItemEntity( @PrimaryKey val id: String, val title: String, val description: String, val tags: String, val status: String, val category: String ) @Dao interface ItemDao { @Query("SELECT * FROM items WHERE category = :category") suspend fun getByCategory(category: String): List<ItemEntity> @Upsert suspend fun upsert(items: List<ItemEntity>) @Query("SELECT * FROM items") fun observeAll(): Flow<List<ItemEntity>> } ``` ### SQLDelight (KMP) ```sql -- Item.sq CREATE TABLE ItemEntity ( id TEXT NOT NULL PRIMARY KEY, title TEXT NOT NULL, description TEXT NOT NULL, tags TEXT NOT NULL, status TEXT NOT NULL, category TEXT NOT NULL ); getByCategory: SELECT * FROM ItemEntity WHERE category = ?; upsert: INSERT OR REPLACE INTO ItemEntity (id, title, description, tags, status, category) VALUES (?, ?, ?, ?, ?, ?); observeAll: SELECT * FROM ItemEntity; ``` ### Ktor Network Client (KMP) ```kotlin class ItemRemoteDataSource(private val client: HttpClient) { suspend fun fetchItems(category: String): List<ItemDto> { return client.get("api/items") { parameter("category", category) }.body() } } // HttpClient setup with content negotiation val httpClient = HttpClient { install(ContentNegotiation) { json(Json { ignoreUnknownKeys = true }) } install(Logging) { level = LogLevel.HEADERS } defaultRequest { url("https://api.example.com/") } } ``` ## Dependency Injection ### Koin (KMP-friendly) ```kotlin // Domain module val domainModule = module { factory { GetItemsByCategoryUseCase(get()) } factory { ObserveUserProgressUseCase(get()) } } // Data module val dataModule = module { single<ItemRepository> { ItemRepositoryImpl(get(), get()) } single { ItemLocalDataSource(get()) } single { ItemRemoteDataSource(get()) } } // Presentation module val presentationModule = module { viewModelOf(::ItemListViewModel) viewModelOf(::DashboardViewModel) } ``` ### Hilt (Android-only) ```kotlin @Module @InstallIn(SingletonComponent::class) abstract class RepositoryModule { @Binds abstract fun bindItemRepository(impl: ItemRepositoryImpl): ItemRepository } @HiltViewModel class ItemListViewModel @Inject constructor( private val getItems: GetItemsByCategoryUseCase ) : ViewModel() ``` ## Error Handling ### Result/Try Pattern Use `Result<T>` or a custom sealed type for error propagation: ```kotlin sealed interface Try<out T> { data class Success<T>(val value: T) : Try<T> data class Failure(val error: AppError) : Try<Nothing> } sealed interface AppError { data class Network(val message: String) : AppError data class Database(val message: String) : AppError data object Unauthorized : AppError } // In ViewModel — map to UI state viewModelScope.launch { when (val result = getItems(category)) { is Try.Success -> _state.update { it.copy(items = result.value, isLoading = false) } is Try.Failure -> _state.update { it.copy(error = result.error.toMessage(), isLoading = false) } } } ``` ## Convention Plugins (Gradle) For KMP projects, use convention plugins to reduce build file duplication: ```kotlin // build-logic/src/main/kotlin/kmp-library.gradle.kts plugins { id("org.jetbrains.kotlin.multiplatform") } kotlin { androidTarget() iosX64(); iosArm64(); iosSimulatorArm64() sourceSets { commonMain.dependencies { /* shared deps */ } commonTest.dependencies { implementation(kotlin("test")) } } } ``` Apply in modules: ```kotlin // domain/build.gradle.kts plugins { id("kmp-library") } ``` ## Anti-Patterns to Avoid - Importing Android framework classes in `domain` — keep it pure Kotlin - Exposing database entities or DTOs to the UI layer — always map to domain models - Putting business logic in ViewModels — extract to UseCases - Using `GlobalScope` or unstructured coroutines — use `viewModelScope` or structured concurrency - Fat repository implementations — split into focused DataSources - Circular module dependencies — if A depends on B, B must not depend on A ## References See skill: `compose-multiplatform-patterns` for UI patterns. See skill: `kotlin-coroutines-flows` for async patterns.