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: class GetItemsByCategoryUseCase( private val repository: ItemRepository ) { suspend operator fun invoke(category: String): Result<List> { return repository.getItemsByCategory(category) } } // Flow-based UseCase for reactive streams class ObserveUserProgressUseCase( private val repository: UserRepository ) { operator fun invoke(userId: String): Flow { return repository.observeProgress(userId) } } Domain Models Domain models are plain Kotlin data classes — no framework annotations: data class Item( val id: String, val title: String, val description: String, val tags: List, val status: Status, val category: String ) enum class Status { DRAFT, ACTIVE, ARCHIVED } Repository Interfaces Defined in domain, implemented in data: interface ItemRepository { suspend fun getItemsByCategory(category: String): Result<List> suspend fun saveItem(item: Item): Result fun observeItems(): Flow<List> } Data Layer Repository Implementation Coordinates between local and remote data sources: class ItemRepositoryImpl( private val localDataSource: ItemLocalDataSource, private val remoteDataSource: ItemRemoteDataSource ) : ItemRepository { override suspend fun getItemsByCategory(category: String): Result<List> { 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 { return runCatching { localDataSource.insertItems(listOf(item.toEntity())) } } override fun observeItems(): Flow<List> { return localDataSource.observeAll().map { entities -> entities.map { it.toDomain() } } } } Mapper Pattern Keep mappers as extension functions near the data models: // 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) @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 @Upsert suspend fun upsert(items: List) @Query("SELECT * FROM items") fun observeAll(): Flow<List> } SQLDelight (KMP) -- 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) class ItemRemoteDataSource(private val client: HttpClient) { suspend fun fetchItems(category: String): List { 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) // Domain module val domainModule = module { factory { GetItemsByCategoryUseCase(get()) } factory { ObserveUserProgressUseCase(get()) } } // Data module val dataModule = module { single { ItemRepositoryImpl(get(), get()) } single { ItemLocalDataSource(get()) } single { ItemRemoteDataSource(get()) } } // Presentation module val presentationModule = module { viewModelOf(::ItemListViewModel) viewModelOf(::DashboardViewModel) } Hilt (Android-only) @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 or a custom sealed type for error propagation: sealed interface Try { data class Success(val value: T) : Try data class Failure(val error: AppError) : Try } 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: // 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: // 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. 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