KMP Repository Pattern

Implement the repository pattern with shared interfaces in commonMain and platform-specific implementations.

Architecture Overview

┌─────────────────────────────────────────────────────────┐
│                    Presentation Layer                    │
│              (ViewModels, Composables, SwiftUI)          │
└──────────────────────────┬──────────────────────────────┘
                           │
┌──────────────────────────▼──────────────────────────────┐
│                    Domain Layer                          │
│                    (Use Cases)                           │
└──────────────────────────┬──────────────────────────────┘
                           │
┌──────────────────────────▼──────────────────────────────┐
│                    Data Layer                            │
│  ┌──────────────────────────────────────────────────┐   │
│  │         Repository Interface (commonMain)         │   │
│  └──────────────────────┬───────────────────────────┘   │
│                         │                               │
│  ┌──────────────────────▼───────────────────────────┐   │
│  │      Repository Implementation (commonMain)      │   │
│  │  - Coordinates data sources                      │   │
│  │  - Caching strategy                              │   │
│  │  - Error mapping                                 │   │
│  └──────────────────────┬───────────────────────────┘   │
│                         │                               │
│  ┌──────────────────────▼───────────────────────────┐   │
│  │              Data Sources                        │   │
│  │  ┌────────────┐  ┌────────────┐  ┌───────────┐  │   │
│  │  │   Remote   │  │   Local    │  │   Memory  │  │   │
│  │  │ (Network)  │  │ (Database) │  │  (Cache)  │  │   │
│  │  └────────────┘  └────────────┘  └───────────┘  │   │
│  └───────────────────────────────────────────────────┘   │
└───────────────────────────────────────────────────────────┘

Repository Interface

// commonMain/kotlin/data/repository/UserRepository.kt
interface UserRepository {
    // Single shot
    suspend fun getUser(id: String): Result<User>
    suspend fun getUsers(page: Int): Result<PaginatedResponse<User>>
    suspend fun saveUser(user: User): Result<User>
    suspend fun deleteUser(id: String): Result<Unit>

    // Streams
    fun observeUser(id: String): Flow<Result<User>>
    fun observeUsers(): Flow<Result<List<User>>>

    // Cache control
    suspend fun refreshUser(id: String): Result<User>
    suspend fun refreshAll(): Result<Unit>
    suspend fun clearCache(): Result<Unit>
}

Base Repository

// commonMain/kotlin/data/repository/BaseRepository.kt
abstract class BaseRepository {
    // Execute with error handling
    protected suspend fun <T> execute(
        block: suspend () -> T
    ): Result<T> = try {
        Result.success(block())
    } catch (e: CancellationException) {
        throw e
    } catch (e: NetworkException) {
        Result.failure(e)
    } catch (e: Exception) {
        Result.failure(DataException(cause = e))
    }

    // Flow with error handling
    protected fun <T> flowResult(
        block: suspend () -> Flow<T>
    ): Flow<Result<T>> = flow {
        try {
            block().collect { emit(Result.success(it)) }
        } catch (e: CancellationException) {
            throw e
        } catch (e: Exception) {
            emit(Result.failure(e))
        }
    }

    // Cache-first strategy
    protected suspend fun <T> cacheFirst(
        cacheKey: String,
        ttl: Duration,
        cacheBlock: suspend () -> T?,
        networkBlock: suspend () -> T
    ): Result<T> = execute {
        // Try cache
        cacheBlock()?.let { return@execute it }

        // Fetch from network
        val result = networkBlock()

        // Update cache
        // saveToCache(cacheKey, result)

        result
    }

    // Network-first strategy
    protected suspend fun <T> networkFirst(
        cacheKey: String,
        cacheBlock: suspend () -> T?,
        networkBlock: suspend () -> T
    ): Result<T> = execute {
        try {
            // Try network first
            val result = networkBlock()
            // Update cache
            result
        } catch (e: NetworkException) {
            // Fall back to cache
            cacheBlock() ?: throw e
        }
    }

    // Refresh strategy
    protected suspend fun <T> refresh(
        networkBlock: suspend () -> T,
        cacheBlock: suspend (T) -> Unit
    ): Result<T> = execute {
        val result = networkBlock()
        cacheBlock(result)
        result
    }
}

Concrete Repository

// commonMain/kotlin/data/repository/UserRepositoryImpl.kt
class UserRepositoryImpl(
    private val remoteDataSource: UserRemoteDataSource,
    private val localDataSource: UserLocalDataSource,
    private val memoryCache: MemoryCache,
    private val dispatcher: CoroutineDispatcher = PlatformDispatchers.io
) : BaseRepository(), UserRepository {

    override suspend fun getUser(id: String): Result<User> =
        networkFirst(
            cacheKey = "user:$id",
            cacheBlock = { localDataSource.getUser(id) },
            networkBlock = { remoteDataSource.getUser(id).also { user ->
                localDataSource.saveUser(user)
                memoryCache.put("user:$id", user)
            }}
        )

    override suspend fun getUsers(page: Int): Result<PaginatedResponse<User>> =
        execute {
            remoteDataSource.getUsers(page).also { response ->
                localDataSource.saveUsers(response.items)
            }
        }

    override suspend fun saveUser(user: User): Result<User> =
        execute {
            remoteDataSource.createUser(user).also { savedUser ->
                localDataSource.saveUser(savedUser)
                memoryCache.put("user:${savedUser.id}", savedUser)
            }
        }

    override suspend fun deleteUser(id: String): Result<Unit> =
        execute {
            remoteDataSource.deleteUser(id).also {
                localDataSource.deleteUser(id)
                memoryCache.remove("user:$id")
            }
        }

    override fun observeUser(id: String): Flow<Result<User>> =
        flowResult {
            localDataSource.observeUser(id)
        }

    override fun observeUsers(): Flow<Result<List<User>>> =
        flowResult {
            localDataSource.observeAllUsers()
        }

    override suspend fun refreshUser(id: String): Result<User> =
        refresh(
            networkBlock = { remoteDataSource.getUser(id) },
            cacheBlock = { user -> localDataSource.saveUser(user) }
        )

    override suspend fun refreshAll(): Result<Unit> =
        execute {
            remoteDataSource.getUsers(page = 1).items.forEach { user ->
                localDataSource.saveUser(user)
            }
        }

    override suspend fun clearCache(): Result<Unit> =
        execute {
            localDataSource.clearAll()
            memoryCache.clear()
        }
}

Data Sources

Remote Data Source

// commonMain/kotlin/data/datasource/remote/UserRemoteDataSource.kt
interface UserRemoteDataSource {
    suspend fun getUser(id: String): User
    suspend fun getUsers(page: Int): PaginatedResponse<User>
    suspend fun createUser(user: User): User
    suspend fun updateUser(id: String, user: User): User
    suspend fun deleteUser(id: String)
}

class UserRemoteDataSourceImpl(
    private val api: UserApi
) : UserRemoteDataSource {
    override suspend fun getUser(id: String): User = api.getUser(id)
    override suspend fun getUsers(page: Int): PaginatedResponse<User> = api.getUsers(page)
    override suspend fun createUser(user: User): User = api.createUser(user)
    override suspend fun updateUser(id: String, user: User): User = api.updateUser(id, user)
    override suspend fun deleteUser(id: String) = api.deleteUser(id)
}

Local Data Source

// commonMain/kotlin/data/datasource/local/UserLocalDataSource.kt
interface UserLocalDataSource {
    suspend fun getUser(id: String): User?
    suspend fun getAllUsers(): List<User>
    suspend fun saveUser(user: User)
    suspend fun saveUsers(users: List<User>)
    suspend fun deleteUser(id: String)
    suspend fun clearAll()
    fun observeUser(id: String): Flow<User>
    fun observeAllUsers(): Flow<List<User>>
}

class UserLocalDataSourceImpl(
    private val database: AppDatabase
) : UserLocalDataSource {
    override suspend fun getUser(id: String): User? =
        database.userQueries().getById(id).executeAsOneOrNull()?.toDomain()

    override suspend fun getAllUsers(): List<User> =
        database.userQueries().getAll().executeAsList().map { it.toDomain() }

    override suspend fun saveUser(user: User) =
        database.userQueries().insert(user.toEntity())

    override suspend fun saveUsers(users: List<User>) =
        database.transaction {
            users.forEach { saveUser(it) }
        }

    override suspend fun deleteUser(id: String) =
        database.userQueries().deleteById(id)

    override suspend fun clearAll() =
        database.userQueries().deleteAll()

    override fun observeUser(id: String): Flow<User> =
        database.userQueries().getById(id).asFlow().map { it.executeAsOneOrNull()?.toDomain() }
        .filterNotNull()

    override fun observeAllUsers(): Flow<List<User>> =
        database.userQueries().getAll().asFlow().map { it.executeAsList().map { entity -> entity.toDomain() } }
}

Use Cases

// commonMain/kotlin/domain/usecase/GetUserUseCase.kt
class GetUserUseCase(
    private val repository: UserRepository
) {
    suspend operator fun invoke(id: String): Result<User> =
        repository.getUser(id)
}

// commonMain/kotlin/domain/usecase/ObserveUserUseCase.kt
class ObserveUserUseCase(
    private val repository: UserRepository
) {
    operator fun invoke(id: String): Flow<Result<User>> =
        repository.observeUser(id)
}

// commonMain/kotlin/domain/usecase/RefreshUserDataUseCase.kt
class RefreshUserDataUseCase(
    private val repository: UserRepository
) {
    suspend operator fun invoke(id: String): Result<User> =
        repository.refreshUser(id)
}

Dependency Injection

// commonMain/kotlin/di/DataModule.kt
val dataModule = module {
    // Data sources
    single { UserApi(get()) }
    single<UserRemoteDataSource> { UserRemoteDataSourceImpl(get()) }

    // Local data source
    single { createDatabase(get()) }
    single<UserLocalDataSource> { UserLocalDataSourceImpl(get()) }

    // Memory cache
    single { MemoryCache() }

    // Repositories
    single<UserRepository> { UserRepositoryImpl(get(), get(), get()) }

    // Use cases
    factory { GetUserUseCase(get()) }
    factory { ObserveUserUseCase(get()) }
    factory { RefreshUserDataUseCase(get()) }
}

Platform-Specific Database

// commonMain/kotlin/database/DatabaseFactory.kt
expect class DatabaseFactory {
    fun create(): AppDatabase
}

// Android implementation
actual class DatabaseFactory(private val context: Context) {
    actual fun create(): AppDatabase =
        Room.databaseBuilder(
            context,
            AppDatabase::class.java,
            "app.db"
        ).build()
}

// iOS implementation
actual class DatabaseFactory {
    actual fun create(): AppDatabase {
        val dbPath = NSSearchPathForDirectoriesInDomains(
            NSDocumentDirectory,
            NSUserDomainMask,
            true
        ).first() as String
        return AppDatabase("$dbPath/app.db")
    }
}

Repository Testing

// commonTest/kotlin/data/repository/UserRepositoryTest.kt
class UserRepositoryTest {
    private lateinit var repository: UserRepository
    private lateinit var mockRemote: UserRemoteDataSource
    private lateinit var mockLocal: UserLocalDataSource

    @BeforeTest
    fun setup() {
        mockRemote = mockk()
        mockLocal = mockk()
        repository = UserRepositoryImpl(
            remoteDataSource = mockRemote,
            localDataSource = mockLocal,
            memoryCache = MemoryCache()
        )
    }

    @Test
    fun `returns cached user when available`() = runTest {
        val cachedUser = User(id = "123", name = "John")
        coEvery { mockLocal.getUser("123") } returns cachedUser

        val result = repository.getUser("123")

        assertTrue(result.isSuccess)
        assertEquals(cachedUser, result.getOrNull())
        coVerify { mockRemote wasNot Called }
    }

    @Test
    fun `fetches from network when cache empty`() = runTest {
        val networkUser = User(id = "123", name = "John")
        coEvery { mockLocal.getUser("123") } returns null
        coEvery { mockRemote.getUser("123") } returns networkUser
        coEvery { mockLocal.saveUser(any()) } just Runs

        val result = repository.getUser("123")

        assertTrue(result.isSuccess)
        assertEquals(networkUser, result.getOrNull())
        coVerify { mockRemote.getUser("123") }
        coVerify { mockLocal.saveUser(networkUser) }
    }
}

Best Practices

✅ DO

// ✅ Define interfaces in commonMain
interface UserRepository { ... }

// ✅ Return Result for error handling
suspend fun getUser(id: String): Result<User>

// ✅ Expose Flow for state
fun observeUser(id: String): Flow<User>

// ✅ Use base repository for common patterns
class UserRepositoryImpl : BaseRepository() { ... }

// ✅ Keep repositories platform-agnostic
// No Android/iOS specific code in repository implementations

❌ DON'T

// ❌ Don't expose platform types
suspend fun getUser(): SQLiteDatabase  // ❌
suspend fun getUser(): User  // ✅

// ❌ Don't put business logic in repositories
if (user.age < 18) throw Exception()  // ❌ belongs in use case

// ❌ Don't make repositories depend on each other
// Use use cases to coordinate

// ❌ Don't use global state
object UserRepository { ... }  // ❌ use DI

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Remember: Repositories are your data gatekeepers. Keep them platform-agnostic, testable, and focused on data coordination.