Android Data Layer

Android Data Layer

The data layer coordinates data from multiple sources; its public API is repository interfaces, and its internals (DAOs, API services, DTOs) never leak upward. This skill is the canonical home for the error-propagation model, plus the KMP-Room setup. Related: android-skills:android-retrofit (Retrofit service/OkHttp/Hilt wiring), android-skills:android-dev (overall app architecture).

Error propagation — the layered model

The goal: data-layer errors (IOException / HttpException / SQLiteException) must never reach the ViewModel. Map them to a domain error type at the repository boundary. If the project already has an error convention, match it; otherwise a sealed DataError hierarchy is a reasonable default:

sealed class DataError(message: String, cause: Throwable? = null) : Exception(message, cause) {
    class Network(cause: Throwable) : DataError("Network error", cause)
    class Server(val code: Int, message: String?) : DataError("Server error $code: $message")
    class Local(cause: Throwable) : DataError("Local storage error", cause)
}

// repository = error boundary: catch platform exceptions, return Result<T> (no domain layer) or throw DataError (with one)
suspend fun refreshNews(): Result<Unit> = withContext(io) {
    try { newsDao.insertAll(newsApi.fetchLatest().map(NewsDto::toEntity)); Result.success(Unit) }
    catch (e: IOException) { Result.failure(DataError.Network(e)) }
    catch (e: HttpException) { Result.failure(DataError.Server(e.code(), e.message())) }
}

The boundary moves outward by one layer when a domain layer exists:

1. Data sources throw platform/library exceptions (IOException, HttpException, SQLiteException).
2. Repository is the error boundary — catch those and remap to DataError; never let raw platform types leak. Without a domain layer (simple MVVM): the repository returns Result<T> directly and the ViewModel handles it without knowing about platform exceptions.
3. Use cases (when present) are the Result boundary — the repository instead throws DataError, and the use case catches it and returns Result<T> with a domain-specific error model. Use cases never catch platform exception types.
4. ViewModels handle Result<T> and map it to UI state (explicit loading / success / error).

A @Serializable value class encodes as the wrapped value, not an object. Changing data class UserId(val value: String) to @JvmInline value class UserId(val value: String) changes the wire format from {"value":"x"} to "x" — a breaking change for a stored or network payload.

(Default to the Entity suffix — ArticleEntity — for Room types; match an existing convention such as a Cached prefix if the project already uses one.)

Room in KMP (commonMain)

Room has been KMP-stable since 2.7.0. The shared setup differs from the Android-only setup in three places:

1. @ConstructedBy(...) on the @Database, paired with an expect object Room generates per-platform actuals for.
2. BundledSQLiteDriver (from androidx.sqlite:sqlite-bundled) — pins the same SQLite version across Android/iOS/JVM/web (Android's system SQLite drifts between API levels and devices).
3. setQueryCoroutineContext(Dispatchers.IO) — Android Room defaults this; KMP doesn't.

// commonMain
@Database(entities = [ArticleEntity::class], version = 1, exportSchema = true)
@ConstructedBy(AppDatabaseConstructor::class)
abstract class AppDatabase : RoomDatabase() { abstract fun articleDao(): ArticleDao }

@Suppress("KotlinNoActualForExpect")
expect object AppDatabaseConstructor : RoomDatabaseConstructor<AppDatabase>

fun getRoomDatabase(builder: RoomDatabase.Builder<AppDatabase>): AppDatabase =
    builder.setDriver(BundledSQLiteDriver()).setQueryCoroutineContext(Dispatchers.IO).build()

Each platform provides its own builder — Android Room.databaseBuilder(context, AppDatabase::class.java, "app.db"); iOS/JVM Room.databaseBuilder<AppDatabase>(databasePath = …). KSP must be wired per target (ksp(libs.androidx.room.compiler) is Android-only):

dependencies {
    add("kspAndroid", libs.androidx.room.compiler)
    add("kspIosArm64", libs.androidx.room.compiler)
    add("kspIosSimulatorArm64", libs.androidx.room.compiler)
    // … one per target
}
room { schemaDirectory("$projectDir/schemas") }   // export the schema for migration-history tracking

For pure-Android projects, skip @ConstructedBy and call Room.databaseBuilder(context, AppDatabase::class.java, "app.db") directly — Room behaves identically; the KMP setup is opt-in when the data layer must live in commonMain.