data-layer

You are an expert in Android data layer architecture. Follow offline-first principles, use Room for persistence, DataStore for preferences, and maintain strict layer separation with dedicated models per layer.

Repository Pattern

Source: https://developer.android.com/topic/architecture/data-layer

DO:

• Repository is the single entry point to all data for a given type:

class ArticleRepository @Inject constructor(
    private val articleRemoteDataSource: ArticleRemoteDataSource,
    private val articleLocalDataSource: ArticleLocalDataSource,
) {
    fun getArticles(): Flow<List<Article>> =
        articleLocalDataSource.observeArticles()
            .map { entities -> entities.map { it.toArticle() } }

    suspend fun refreshArticles() {
        val networkArticles = articleRemoteDataSource.fetchArticles()
        articleLocalDataSource.upsertArticles(networkArticles.map { it.toEntity() })
    }
}

• Expose Flow<T> for data that changes over time
• Expose suspend functions for one-shot operations
• Return immutable types only
• One repository per data type (UserRepository, ArticleRepository)

DON'T:

• Expose DataSource classes outside the data layer
• Return mutable collections (MutableList, MutableMap)
• Create a single repository for the entire app
• Put UI formatting logic in repositories

Model Separation

Source: https://developer.android.com/topic/architecture/data-layer

DO:

• Create separate model classes for each layer boundary:

// Network layer — matches API JSON
@Serializable
data class NetworkArticle(
    @SerialName("article_id") val id: String,
    val title: String,
    @SerialName("body_text") val body: String,
    @SerialName("created_at") val createdAt: String,
)

// Database layer — matches Room schema
@Entity(tableName = "articles")
data class ArticleEntity(
    @PrimaryKey val id: String,
    val title: String,
    val body: String,
    val createdAt: Long,
    val lastRefreshed: Long = System.currentTimeMillis(),
)

// Domain/UI layer — what the app works with
data class Article(
    val id: String,
    val title: String,
    val body: String,
    val createdAt: Instant,
)

• Create extension functions for mapping between layers:

fun NetworkArticle.toEntity() = ArticleEntity(...)
fun ArticleEntity.toArticle() = Article(...)