---name: android-data-layerdescription: Guidance on implementing the Data Layer using Repository pattern, Room (Local), and Retrofit (Remote) with offline-first synchronization.---

Android Data Layer & Offline-First

Instructions

The Data Layer coordinates data from multiple sources.

1. Repository Pattern

• Role: Single Source of Truth (SSOT).
• Logic: The repository decides whether to return cached data or fetch fresh data.
• Implementation:

  class NewsRepository @Inject constructor(
      private val newsDao: NewsDao,
      private val newsApi: NewsApi
  ) {
      // Expose data from Local DB as the source of truth
      val newsStream: Flow<List<News>> = newsDao.getAllNews()
  
      // Sync operation
      suspend fun refreshNews() {
          val remoteNews = newsApi.fetchLatest()
          newsDao.insertAll(remoteNews)
      }
  }
  

2. Local Persistence (Room)

• Usage: Primary cache and offline storage.
• Entities: Define @Entity data classes.
• DAOs: Return Flow<T> for observable data.

3. Remote Data (Retrofit)

• Usage: Fetching data from backend.
• Response: Use suspend functions in interfaces.
• Error Handling: Wrap network calls in try-catch blocks or a Result wrapper to handle exceptions (NoInternet, 404, etc.) gracefully.

4. Synchronization

• Read: "Stale-While-Revalidate". Show local data immediately, trigger a background refresh.
• Write: "Outbox Pattern" (Advanced). Save local change immediately, mark as "unsynced", use WorkManager to push changes to server.

5. Dependency Injection

• Bind Repository interfaces to implementations in a Hilt Module.

  @Binds
  abstract fun bindNewsRepository(impl: OfflineFirstNewsRepository): NewsRepository