Android Development

Build Android applications following Google's official architecture guidance, as demonstrated in the NowInAndroid reference app.

Quick Reference

Task	Reference File
Project structure & modules	modularization.md
Architecture layers (UI, Domain, Data)	architecture.md
Jetpack Compose patterns	compose-patterns.md
Coroutines & Flow patterns	coroutines.md
Gradle & build configuration	gradle-setup.md
Testing approach	testing.md

Workflow Decision Tree

Creating a new project? → Read modularization.md for project structure → Use templates in assets/templates/

Adding a new feature? → Create feature module with api and impl submodules → Follow patterns in architecture.md

Building UI screens? → Read compose-patterns.md → Create Screen + ViewModel + UiState

Setting up data layer? → Read data layer section in architecture.md → Create Repository + DataSource + DAO

Working with coroutines/Flow? → Read coroutines.md → Follow dispatcher and structured concurrency patterns

Core Principles

1. Offline-first: Local database is source of truth, sync with remote
2. Unidirectional data flow: Events flow down, data flows up
3. Reactive streams: Use Kotlin Flow for all data exposure
4. Modular by feature: Each feature is self-contained with clear boundaries
5. Testable by design: Use interfaces and test doubles, no mocking libraries

Architecture Layers

┌─────────────────────────────────────────┐
│              UI Layer                    │
│  (Compose Screens + ViewModels)          │
├─────────────────────────────────────────┤
│           Domain Layer                   │
│  (Use Cases - optional, for reuse)       │
├─────────────────────────────────────────┤
│            Data Layer                    │
│  (Repositories + DataSources)            │
└─────────────────────────────────────────┘

Module Types

app/                    # App module - navigation, scaffolding
feature/
  ├── featurename/
  │   ├── api/          # Navigation keys (public)
  │   └── impl/         # Screen, ViewModel, DI (internal)
core/
  ├── data/             # Repositories
  ├── database/         # Room DAOs, entities
  ├── network/          # Retrofit, API models
  ├── model/            # Domain models (pure Kotlin)
  ├── common/           # Shared utilities
  ├── ui/               # Reusable Compose components
  ├── designsystem/     # Theme, icons, base components
  ├── datastore/        # Preferences storage
  └── testing/          # Test utilities

Creating a New Feature

1. Create feature:myfeature:api module with navigation key
2. Create feature:myfeature:impl module with:
   • MyFeatureScreen.kt - Composable UI
   • MyFeatureViewModel.kt - State holder
   • MyFeatureUiState.kt - Sealed interface for states
   • MyFeatureNavigation.kt - Navigation setup
   • MyFeatureModule.kt - Hilt DI module

Standard File Patterns

ViewModel Pattern

@HiltViewModel
class MyFeatureViewModel @Inject constructor(
    private val myRepository: MyRepository,
) : ViewModel() {

    val uiState: StateFlow<MyFeatureUiState> = myRepository
        .getData()
        .map { data -> MyFeatureUiState.Success(data) }
        .stateIn(
            scope = viewModelScope,
            started = SharingStarted.WhileSubscribed(5_000),
            initialValue = MyFeatureUiState.Loading,
        )

    fun onAction(action: MyFeatureAction) {
        when (action) {
            is MyFeatureAction.ItemClicked -> handleItemClick(action.id)
            is MyFeatureAction.RefreshRequested -> refresh()
        }
    }

    private fun refresh() {
        viewModelScope.launch {
            myRepository.sync()
        }
    }
}

UiState Pattern

sealed interface MyFeatureUiState {
    data object Loading : MyFeatureUiState
    data class Success(val items: List<Item>) : MyFeatureUiState
    data class Error(val message: String) : MyFeatureUiState
}

When to use individual StateFlows instead: The sealed UiState pattern is idiomatic in Android because collectAsStateWithLifecycle() collects the entire object. However, for independent UI elements that update at different cadences (e.g., a snackbar message, a separate loading indicator for pull-to-refresh while content is visible, or a dialog), use separate StateFlow properties to avoid unnecessary recompositions of unrelated UI.

Action Pattern

sealed interface MyFeatureAction {
    data class ItemClicked(val id: String) : MyFeatureAction
    data object RefreshRequested : MyFeatureAction
}

Screen Pattern

@Composable
internal fun MyFeatureRoute(
    onNavigateToDetail: (String) -> Unit,
    viewModel: MyFeatureViewModel = hiltViewModel(),
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()
    MyFeatureScreen(
        uiState = uiState,
        onAction = viewModel::onAction,
        onNavigateToDetail = onNavigateToDetail,
    )
}

@Composable
internal fun MyFeatureScreen(
    uiState: MyFeatureUiState,
    onAction: (MyFeatureAction) -> Unit,
    onNavigateToDetail: (String) -> Unit,
) {
    when (uiState) {
        is MyFeatureUiState.Loading -> LoadingIndicator()
        is MyFeatureUiState.Success -> ContentList(uiState.items, onAction)
        is MyFeatureUiState.Error -> ErrorMessage(uiState.message)
    }
}

Repository Pattern

interface MyRepository {
    fun getData(): Flow<List<MyModel>>
    suspend fun updateItem(id: String, data: MyModel)
    suspend fun sync()
}

internal class OfflineFirstMyRepository @Inject constructor(
    private val localDataSource: MyDao,
    private val networkDataSource: MyNetworkApi,
) : MyRepository {

    override fun getData(): Flow<List<MyModel>> =
        localDataSource.getAll().map { entities ->
            entities.map { it.toModel() }
        }

    override suspend fun updateItem(id: String, data: MyModel) {
        localDataSource.upsert(data.toEntity())
    }

    override suspend fun sync() {
        val remoteItems = networkDataSource.getItems()
        localDataSource.upsertAll(remoteItems.map { it.toEntity() })
    }
}

Navigation Pattern (Type-Safe)

// In api module
@Serializable
data class MyFeatureRoute(val id: String? = null)

fun NavController.navigateToMyFeature(id: String? = null) {
    navigate(MyFeatureRoute(id))
}

// In impl module
fun NavGraphBuilder.myFeatureScreen(
    onNavigateToDetail: (String) -> Unit,
) {
    composable<MyFeatureRoute> {
        MyFeatureRoute(onNavigateToDetail = onNavigateToDetail)
    }
}

Hilt DI Module Pattern

@Module
@InstallIn(SingletonComponent::class)
abstract class DataModule {

    @Binds
    abstract fun bindMyRepository(
        impl: OfflineFirstMyRepository,
    ): MyRepository
}

Target SDK & Language

Setting	Value
compileSdk / targetSdk	36 (Android 16 "Baklava")
minSdk	26 (Android 8.0)
Kotlin	2.3.10
Compose BOM	2026.02.01
Java	17
AGP	9.0.1 (Kotlin built-in, Gradle 9.1.0+ required)
Compose Compiler	Built into Kotlin 2.0+ (no separate version)

Key Dependencies

// Gradle version catalog (libs.versions.toml)
[versions]
kotlin = "2.3.10"
compose-bom = "2026.02.01"
hilt = "2.59.2"
room = "2.8.4"
coroutines = "1.10.1"

[libraries]
androidx-compose-bom = { group = "androidx.compose", name = "compose-bom", version.ref = "compose-bom" }
hilt-android = { group = "com.google.dagger", name = "hilt-android", version.ref = "hilt" }
room-runtime = { group = "androidx.room", name = "room-runtime", version.ref = "room" }

Build Configuration

Use convention plugins in build-logic/ for consistent configuration:

• AndroidApplicationConventionPlugin - App modules
• AndroidLibraryConventionPlugin - Library modules
• AndroidFeatureConventionPlugin - Feature modules
• AndroidComposeConventionPlugin - Compose setup (uses kotlin-compose plugin)
• AndroidHiltConventionPlugin - Hilt setup

AGP 9.0 note: Kotlin support is built into AGP 9.0 — remove id("org.jetbrains.kotlin.android") from plugin blocks.

See gradle-setup.md for complete build configuration.