Google App Development

Google App Development (Jetpack Compose / Kotlin)

Modern best practices for building apps across Google platforms. Targets Jetpack Compose as the primary UI framework with Kotlin. Covers Android (phone), tablet, foldables, Wear OS, Google TV / Android TV, Android Auto, Android Automotive OS, and AOSP-based platforms (Meta Quest, Amazon Fire TV, Amazon Fire Tablets).

For Kotlin language fundamentals (null safety, coroutines, data modeling, error handling, idiomatic patterns), see the kotlin-development skill. This skill focuses on platform and framework patterns.

Important Rules

• Always generate Kotlin. Only write Java when the project explicitly requires it (legacy codebase, Java-only API). See references/java-interop.md for bridging patterns.
• Compose-first. Use View-based UI only when Compose lacks the capability or the project has an existing View-based codebase. See references/view-interop.md for interop patterns.
• Check the project context. Before applying patterns, check the minSdk, Compose BOM version, and existing architecture. Adapt recommendations accordingly.

Reference Files

• references/compose-patterns.md — Composables, state hoisting, recomposition, remember, LazyColumn, navigation, theming, Material 3, side effects
• references/concurrency.md — Coroutines in Android context: viewModelScope, lifecycleScope, repeatOnLifecycle, WorkManager, testing
• references/android-background-work.md — Foreground Services (types, permissions, restrictions), Background Limits (Doze, App Standby Buckets), AlarmManager (exact/inexact, permissions)
• references/android-lifecycle.md — Activity/Fragment lifecycle, ViewModel, saved state, process death, configuration changes
• references/view-interop.md — AndroidView, ComposeView, embedding Compose in Views and Views in Compose, migration strategies
• references/project-structure.md — Gradle setup, multi-module architecture, build variants, version catalogs, convention plugins
• references/tablet-patterns.md — Adaptive layouts, WindowSizeClass (isWidthAtLeastBreakpoint), multi-window, foldable devices, large screen guidelines
• references/wear-os-patterns.md — Compose for Wear OS, Tiles, complications, Health Services, watch face
• references/tv-patterns.md — Compose for TV, focus management, Leanback, D-pad navigation, Google TV / Android TV
• references/car-app-library.md — Car App Library shared API: CarAppService, Session, Screen, ScreenManager, templates, constraints, lifecycle, CarContext, testing
• references/android-auto-patterns.md — Android Auto phone projection: Media3 MediaLibraryService, messaging notifications, navigation/DHU, POI, distribution
• references/android-automotive-patterns.md — Android Automotive OS (AAOS): AAOS-specific Car App Library diffs, car hardware APIs, HVAC, system UI, multi-user, OEM customization, multi-zone audio
• references/java-interop.md — Calling Java from Kotlin, nullability annotations, SAM conversions, incremental migration
• references/meta-quest-patterns.md — Adapting Android APK for Meta Quest, spatial UI, entitlement check, VR input, passthrough
• references/local-storage.md — Room database (entities, DAOs, relations, migrations, testing), DataStore (Preferences and Proto), encrypted storage (Android Keystore, DataStore + Tink, SQLCipher), file storage (internal, scoped storage, FileProvider), storage selection guide
• references/networking-api.md — Retrofit, OkHttp, Ktor Client, JSON serialization, repository pattern, error handling, interceptors, certificate pinning, caching, connectivity, pagination (custom + Paging 3), file upload/download, testing
• references/fire-tv-patterns.md — Amazon Fire TV, Appstore, Amazon IAP, Alexa integration, missing Google Play Services
• references/media-playback.md — Media3 / ExoPlayer, MediaSession, audio focus, Picture-in-Picture, offline downloads, DRM, streaming formats, caching
• references/billing-payments.md — Google Play Billing Library (PBL 8), BillingClient, one-time purchases (consumable / non-consumable), subscriptions (base plans, offers, replacement modes), subscription offers (eligibility types, pricing phases, offer tags, developer-determined offers, winback offers, promo codes), purchase verification, RTDN, subscription lifecycle (grace period, account hold, pause), alternative billing, testing
• references/fire-tablet-patterns.md — Amazon Fire Tablets, device capabilities, Show Mode, Kids Edition, Special Offers
• references/testing.md — JUnit 5, MockK, Turbine, ViewModel testing, Compose UI testing, Robolectric, Room in-memory testing, Hilt testing, Espresso, coroutine testing (runTest, TestDispatcher), test architecture (pyramid, MVI/MVVM strategies), fakes vs mocks, CI integration
• references/code-quality.md — Android Lint configuration (lint {} block, lint.xml, severity levels), baseline management, suppression (@SuppressLint, tools:ignore), built-in check categories (correctness, security, performance, accessibility), Compose lint checks, custom lint rules (Detector, Issue, IssueRegistry), CI integration (SARIF, GitHub Code Scanning), multi-module convention plugin. For Detekt/ktlint, see kotlin-development skill's references/static-analysis.md

Code Style

For Kotlin language style (naming, null safety, type annotations), follow the kotlin-development skill. Below are Android/Compose-specific conventions.

• Composable naming — PascalCase for UI-emitting composables, camelCase for composables that return values.
• Modifier parameter — always the first optional parameter, default to Modifier.
• Preview functions — prefix with Preview, annotate with @Preview.
• Define a custom app theme and apply it at the root. Every app must have a custom AppTheme composable that wraps MaterialTheme with app-specific colors, typography, and shapes. Apply it once at the top level (setContent { AppTheme { ... } }). All UI code must reference theme tokens — never raw values.

// 1. Define custom theme (once, in ui/theme/)
@Composable
fun AppTheme(
    darkTheme: Boolean = isSystemInDarkTheme(),
    content: @Composable () -> Unit,
) {
    val colorScheme = if (darkTheme) AppDarkColorScheme else AppLightColorScheme
    MaterialTheme(
        colorScheme = colorScheme,
        typography = AppTypography,
        shapes = AppShapes,
        content = content,
    )
}

// 2. Apply at root (once)
setContent { AppTheme { AppNavGraph() } }

// 3. Use theme tokens everywhere — never raw values
Text(
    text = "Hello",
    style = MaterialTheme.typography.bodyMedium,             // not fontSize = 14.sp
    color = MaterialTheme.colorScheme.onSurface,             // not Color(0xFF1C1B1F)
)
Card(shape = MaterialTheme.shapes.medium) { ... }           // not RoundedCornerShape(8.dp)

For values not covered by Material tokens (spacing, elevation, icon sizes), define app-level design tokens:

object AppSpacing {
    val small = 8.dp
    val medium = 16.dp
    val large = 24.dp
}

No magic numbers in UI code. If a numeric value appears in UI, it must be either a Material theme token or an app-defined design constant. For theming details see references/compose-patterns.md.

Naming Conventions (Android-Specific)

Jetpack Compose Essentials

// UI State
data class HomeUiState(
    val items: List<Item> = emptyList(),
    val isLoading: Boolean = false,
    val error: String? = null,
)

// ViewModel
class HomeViewModel(
    private val repository: ItemRepository,
) : ViewModel() {
    private val _uiState = MutableStateFlow(HomeUiState())
    val uiState: StateFlow<HomeUiState> = _uiState.asStateFlow()

    fun load() {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true) }
            repository.getItems()
                .onSuccess { items -> _uiState.update { it.copy(items = items, isLoading = false) } }
                .onFailure { e -> _uiState.update { it.copy(error = e.message, isLoading = false) } }
        }
    }
}

// Composable
@Composable
fun HomeScreen(
    viewModel: HomeViewModel = viewModel(),
    onItemClick: (Item) -> Unit,
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()

    when {
        uiState.isLoading -> LoadingIndicator()
        uiState.error != null -> ErrorMessage(uiState.error!!)
        else -> ItemList(items = uiState.items,
    }
}

Rules:

• Use StateFlow + collectAsStateWithLifecycle() for state in ViewModels.
• State hoisting — composables receive state and emit events, they don't own state.
• Use remember for composable-local state, rememberSaveable for state surviving config changes.
• Keep composables small. Extract when a composable exceeds ~40 lines.

For navigation, theming, side effects, lists, animations see references/compose-patterns.md.

Android Concurrency

For coroutines language fundamentals (Flow, structured concurrency, cancellation), see the kotlin-development skill. Below are Android-specific patterns.

// ViewModel scope — cancelled when ViewModel is cleared
class UserViewModel(private val repo: UserRepository) : ViewModel() {
    val users = repo.observeUsers()
        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), emptyList())
}

// Lifecycle-aware collection in Compose
@Composable
fun UserScreen(viewModel: UserViewModel = viewModel()) {
    val users by viewModel.users.collectAsStateWithLifecycle()
    UserList(users)
}

// Lifecycle-aware collection in Activity/Fragment (legacy)
lifecycleScope.launch {
    repeatOnLifecycle(Lifecycle.State.STARTED) {
        viewModel.users.collect { updateUI(it) }
    }
}

Rules:

• viewModelScope for ViewModel operations — auto-cancelled on clear.
• collectAsStateWithLifecycle() in Compose — lifecycle-aware, stops collection when not visible.
• repeatOnLifecycle in Activities/Fragments — restarts collection on lifecycle transitions.
• WhileSubscribed(5000) for stateIn — keeps upstream active 5s after last subscriber (survives rotation).
• WorkManager for deferrable background work. Foreground services for user-visible ongoing tasks.

For WorkManager, foreground services, and advanced patterns see references/concurrency.md.

Architecture

Recommended: MVI (Model-View-Intent) for new projects. MVI enforces unidirectional data flow with a single immutable state and explicit user intents, which maps naturally to Compose. If the project already uses MVVM, MVP, or MVC — adapt to the existing architecture instead of forcing a rewrite.

MVI Pattern

// 1. State — single immutable data class per screen
data class HomeUiState(
    val items: List<Item> = emptyList(),
    val isLoading: Boolean = false,
    val error: String? = null,
)

// 2. Intent — sealed interface of all user actions
sealed interface HomeIntent {
    data object LoadItems : HomeIntent
    data class DeleteItem(val id: String) : HomeIntent
    data object RetryLoad : HomeIntent
}

// 3. ViewModel — reduces intents into state
@HiltViewModel
class HomeViewModel @Inject constructor(
    private val repository: ItemRepository,
) : ViewModel() {
    private val _uiState = MutableStateFlow(HomeUiState())
    val uiState: StateFlow<HomeUiState> = _uiState.asStateFlow()

    fun onIntent(intent: HomeIntent) {
        when (intent) {
            is HomeIntent.LoadItems -> loadItems()
            is HomeIntent.DeleteItem -> deleteItem(intent.id)
            is HomeIntent.RetryLoad -> loadItems()
        }
    }

    private fun loadItems() {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true, error = null) }
            repository.getItems()
                .onSuccess { items -> _uiState.update { it.copy(items = items, isLoading = false) } }
                .onFailure { e -> _uiState.update { it.copy(error = e.message, isLoading = false) } }
        }
    }

    private fun deleteItem(id: String) {
        viewModelScope.launch {
            repository.delete(id)
            _uiState.update { it.copy(items = it.items.filter { item -> item.id != id }) }
        }
    }
}

// 4. View — renders state, emits intents
@Composable
fun HomeScreen(viewModel: HomeViewModel = hiltViewModel()) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()

    LaunchedEffect(Unit) { viewModel.onIntent(HomeIntent.LoadItems) }

    when {
        uiState.isLoading -> LoadingIndicator()
        uiState.error != null -> ErrorScreen(
            message = uiState.error!!,
            viewModel.onIntent(HomeIntent.RetryLoad) },
        )
        else -> ItemList(
            items = uiState.items,
            id -> viewModel.onIntent(HomeIntent.DeleteItem(id)) },
        )
    }
}

Project Structure

app/
├── MainActivity.kt
├── navigation/
│   └── AppNavGraph.kt
├── feature/
│   ├── home/
│   │   ├── HomeScreen.kt
│   │   ├── HomeViewModel.kt
│   │   ├── HomeUiState.kt
│   │   └── HomeIntent.kt
│   ├── profile/
│   └── settings/
├── core/
│   ├── data/
│   │   ├── repository/
│   │   └── model/
│   ├── network/
│   └── database/
└── ui/
    ├── theme/
    └── components/

Rules

• Organize by feature, not by technical layer.
• Unidirectional Data Flow (UDF) — intents flow up, state flows down.
• Single state per screen — one UiState data class, one StateFlow.
• Explicit intents — all user actions are modeled as sealed interface members. No ad-hoc methods on ViewModel.
• Repository pattern — single source of truth for data. Repositories expose Flows.
• Use case classes (optional) — encapsulate complex business logic. Skip for simple CRUD.
• One screen composable per file. ViewModel per screen.
• Adapt to existing architecture. If the project uses MVVM/MVP/MVC, follow the established pattern. Propose MVI for new screens or new projects.

For multi-module architecture, Gradle setup, and build variants see references/project-structure.md.

Dependency Injection

// Hilt (recommended)
@HiltViewModel
class HomeViewModel @Inject constructor(
    private val repository: ItemRepository,
) : ViewModel() { ... }

@Module
@InstallIn(SingletonComponent::class)
abstract class RepositoryModule {
    @Binds
    abstract fun bindItemRepository(impl: DefaultItemRepository): ItemRepository
}

// Manual DI (for small projects or libraries)
class AppContainer {
    private val api: ApiService by lazy { RetrofitApiService() }
    val repository: ItemRepository by lazy { DefaultItemRepository(api) }
}

Rules:

• Hilt for apps — standard Android DI, integrates with ViewModel, WorkManager, Navigation.
• Manual DI or Koin for libraries or KMP shared modules.
• Inject interfaces, not implementations.
• Use @Singleton sparingly — scope to the narrowest lifecycle.

Testable Design

• Inject dependencies via constructor. ViewModels receive repositories, not context.
• Repository interfaces — swap real implementations with fakes in tests.
• UI state as data class — easy to assert in unit tests.
• Compose testing — createComposeRule(), semantic matchers, onNodeWithText.

class HomeViewModelTest {
    private val fakeRepository = FakeItemRepository()
    private val viewModel = HomeViewModel(fakeRepository)

    @Test
    fun `load items updates state`() = runTest {
        fakeRepository.emit(listOf(Item("1", "Test")))
        viewModel.load()
        assertEquals(listOf(Item("1", "Test")), viewModel.uiState.value.items)
    }
}

Test naming: fun 'description of behavior'() with backtick syntax, or test_method_condition_expected().

Platform-Specific Guidance

The core skill covers Android phone by default. For other platforms, consult the corresponding reference:

Quick Reference: Common Mistakes