kotlin-patterns

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# Kotlin Patterns

Modern Kotlin patterns for Android, server-side, and multiplatform development.

## Coroutine Patterns

```kotlin
import kotlinx.coroutines.*

// Structured concurrency: child failures cancel siblings
suspend fun loadDashboard(): Dashboard = coroutineScope {
    val profile = async { api.fetchProfile() }
    val orders = async { api.fetchRecentOrders() }
    val stats = async { api.fetchStats() }

    // All run concurrently; if one fails, others are cancelled
    Dashboard(
        profile = profile.await(),
        orders = orders.await(),
        stats = stats.await()
    )
}

// SupervisorScope: child failures don't cancel siblings
suspend fun loadOptionalData(): HomeScreen = supervisorScope {
    val required = async { api.fetchRequiredData() }
    val optional = async {
        try { api.fetchRecommendations() }
        catch (e: Exception) { emptyList() }  // Graceful fallback
    }

    HomeScreen(
        data = required.await(),
        recommendations = optional.await()
    )
}

// Retry with exponential backoff
suspend fun <T> retryWithBackoff(
    maxRetries: Int = 3,
    initialDelayMs: Long = 1000,
    factor: Double = 2.0,
    block: suspend () -> T
): T {
    var currentDelay = initialDelayMs
    repeat(maxRetries - 1) { attempt ->
        try {
            return block()
        } catch (e: Exception) {
            if (e is CancellationException) throw e  // Never swallow cancellation
            delay(currentDelay)
            currentDelay = (currentDelay * factor).toLong()
        }
    }
    return block()  // Last attempt, let exception propagate
}
```

## Flow Operators

```kotlin
import kotlinx.coroutines.flow.*

// Repository pattern with Flow
class OrderRepository(private val api: OrderApi, private val db: OrderDao) {

    // Offline-first: emit cached, then fetch fresh
    fun getOrders(): Flow<List<Order>> = flow {
        // Emit cached data first
        emit(db.getAllOrders())

        // Fetch fresh data
        val fresh = api.fetchOrders()
        db.insertAll(fresh)
        emit(fresh)
    }.catch { e ->
        // On network error, emit cached data
        emit(db.getAllOrders())
    }

    // Debounce search input
    fun search(queryFlow: Flow<String>): Flow<List<Order>> =
        queryFlow
            .debounce(300)
            .distinctUntilChanged()
            .filter { it.length >= 2 }
            .flatMapLatest { query ->
                flow { emit(api.search(query)) }
                    .catch { emit(emptyList()) }
            }

    // Combine multiple flows
    fun getDashboard(): Flow<DashboardState> =
        combine(
            getOrders(),
            getStats(),
            getNotifications()
        ) { orders, stats, notifications ->
            DashboardState(orders, stats, notifications)
        }
}

// StateFlow for UI state (hot flow, always has value)
class OrderViewModel(private val repo: OrderRepository) : ViewModel() {

    private val _uiState = MutableStateFlow<UiState>(UiState.Loading)
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()

    init {
        viewModelScope.launch {
            repo.getOrders()
                .map { orders -> UiState.Success(orders) as UiState }
                .catch { emit(UiState.Error(it.message ?: "Unknown error")) }
                .collect { _uiState.value = it }
        }
    }

    sealed interface UiState {
        data object Loading : UiState

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