Flow/StateFlow Practical Patterns Collection — combine/flatMap/debounce/retry

#android #kotlin #jetpackcompose #programming

What You'll Learn

Flow/StateFlow実践パターン（combine、flatMapLatest、debounce、retry、callbackFlow、テスト）を解説します。

combine: Combining Multiple Flows

@HiltViewModel
class DashboardViewModel @Inject constructor(
    userRepository: UserRepository,
    orderRepository: OrderRepository,
    notificationRepository: NotificationRepository
) : ViewModel() {

    val dashboardState = combine(
        userRepository.getUser(),
        orderRepository.getRecentOrders(),
        notificationRepository.getUnreadCount()
    ) { user, orders, unreadCount ->
        DashboardState(user, orders, unreadCount)
    }.stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), DashboardState())
}

flatMapLatest: Search

@HiltViewModel
class SearchViewModel @Inject constructor(
    private val repository: SearchRepository
) : ViewModel() {
    private val _query = MutableStateFlow("")

    val results = _query
        .debounce(300)  // 300ms待ってから検索
        .distinctUntilChanged()
        .flatMapLatest { query ->
            if (query.isBlank()) flowOf(emptyList())
            else repository.search(query)
        }
        .catch { emit(emptyList()) }
        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), emptyList())

    fun onQueryChange(query: String) { _query.value = query }
}

retry: Error Retry

fun getDataWithRetry(): Flow<List<Item>> = flow {
    emit(api.getItems())
}.retry(3) { cause ->
    cause is IOException && run {
        delay(1000)
        true
    }
}.catch { emit(emptyList()) }

// exponential backoff
fun <T> Flow<T>.retryWithBackoff(
    maxRetries: Int = 3,
    initialDelay: Long = 1000
): Flow<T> = retryWhen { cause, attempt ->
    if (attempt < maxRetries && cause is IOException) {
        delay(initialDelay * (1 shl attempt.toInt()))
        true
    } else false
}

callbackFlow: Callback to Flow Conversion

fun locationUpdates(context: Context): Flow<Location> = callbackFlow {
    val client = LocationServices.getFusedLocationProviderClient(context)
    val request = LocationRequest.Builder(Priority.PRIORITY_HIGH_ACCURACY, 5000).build()

    val callback = object : LocationCallback() {
        override fun onLocationResult(result: LocationResult) {
            result.lastLocation?.let { trySend(it) }
        }
    }

    client.requestLocationUpdates(request, callback, Looper.getMainLooper())
    awaitClose { client.removeLocationUpdates(callback) }
}

stateIn vs shareIn

// stateIn: 最新値を保持（UI状態向け）
val uiState = flow.stateIn(
    scope = viewModelScope,
    started = SharingStarted.WhileSubscribed(5000),  // 5秒キャッシュ
    initialValue = UiState.Loading
)

// shareIn: イベントストリーム（通知向け）
val events = flow.shareIn(
    scope = viewModelScope,
    started = SharingStarted.Eagerly,
    replay = 0  // 新しいサブスクライバに過去のイベントを送らない
)

Testing (Turbine)

@Test
fun searchReturnsResults() = runTest {
    val viewModel = SearchViewModel(FakeSearchRepository())

    viewModel.results.test {
        assertEquals(emptyList<SearchResult>(), awaitItem()) // 初期値

        viewModel.onQueryChange("kotlin")
        advanceTimeBy(300) // debounce待ち

        val results = awaitItem()
        assertTrue(results.isNotEmpty())
    }
}

Summary

パターン	用途
`combine`	Combine multiple flows
`flatMapLatest`	Process latest only
`debounce`	Wait for input
`retry`	Error retry
`callbackFlow`	Callback conversion
`stateIn`	Share UI state

combineでIntegrate multiple data sources
debounce + flatMapLatestでSearch optimization
callbackFlowでLegacy API to Flow
WhileSubscribed(5000)でBackground optimization

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