modern-app-patterns

Coroutines + Flow Basics

Use structured concurrency and cold streams for async work.

Snippets

suspend fun <T> retry(times: Int, block: suspend () -> T): T {
  repeat(times - 1) { runCatching { return block() } }
  return block()
}

fun <T> Flow<T>.asState(initial: T): StateFlow<T> =
  this.stateIn(scope = CoroutineScope(SupervisorJob() + Dispatchers.Main.immediate), started = SharingStarted.WhileSubscribed(5_000), initialValue = initial)

viewModelScope.launch { withContext(Dispatchers.IO) { repo.sync() } }

Notes

• Prefer SupervisorJob for UI scopes.
• Always cancel in onCleared() if you create custom scopes.

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Live end-to-end example (copy/paste)

Flow from repository → ViewModel StateFlow → Compose UI.

// data/TickerRepo.kt
interface TickerRepo { fun stream(): Flow<Int> }
class FakeTickerRepo : TickerRepo {
  override fun stream(): Flow<Int> = flow {
    var i = 0
    while (true) { emit(i++); delay(1000) }
  }
}

// ui/TickerViewModel.kt
@HiltViewModel
class TickerViewModel @Inject constructor(repo: TickerRepo): ViewModel() {
  val count: StateFlow<Int> = repo.stream()
    .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), 0)
}

// ui/TickerScreen.kt
@Composable
fun TickerScreen(vm: TickerViewModel = hiltViewModel()) {
  val c by vm.count.collectAsStateWithLifecycle()
  Text("Tick: $c")
}

Notes

• stateIn converts a cold flow to a hot StateFlow bound to the VM scope.

Sandbox copy map

Paste into Android Studio project (see sandboxes/android-compose):

• data/TickerRepo.kt — flow source
• ui/TickerViewModel.kt — StateFlow exposure
• ui/TickerScreen.kt — UI consuming the flow

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