Coroutines Patterns

---
name: coroutines-patterns
description: Kotlin Coroutines and Flow patterns for structured concurrency, error handling, and async operations.
---

# Coroutines Patterns

Structured concurrency for Kotlin.

## Coroutine Scopes

```kotlin
// ✅ ViewModel scope (auto-cancelled)
class HomeViewModel : ViewModel() {
    fun loadData() {
        viewModelScope.launch {
            // Cancelled when ViewModel cleared
        }
    }
}

// ✅ Lifecycle scope
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        lifecycleScope.launch {
            // Cancelled when lifecycle destroyed
        }
    }
}

// ❌ AVOID: GlobalScope
GlobalScope.launch { }  // Never cancelled, memory leaks
```

## Dispatchers

```kotlin
// Main - UI operations
withContext(Dispatchers.Main) {
    textView.text = "Updated"
}

// IO - Network, disk
withContext(Dispatchers.IO) {
    api.fetchData()
    database.query()
}

// Default - CPU intensive
withContext(Dispatchers.Default) {
    list.sortedBy { it.score }
}
```

## Flow Patterns

```kotlin
// StateFlow for UI state
private val _state = MutableStateFlow(HomeState())
val state: StateFlow<HomeState> = _state.asStateFlow()

// SharedFlow for events
private val _events = MutableSharedFlow<Event>()
val events: SharedFlow<Event> = _events.asSharedFlow()

// Collect with lifecycle
@Composable
fun HomeScreen(viewModel: HomeViewModel) {
    val state by viewModel.state.collectAsStateWithLifecycle()
}
```

## Flow Operators

```kotlin
flow
    .filter { it.isActive }
    .map { transform(it) }
    .distinctUntilChanged()
    .debounce(300)
    .catch { emit(fallback) }
    .collect { process(it) }
```

## Error Handling

```kotlin
// Try-catch in coroutine
viewModelScope.launch {
    try {
        val result = repository.fetchData()
        _state.value = Success(result)
    } catch (e: Exception) {
        _state.value = Error(e.message)
    }
}

// supervisorScope - siblings don't cancel
supervisorScope {
    launch { task1() }  // Failure doesn't cancel task2
    launch { task2() }
}
```

## Cancellation

```kotlin
// Cooperative cancellation
suspend fun processItems(items: List<Item>) {
    items.forEach { item ->
        ensureActive()  // Check cancellation
        process(item)
    }
}

// CancellationException handling
try {
    coroutineWork()
} catch (e: CancellationException) {
    throw e  // Don't swallow!
} catch (e: Exception) {
    handleError(e)
}
```

---

**Remember**: Structured concurrency = lifecycle-bound, cancellable, debuggable.