feat: add Kotlin, Android, and KMP rules, agent, skills, and command

skills/kotlin-coroutines-flows/SKILL.md

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+---
+name: kotlin-coroutines-flows
+description: Kotlin Coroutines and Flow patterns for Android and KMP — structured concurrency, Flow operators, StateFlow, error handling, and testing.
+origin: ECC
+---
+
+# Kotlin Coroutines & Flows
+
+Patterns for structured concurrency, Flow-based reactive streams, and coroutine testing in Android and Kotlin Multiplatform projects.
+
+## When to Activate
+
+- Writing async code with Kotlin coroutines
+- Using Flow, StateFlow, or SharedFlow for reactive data
+- Handling concurrent operations (parallel loading, debounce, retry)
+- Testing coroutines and Flows
+- Managing coroutine scopes and cancellation
+
+## Structured Concurrency
+
+### Scope Hierarchy
+
+```
+Application
+  └── viewModelScope (ViewModel)
+        └── coroutineScope { } (structured child)
+              ├── async { } (concurrent task)
+              └── async { } (concurrent task)
+```
+
+Always use structured concurrency — never `GlobalScope`:
+
+```kotlin
+// BAD
+GlobalScope.launch { fetchData() }
+
+// GOOD — scoped to ViewModel lifecycle
+viewModelScope.launch { fetchData() }
+
+// GOOD — scoped to composable lifecycle
+LaunchedEffect(key) { fetchData() }
+```
+
+### Parallel Decomposition
+
+Use `coroutineScope` + `async` for parallel work:
+
+```kotlin
+suspend fun loadDashboard(): Dashboard = coroutineScope {
+    val items = async { itemRepository.getRecent() }
+    val stats = async { statsRepository.getToday() }
+    val profile = async { userRepository.getCurrent() }
+    Dashboard(
+        items = items.await(),
+        stats = stats.await(),
+        profile = profile.await()
+    )
+}
+```
+
+### SupervisorScope
+
+Use `supervisorScope` when child failures should not cancel siblings:
+
+```kotlin
+suspend fun syncAll() = supervisorScope {
+    launch { syncItems() }       // failure here won't cancel syncStats
+    launch { syncStats() }
+    launch { syncSettings() }
+}
+```
+
+## Flow Patterns
+
+### Cold Flow — One-Shot to Stream Conversion
+
+```kotlin
+fun observeItems(): Flow<List<Item>> = flow {
+    // Re-emits whenever the database changes
+    itemDao.observeAll()
+        .map { entities -> entities.map { it.toDomain() } }
+        .collect { emit(it) }
+}
+```
+
+### StateFlow for UI State
+
+```kotlin
+class DashboardViewModel(
+    observeProgress: ObserveUserProgressUseCase
+) : ViewModel() {
+    val progress: StateFlow<UserProgress> = observeProgress()
+        .stateIn(
+            scope = viewModelScope,
+            started = SharingStarted.WhileSubscribed(5_000),
+            initialValue = UserProgress.EMPTY
+        )
+}
+```
+
+`WhileSubscribed(5_000)` keeps the upstream active for 5 seconds after the last subscriber leaves — survives configuration changes without restarting.
+
+### Combining Multiple Flows
+
+```kotlin
+val uiState: StateFlow<HomeState> = combine(
+    itemRepository.observeItems(),
+    settingsRepository.observeTheme(),
+    userRepository.observeProfile()
+) { items, theme, profile ->
+    HomeState(items = items, theme = theme, profile = profile)
+}.stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), HomeState())
+```
+
+### Flow Operators
+
+```kotlin
+// Debounce search input
+searchQuery
+    .debounce(300)
+    .distinctUntilChanged()
+    .flatMapLatest { query -> repository.search(query) }
+    .catch { emit(emptyList()) }
+    .collect { results -> _state.update { it.copy(results = results) } }
+
+// Retry with exponential backoff
+fun fetchWithRetry(): Flow<Data> = flow { emit(api.fetch()) }
+    .retry(3) { cause ->
+        cause is IOException && run { delay(1000L * (1 shl (3 - remainingAttempts))) ; true }
+    }
+```
+
+### SharedFlow for One-Time Events
+
+```kotlin
+class ItemListViewModel : ViewModel() {
+    private val _effects = MutableSharedFlow<Effect>()
+    val effects: SharedFlow<Effect> = _effects.asSharedFlow()
+
+    sealed interface Effect {
+        data class ShowSnackbar(val message: String) : Effect
+        data class NavigateTo(val route: String) : Effect
+    }
+
+    private fun deleteItem(id: String) {
+        viewModelScope.launch {
+            repository.delete(id)
+            _effects.emit(Effect.ShowSnackbar("Item deleted"))
+        }
+    }
+}
+
+// Collect in Composable
+LaunchedEffect(Unit) {
+    viewModel.effects.collect { effect ->
+        when (effect) {
+            is Effect.ShowSnackbar -> snackbarHostState.showSnackbar(effect.message)
+            is Effect.NavigateTo -> navController.navigate(effect.route)
+        }
+    }
+}
+```
+
+## Dispatchers
+
+```kotlin
+// CPU-intensive work
+withContext(Dispatchers.Default) { parseJson(largePayload) }
+
+// IO-bound work
+withContext(Dispatchers.IO) { database.query() }
+
+// Main thread (UI) — default in viewModelScope
+withContext(Dispatchers.Main) { updateUi() }
+```
+
+In KMP, use `Dispatchers.Default` and `Dispatchers.Main` (available on all platforms). `Dispatchers.IO` is JVM/Android only — use `Dispatchers.Default` on other platforms or provide via DI.
+
+## Cancellation
+
+### Cooperative Cancellation
+
+Long-running loops must check for cancellation:
+
+```kotlin
+suspend fun processItems(items: List<Item>) {
+    for (item in items) {
+        ensureActive()  // throws CancellationException if cancelled
+        process(item)
+    }
+}
+```
+
+### Cleanup with try/finally
+
+```kotlin
+viewModelScope.launch {
+    try {
+        _state.update { it.copy(isLoading = true) }
+        val data = repository.fetch()
+        _state.update { it.copy(data = data) }
+    } finally {
+        _state.update { it.copy(isLoading = false) }  // always runs, even on cancellation
+    }
+}
+```
+
+## Testing
+
+### Testing StateFlow with Turbine
+
+```kotlin
+@Test
+fun `search updates item list`() = runTest {
+    val fakeRepository = FakeItemRepository(items = testItems)
+    val viewModel = ItemListViewModel(GetItemsUseCase(fakeRepository))
+
+    viewModel.state.test {
+        assertEquals(ItemListState(), awaitItem())  // initial
+
+        viewModel.onSearch("query")
+        val loading = awaitItem()
+        assertTrue(loading.isLoading)
+
+        val loaded = awaitItem()
+        assertFalse(loaded.isLoading)
+        assertEquals(1, loaded.items.size)
+    }
+}
+```
+
+### Testing with TestDispatcher
+
+```kotlin
+@Test
+fun `parallel load completes correctly`() = runTest {
+    val viewModel = DashboardViewModel(
+        itemRepo = FakeItemRepo(),
+        statsRepo = FakeStatsRepo()
+    )
+
+    viewModel.load()
+    advanceUntilIdle()
+
+    val state = viewModel.state.value
+    assertNotNull(state.items)
+    assertNotNull(state.stats)
+}
+```
+
+### Faking Flows
+
+```kotlin
+class FakeItemRepository : ItemRepository {
+    private val _items = MutableStateFlow<List<Item>>(emptyList())
+
+    override fun observeItems(): Flow<List<Item>> = _items
+
+    fun emit(items: List<Item>) { _items.value = items }
+
+    override suspend fun getItemsByCategory(category: String): Result<List<Item>> {
+        return Result.success(_items.value.filter { it.category == category })
+    }
+}
+```
+
+## Anti-Patterns to Avoid
+
+- Using `GlobalScope` — leaks coroutines, no structured cancellation
+- Collecting Flows in `init {}` without a scope — use `viewModelScope.launch`
+- Using `MutableStateFlow` with mutable collections — always use immutable copies: `_state.update { it.copy(list = it.list + newItem) }`
+- Catching `CancellationException` — let it propagate for proper cancellation
+- Using `flowOn(Dispatchers.Main)` to collect — collection dispatcher is the caller's dispatcher
+- Creating `Flow` in `@Composable` without `remember` — recreates the flow every recomposition
+
+## References
+
+See skill: `compose-multiplatform-patterns` for UI consumption of Flows.
+See skill: `android-clean-architecture` for where coroutines fit in layers.