Kotlin Coroutines

Kotlin Coroutines

Built on structured concurrency: every coroutine runs in a scope; cancellation and errors propagate through the parent–child hierarchy. This reference focuses on the two disciplines most often gotten wrong: who owns the scope, and exception handling that doesn't break cancellation.

Main-safety: the function doing blocking work owns the withContext(ioDispatcher); callers never switch dispatchers before calling a suspend function.

Scope ownership — prefer suspend fun, let the caller own the scope

A stored CoroutineScope on a non-UI class (repository, manager, use case, data source) is a strong review signal: the class would have to prove it owns cancellation, error reporting, restart, and lifecycle — most can't. The fix is almost always make the API suspend and let the caller own the scope.

// DO — suspend fun; the caller owns the scope, cancellation propagates, exceptions surface
class ArticlesRepository(private val dataSource: ArticlesDataSource, private val io: CoroutineDispatcher) {
    suspend fun bookmark(article: Article) = withContext(io) { dataSource.bookmark(article) }
}
class BookmarkViewModel(private val repo: ArticlesRepository) : ViewModel() {
    fun onBookmark(a: Article) { viewModelScope.launch { repo.bookmark(a) } }
}

// DO NOT — store a scope and launch inside the repository
class ArticlesRepository(private val externalScope: CoroutineScope, /* ... */) {
    suspend fun bookmark(a: Article) { externalScope.launch { dataSource.bookmark(a) }.join() }
}

Why stored scopes are dangerous: once that scope is cancelled, every future launch on it completes silently as cancelled — no exception, no log. If the cancellation came from process death or a misconfigured DI graph, the class keeps accepting calls and silently does nothing.

When work must outlive the caller

A write that must survive the user navigating away doesn't belong to the repository — it belongs to a WorkManager job, or a named application-scoped class that deliberately owns its scope (an OfflineBookmarkQueue with an injected applicationScope, cancelled only on process death). The named class makes the lifetime explicit, testable, and observable — unlike a buried externalScope.launch.

State-holder carve-out — when launch from a non-suspending method is OK

A UI state holder may launch from a non-suspending event callback only under all three: (1) it actually owns UI state the view layer collects; (2) it uses a lifecycle-bound scope (viewModelScope / rememberCoroutineScope); (3) the trigger is a UI event (click, swipe, lifecycle) — not a repository call, background timer, or DI hook. If any fails, expose a suspend fun and let the real state holder own the scope.

Anti-patterns: init { launch } and DI-singleton launch

init { viewModelScope.launch { while (isActive) { … } } } makes the work invisible — no named trigger, no restart path, and a nav back/forward silently re-launches it. Expose a StateFlow via stateIn instead. Likewise a @Singleton (or a Hilt Initializer) that launches from its constructor starts coroutines "wherever DI realizes me," observable by no one — use WorkManager.enqueueUniquePeriodicWork, invert the work into the consumer, or put it in a named class with an explicit startX() method.

Diagnostic for any DI-bound or stored-scope launch: where is the start moment defined? who can observe whether it's running? can it be restarted without restarting the process? Answers of "wherever / no one / no" mean refactor.

Cancellation

Cooperative: ensureActive() at the top of long loops (throws if cancelled), isActive to check without throwing, yield() to also yield the thread. delay / withContext are already cancellable. Cleanup that must run after cancellation goes in finally { withContext(NonCancellable) { … } } — NonCancellable only there, never as a general escape hatch. TimeoutCancellationException (from withTimeout) is a CancellationException — never catch it without rethrowing. (Don't reach for withTimeout for network timeouts — OkHttp/Retrofit/Ktor own those.)

Exception handling — don't break cancellation

// DO — specific types
try { repo.login(u, p) } catch (e: IOException) { _state.value = Error("Network") }

// DO NOT — catch (e: Exception) / Throwable swallows CancellationException and breaks structured cancellation

• Catch specific types (IOException, HttpException), never Exception / Throwable.
• Never catch CancellationException without rethrowing it.
• Never runCatching in a suspend function — it catches CancellationException. Use a suspendRunCatching that rethrows it:

suspend inline fun <R> suspendRunCatching(block: () -> R): Result<R> = try {
    Result.success(block())
} catch (e: CancellationException) { throw e } catch (e: Throwable) { Result.failure(e) }

• try/catch doesn't wrap launch {} — put it inside the coroutine body.
• An unawaited async {} in a supervisorScope swallows its exception — use launch if you don't need the result.
• viewModelScope already uses SupervisorJob — don't add another alongside it.

KMP

Don't use MainScope() — it's unstructured (Dispatchers.Main + SupervisorJob()), with the same problems as GlobalScope. Inject a lifecycle-bound CoroutineScope from the platform layer (viewModelScope on Android; a view-controller-bound scope on iOS). Inject CoroutineDispatcher for platform implementations; expect/actual Dispatchers.Main.immediate if a target lacks it.

Testing

Coroutine and dispatcher testing lives in android-skills:android-testing — notably the two-schedulers trap (runTest + MainDispatcherRule must share one scheduler) and the StandardTestDispatcher default in Compose tests.