kotlin-coroutines

name

kotlin-coroutines

description

Kotlin coroutines for Android/KMP — suspend functions, cancellation safety, structured concurrency, parallel execution, synchronization, scopes, blocking code wrapping, and cooperative cancellation. Use this skill whenever writing or reviewing coroutines, suspend functions, coroutine scopes, or any asynchronous Kotlin code that is NOT primarily about Flows. Trigger on phrases like "coroutine", "suspend", "async", "launch", "withContext", "Dispatchers", "ensureActive", "NonCancellable", "Mutex", "limitedParallelism", "coroutineScope", "supervisorScope", "viewModelScope", "application scope", "parallel execution", "blocking code", "runBlocking", "cancellation", or "yield".

Kotlin Coroutines (Android / KMP)

For Flow types, operators, stateIn/shareIn, backpressure, and lifecycle-aware collection, see kotlin-flows.

Cancellation Safety: ensureActive()

When catching Exception or Throwable in a suspend context that contains multiple sequential suspend calls, always call coroutineContext.ensureActive() as the first line of the catch block. This prevents silently swallowing CancellationException, which would leave the coroutine running after cancellation and cause inconsistent state.

Correct:

suspend fun syncData() {
    try {
        val data = remoteDataSource.fetch()   // suspend call 1
        localDataSource.insert(data)          // suspend call 2
    } catch (e: Exception) {
        coroutineContext.ensureActive()        // ← FIRST LINE — rethrows if cancelled
        // Handle the actual error (network, DB, etc.)
        logger.error("Sync failed", e)
    }
}

Wrong — swallows CancellationException:

suspend fun syncData() {
    try {
        val data = remoteDataSource.fetch()
        localDataSource.insert(data)
    } catch (e: Exception) {
        // BUG: If cancellation happened between fetch() and insert(),
        // CancellationException is caught here and silently swallowed.
        logger.error("Sync failed", e)
    }
}

If the catch block only contains a single suspend call or no suspend calls at all, ensureActive() is still good practice but less critical.

---

NonCancellable and Atomic Operations

Scenario	Approach
Cleanup work (close resource, flush log)	withContext(NonCancellable) { ... }
Multi-step suspend calls that must all complete	Inject applicationScope, launch in it
Multiple database writes that must be atomic	Room withTransaction { }

Cleanup — NonCancellable:

suspend fun disconnect() {
    withContext(NonCancellable) {
        socket.sendCloseFrame()
        socket.awaitClose()
    }
}

Multi-step atomic — application scope:

class SyncRepository(
    private val applicationScope: CoroutineScope,
    private val remoteDataSource: RemoteDataSource,
    private val localDataSource: LocalDataSource
) {
    suspend fun syncAndNotify() {
        // Runs independently of the caller's lifecycle.
        // If the ViewModel is cleared mid-sync, this still completes.
        applicationScope.launch {
            val data = remoteDataSource.fetch()
            localDataSource.insert(data)
            notificationService.notifyComplete()
        }.join() // optional: suspend until done
    }
}

Database transaction — preferred over application scope for DB:

suspend fun moveNote(noteId: String, targetFolderId: String) {
    db.withTransaction {
        noteDao.removeFromCurrentFolder(noteId)
        noteDao.insertIntoFolder(noteId, targetFolderId)
    }
}

Never use withContext(NonCancellable) for business logic or multi-step operations — it prevents the coroutine from being cancelled during that block, which is only appropriate for short cleanup work.

---

Blocking Code and Main Safety

Rule: Never call blocking code directly in a suspend function or coroutine. Always wrap it in a dedicated class that exposes a suspend function with the correct dispatcher.

Work type	Dispatcher
File I/O, network, database, stream reads/writes	Dispatchers.IO
CPU-heavy (image compression, JSON parsing, sorting large lists)	Dispatchers.Default

class FileReader(private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO) {
    suspend fun readFile(path: Path): ByteArray = withContext(ioDispatcher) {
        path.toFile().readBytes()   // blocking call — safe inside withContext(IO)
    }
}

class ImageCompressor(private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default) {
    suspend fun compress(bitmap: Bitmap, quality: Int): ByteArray = withContext(defaultDispatcher) {
        val stream = ByteArrayOutputStream()
        bitmap.compress(Bitmap.CompressFormat.JPEG, quality, stream)
        stream.toByteArray()
    }
}

Never use Thread.sleep() in a coroutine. Use delay() instead — it suspends without blocking the thread.

---

Parallel Execution

Rule: Never execute two independent suspend functions sequentially. Use async or separate launch blocks.

Fail-fast — coroutineScope + async

If one fails, all siblings are cancelled:

suspend fun loadDashboard(): Dashboard = coroutineScope {
    val userDeferred = async { userRepository.getUser() }
    val statsDeferred = async { statsRepository.getStats() }
    val notificationsDeferred = async { notificationRepository.getRecent() }

    Dashboard(
        user = userDeferred.await(),
        stats = statsDeferred.await(),
        notifications = notificationsDeferred.await()
    )
}

Independent — supervisorScope + async

If one fails, the others continue:

suspend fun loadDashboard(): Dashboard = supervisorScope {
    val userDeferred = async { userRepository.getUser() }
    val statsDeferred = async {
        try {
            statsRepository.getStats()
        } catch (e: Exception) {
            coroutineContext.ensureActive()
            null  // stats are optional
        }
    }

    Dashboard(
        user = userDeferred.await(),
        stats = statsDeferred.await()
    )
}

awaitAll for collecting results

suspend fun fetchAllPages(urls: List<String>): List<Page> = coroutineScope {
    urls.map { url ->
        async { api.fetchPage(url) }
    }.awaitAll()
}

Separate launch blocks — when return values aren't needed

viewModelScope.launch {
    launch { analyticsTracker.trackScreenView("profile") }
    launch { prefetchService.prefetchRelatedData(userId) }
    // Both run in parallel, neither blocks the other
}

---

Synchronization

Need	Tool
Protect a critical section of suspend code	Mutex
Confine all execution (including non-suspend) to one thread	limitedParallelism(1)

Mutex — for suspend-only critical sections

class InMemoryCache<K, V> {
    private val mutex = Mutex()
    private val map = mutableMapOf<K, V>()

    suspend fun getOrPut(key: K, compute: suspend () -> V): V {
        mutex.withLock {
            map[key]?.let { return it }
            val value = compute()
            map[key] = value
            return value
        }
    }
}

limitedParallelism(1) — for thread confinement

class TokenStorage(
    private val prefs: SharedPreferences
) {
    // Single-thread dispatcher — safe for non-suspend SharedPreferences calls
    private val dispatcher = Dispatchers.IO.limitedParallelism(1)

    suspend fun saveToken(token: String) = withContext(dispatcher) {
        prefs.edit().putString("token", token).commit()
    }

    suspend fun getToken(): String? = withContext(dispatcher) {
        prefs.getString("token", null)
    }
}

Use Dispatchers.IO.limitedParallelism(1) for I/O-bound confined work, Dispatchers.Default.limitedParallelism(1) for CPU-bound confined work.

---

Custom Scopes and Application Scope

Custom coroutine scopes should use SupervisorJob() so one child failure doesn't cancel the entire scope:

val myScope = CoroutineScope(SupervisorJob() + Dispatchers.Default)

Application scope — Android / KMP

Create in the Application class and inject via Koin:

class MyApp : Application() {
    val applicationScope = CoroutineScope(SupervisorJob() + Dispatchers.Default)
}

// Koin module
val appModule = module {
    single<CoroutineScope>(named("applicationScope")) {
        (androidApplication() as MyApp).applicationScope
    }
}

// Inject where needed
class SyncRepository(
    @Named("applicationScope") private val applicationScope: CoroutineScope,
    private val api: Api,
    private val dao: NoteDao
) {
    suspend fun syncNotes() {
        applicationScope.launch {
            val notes = api.fetchNotes()
            dao.upsertAll(notes)
        }.join()
    }
}

When to use application scope: Multi-step operations that must outlive the ViewModel or screen lifecycle (e.g., sync that started from a screen but must finish even if the user navigates away). If the multi-step work is purely database operations, prefer a Room transaction instead.

---

Cooperative Cancellation

ensureActive() — cheap check in loops

suspend fun processItems(items: List<Item>) {
    for (item in items) {
        ensureActive()  // throws CancellationException if cancelled — cheap, no thread yield
        processItem(item)
    }
}

yield() — when converting blocking code to suspend

suspend fun processLargeFile(file: File) = withContext(Dispatchers.IO) {
    file.bufferedReader().useLines { lines ->
        lines.forEach { line ->
            yield()  // checks cancellation AND yields the thread to other coroutines
            parseLine(line)
        }
    }
}

Pick ensureActive() for loops (cheaper — just checks a flag). Pick yield() when wrapping blocking code (yields the thread so other coroutines on the same dispatcher can run).

---

runBlocking

runBlocking is allowed only in:

1. Tests — runTest is preferred, but runBlocking works for simple cases
2. Plain Kotlin main() function — entry point that bridges to suspend world
3. Contexts where coroutines can't be launched AND you're certain it's a background thread — e.g., a Retrofit Interceptor (OkHttp guarantees this runs on its own thread pool)

// Retrofit interceptor — OK because OkHttp runs interceptors on background threads
class AuthInterceptor(private val tokenProvider: TokenProvider) : Interceptor {
    override fun intercept(chain: Interceptor.Chain): Response {
        val token = runBlocking { tokenProvider.getToken() }
        val request = chain.request().newBuilder()
            .addHeader("Authorization", "Bearer $token")
            .build()
        return chain.proceed(request)
    }
}

NEVER use runBlocking:

• On the Android main thread — it blocks the UI
• Inside an existing coroutine — it blocks the thread and can cause deadlocks
• In any Android production code where a coroutine scope is available

---

Testing

See android-testing skill for full coroutine test patterns including runTest, Turbine, UnconfinedTestDispatcher, and Dispatchers.setMain().

Testability rule: Inject CoroutineDispatcher in any class that dispatches to a non-Main dispatcher and is unit-tested. This lets tests replace Dispatchers.IO / Dispatchers.Default with a test dispatcher:

class NoteRepository(
    private val api: NoteApi,
    private val dao: NoteDao,
    private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO
) {
    suspend fun refreshNotes() = withContext(ioDispatcher) {
        val notes = api.fetchNotes()
        dao.upsertAll(notes)
    }
}

---

Common Mistakes

Mistake	Fix
Catching Exception without ensureActive()	Add coroutineContext.ensureActive() as first catch line
Sequential independent suspend calls	async + awaitAll or parallel launch
Thread.sleep() in coroutine	delay()
withContext(NonCancellable) for business logic	Use application scope or DB transaction
runBlocking on main thread	viewModelScope.launch or proper scope
GlobalScope.launch	Inject application scope via DI

---

Checklist: Writing Coroutine Code

• ensureActive() is the first line in every catch(e: Exception) / catch(e: Throwable) in suspend context with sequential suspend calls
• No blocking calls without withContext(Dispatchers.IO) or withContext(Dispatchers.Default)
• Independent suspend calls run in parallel (async / launch), never sequential
• NonCancellable used only for cleanup — not business logic
• Multi-step DB operations use transactions, not application scope
• No runBlocking on main thread or inside coroutines
• delay() used instead of Thread.sleep() in all coroutine contexts
• Synchronization uses the correct tool (Mutex for suspend-only, limitedParallelism(1) for thread confinement)
• Custom scopes use SupervisorJob() + appropriate dispatcher
• No GlobalScope — use injected application scope
• Loops call ensureActive() for cooperative cancellation
• Blocking-to-suspend conversions use yield() for cooperative cancellation