kotlin-concurrency-expert

Kotlin Concurrency Expert

Overview

Review and fix Kotlin Coroutines issues in Android codebases by applying structured concurrency, lifecycle safety, proper scoping, and modern best practices with minimal behavior changes.

Workflow

1. Triage the Issue

• Capture the exact error, crash, or symptom (ANR, memory leak, race condition, incorrect state).
• Check project coroutines setup: kotlinx-coroutines-android version, lifecycle-runtime-ktx version.
• Identify the current scope context (viewModelScope, lifecycleScope, custom scope, or none).
• Confirm whether the code is UI-bound (Dispatchers.Main) or intended to run off the main thread (Dispatchers.IO, Dispatchers.Default).
• Verify Dispatcher injection patterns for testability.

2. Apply the Smallest Safe Fix

Prefer edits that preserve existing behavior while satisfying structured concurrency and lifecycle safety.

Common fixes:

• ANR / Main thread blocking: Move heavy work to withContext(Dispatchers.IO) or Dispatchers.Default; ensure suspend functions are main-safe.
• Memory leaks / zombie coroutines: Replace GlobalScope with a lifecycle-bound scope (viewModelScope, lifecycleScope, or injected applicationScope).
• Lifecycle collection issues: Replace deprecated launchWhenStarted with repeatOnLifecycle(Lifecycle.State.STARTED).
• State exposure: Encapsulate MutableStateFlow / MutableSharedFlow; expose read-only StateFlow or Flow.
• CancellationException swallowing: Ensure generic catch (e: Exception) blocks rethrow CancellationException.
• Non-cooperative cancellation: Add ensureActive() or yield() in tight loops for cooperative cancellation.
• Callback APIs: Convert listeners to callbackFlow with proper awaitClose cleanup.
• Hardcoded Dispatchers: Inject CoroutineDispatcher via constructor for testability.

Critical Rules

Dispatcher Injection (Testability)

// CORRECT: Inject dispatcher
class UserRepository(
    private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO
) {
    suspend fun fetchUser() = withContext(ioDispatcher) { ... }
}

// INCORRECT: Hardcoded dispatcher
class UserRepository {
    suspend fun fetchUser() = withContext(Dispatchers.IO) { ... }
}

Lifecycle-Aware Collection

// CORRECT: Use repeatOnLifecycle
viewLifecycleOwner.lifecycleScope.launch {
    viewLifecycleOwner.repeatOnLifecycle(Lifecycle.State.STARTED) {
        viewModel.uiState.collect { state -> updateUI(state) }
    }
}

// INCORRECT: Direct collection (unsafe, deprecated)
lifecycleScope.launchWhenStarted {
    viewModel.uiState.collect { state -> updateUI(state) }
}

State Encapsulation

// CORRECT: Expose read-only StateFlow
class MyViewModel : ViewModel() {
    private val _uiState = MutableStateFlow(UiState())
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()
}

// INCORRECT: Exposed mutable state
class MyViewModel : ViewModel() {
    val uiState = MutableStateFlow(UiState()) // Leaks mutability
}

Exception Handling

// CORRECT: Rethrow CancellationException
try {
    doSuspendWork()
} catch (e: CancellationException) {
    throw e // Must rethrow!
} catch (e: Exception) {
    handleError(e)
}

// INCORRECT: Swallows cancellation
try {
    doSuspendWork()
} catch (e: Exception) {
    handleError(e) // CancellationException swallowed!
}

Cooperative Cancellation

// CORRECT: Check for cancellation in tight loops
suspend fun processLargeList(items: List<Item>) {
    items.forEach { item ->
        ensureActive() // Check cancellation
        processItem(item)
    }
}

// INCORRECT: Non-cooperative (ignores cancellation)
suspend fun processLargeList(items: List<Item>) {
    items.forEach { item ->
        processItem(item) // Never checks cancellation
    }
}

Callback Conversion

// CORRECT: callbackFlow with awaitClose
fun locationUpdates(): Flow<Location> = callbackFlow {
    val listener = LocationListener { location ->
        trySend(location)
    }
    locationManager.requestLocationUpdates(listener)
    
    awaitClose { locationManager.removeUpdates(listener) }
}

Scope Guidelines

Scope	Use When	Lifecycle
viewModelScope	ViewModel operations	Cleared with ViewModel
lifecycleScope	UI operations in Activity/Fragment	Destroyed with lifecycle owner
repeatOnLifecycle	Flow collection in UI	Started/Stopped with lifecycle state
applicationScope (injected)	App-wide background work	Application lifetime
GlobalScope	NEVER USE	Breaks structured concurrency

Testing Pattern

@Test
fun `loading data updates state`() = runTest {
    val testDispatcher = StandardTestDispatcher(testScheduler)
    val repository = FakeRepository()
    val viewModel = MyViewModel(repository, testDispatcher)
    
    viewModel.loadData()
    advanceUntilIdle()
    
    assertEquals(UiState.Success(data), viewModel.uiState.value)
}

Reference Material

• Kotlin Coroutines Best Practices
• StateFlow and SharedFlow
• repeatOnLifecycle API