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

viewModelScope

lifecycleScope

repeatOnLifecycle

applicationScope

GlobalScope

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