Kotlin Coroutines Expert

Overview

A guide to mastering asynchronous programming with Kotlin Coroutines. Covers advanced topics like structured concurrency, Flow transformations, exception handling, and testing strategies.

When to Use This Skill

• Use when implementing asynchronous operations in Kotlin.
• Use when designing reactive data streams with Flow.
• Use when debugging coroutine cancellations or exceptions.
• Use when writing unit tests for suspending functions or Flows.

Step-by-Step Guide

1. Structured Concurrency

Always launch coroutines within a defined CoroutineScope. Use coroutineScope or supervisorScope to group concurrent tasks.

suspend fun loadDashboardData(): DashboardData = coroutineScope {
    val userDeferred = async { userRepo.getUser() }
    val settingsDeferred = async { settingsRepo.getSettings() }

    DashboardData(
        user = userDeferred.await(),
        settings = settingsDeferred.await()
    )
}

2. Exception Handling

Use CoroutineExceptionHandler for top-level scopes, but rely on try-catch within suspending functions for granular control.

val handler = CoroutineExceptionHandler { _, exception ->
    println("Caught $exception")
}

viewModelScope.launch(handler) {
    try {
        riskyOperation()
    } catch (e: IOException) {
        // Handle network error specifically
    }
}

3. Reactive Streams with Flow

Use StateFlow for state that needs to be retained, and SharedFlow for events.

// Cold Flow (Lazy)
val searchResults: Flow<List<Item>> = searchQuery
    .debounce(300)
    .flatMapLatest { query -> searchRepo.search(query) }
    .flowOn(Dispatchers.IO)

// Hot Flow (State)
val uiState: StateFlow<UiState> = _uiState.asStateFlow()

Examples

Example 1: Parallel Execution with Error Handling

suspend fun fetchDataWithErrorHandling() = supervisorScope {
    val task1 = async {
        try { api.fetchA() } catch (e: CancellationException) { throw e } catch (e: IOException) { null }
    }
    val task2 = async { api.fetchB() }

    // A child failure does not directly cancel its siblings.
    // But an uncaught exception from await escapes the supervisor block and cancels it.
    val result1 = task1.await()
    val result2 = task2.await() // May throw
}

Best Practices

• ✅ Do: Use Dispatchers.IO for blocking I/O operations.
• ✅ Do: Cancel scopes when they are no longer needed (e.g., ViewModel.onCleared).
• ✅ Do: Use TestScope and runTest for unit testing coroutines.
• ❌ Don't: Use GlobalScope. It breaks structured concurrency and can lead to leaks.
• ❌ Don't: Catch CancellationException unless you rethrow it.

Troubleshooting

Problem: Coroutine test hangs or fails unpredictably. Solution: Ensure you are using runTest and injecting TestDispatcher into your classes so you can control virtual time.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

Examples are fragments: supply imports, repository interfaces, Android lifecycle dependencies and the installed coroutines version. Handle each awaited failure deliberately and rethrow cancellation. SharedFlow replay/buffer choices determine delivery while no collector is active; it is not a guaranteed event queue. StateFlow conflates state changes. Test cancellation and slow/absent collectors, not only happy paths. Official exception behavior.