kotlin-coroutines-expert

# 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. ```kotlin 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. ```kotlin 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. ```kotlin // 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 ```kotlin suspend fun fetchDataWithErrorHandling() = supervisorScope { val task1 = async { try { api.fetchA() } catch (e: Exception) { null } } val task2 = async { api.fetchB() } // If task2 fails, task1 is NOT cancelled because of supervisorScope 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.