kotlin-coroutines-expert

Kotlin Coroutines Expert

Overview

Authoritative guide for asynchronous programming, reactive data streams, and cancellation/exception safety in this Android app.

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1. Structured Concurrency

• Always launch coroutines within a defined CoroutineScope (e.g., viewModelScope with applicationExceptionHandler).
• Use coroutineScope or supervisorScope to group concurrent tasks.
• Never use GlobalScope (causes memory leaks and uncontrollable lifetimes).

suspend fun loadDashboardData(): DashboardData = supervisorScope {
    val userDeferred = async { userRepo.getUser() }
    val stepsDeferred = async { stepsRepo.getTodaySteps() }
    
    DashboardData(
        user = userDeferred.await(),
        steps = stepsDeferred.await()
    )
}

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2. Exception & Cancellation Safety

• Top-level scopes should use CoroutineExceptionHandler.
• Never swallow CancellationException — if caught, rethrow it to allow proper cancellation propagation.
• For network calls, catch specific domain exceptions (IOException, HttpException) instead of a generic empty catch (e: Exception).

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3. Reactive Streams (Flow & StateFlow)

• Expose read-only StateFlow<T> or SharedFlow<T> from ViewModels; keep MutableStateFlow private.
• For One-Shot UI events (navigation, toast, dialog), use Channel<Event> with receiveAsFlow() or SharedFlow<Event>(replay = 0). Never use sticky MutableLiveData.
• Use flowOn(Dispatchers.IO) for heavy data transformations before collecting on the Main thread.

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4. Testing Coroutines

• Use runTest from kotlinx-coroutines-test.
• Inject TestDispatcher (e.g. StandardTestDispatcher or UnconfinedTestDispatcher) into ViewModels/UseCases instead of hardcoding Dispatchers.IO.