coroutines

Android Coroutines Expert Skill

This skill provides authoritative rules and patterns for writing production-quality Kotlin Coroutines code on Android. It enforces structured concurrency, lifecycle safety, and modern best practices (2025 standards).

Responsibilities

• Asynchronous Logic: Implementing suspend functions, Dispatcher management, and parallel execution.
• Reactive Streams: Implementing Flow, StateFlow, SharedFlow, and callbackFlow.
• Lifecycle Integration: Managing scopes (viewModelScope, lifecycleScope) and safe collection (repeatOnLifecycle).
• Error Handling: Implementing CoroutineExceptionHandler, SupervisorJob, and proper try-catch hierarchies.
• Cancellability: Ensuring long-running operations are cooperative using ensureActive().
• Testing: Setting up TestDispatcher and runTest.

Applicability

Activate this skill when the user asks to:

• "Fetch data from an API/Database."
• "Perform background processing."
• "Fix a memory leak" related to threads/tasks.
• "Convert a listener/callback to Coroutines."
• "Implement a ViewModel."
• "Handle UI state updates."

Critical Rules & Constraints

1. Dispatcher Injection (Testability)

• NEVER hardcode Dispatchers (e.g., Dispatchers.IO, Dispatchers.Default) inside classes.
• ALWAYS inject a CoroutineDispatcher via the constructor.
• DEFAULT to Dispatchers.IO in the constructor argument for convenience, but allow it to be overridden.

// CORRECT
class UserRepository(
    private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO
) { ... }

// INCORRECT
class UserRepository {
    fun getData() = withContext(Dispatchers.IO) { ... }
}

2. Main-Safety

• All suspend functions defined in the Data or Domain layer must be main-safe.
• One-shot calls should be exposed as suspend functions.
• Data changes should be exposed as Flow.
• The caller (ViewModel) should be able to call them from Dispatchers.Main without blocking the UI.
• Use withContext(dispatcher) inside the repository implementation to move execution to the background.