Kotlin

Kotlin & KMP Architecture Standards

Use this skill for pure Kotlin or Kotlin Multiplatform modules where clean boundaries and testable async code matter.

1. Clean Architecture Rules (Domain Layer)

When writing Kotlin business logic (Core/Domain modules):

1. UseCases (Interactors):

   • A UseCase MUST contain only one public function: fun execute(): Result<T>.
   • Do NOT use operator fun invoke.
   • UseCases MUST always return a Kotlin Result<T> (success or failure) to explicitly model outcomes.
   • UseCases can depend on other UseCases or Repositories.
   • Any external Helper or Manager MUST be injected into the UseCase, not the Repository.

2. Repositories and DataSources:

   • Repositories return raw data types (e.g., User, List<Item>), NOT Result<T>. The UseCase handles error mapping into Result.
   • Repositories depend ONLY on DataSources. A Repository MUST NOT depend on another Repository.
   • DataSources depend only on external APIs/DBs. A DataSource MUST NOT depend on another DataSource.
   • Prefer concrete classes for Repositories unless multi-platform swapping (expect/actual) or strict test mocking requires an interface.

3. Dependency Graph:

   • UI/Component -> UseCase -> Repository -> DataSource

2. Kotlin Language Patterns

• Coroutines & Flow:
  • Prefer suspend functions for one-shot async operations and Flow for streams of reactive data.
  • Dispatcher Injection: NEVER hardcode Dispatchers.IO or Dispatchers.Default inside UseCases or Repositories. Pass them via constructor injection so they can be easily replaced with UnconfinedTestDispatcher in unit tests.
  • Structured Concurrency: Prefer coroutineScope and supervisorScope over GlobalScope. Never launch unmanaged coroutines.
  • Context-Preservation (Safe Suspend): A suspend function should be safe to call from any dispatcher (main-safe). If a function performs blocking I/O, it MUST shift its own execution context internally using withContext(dispatcher).
  • Coroutine Error Handling:
    • Never swallow CancellationException. If catching a generic Exception inside a coroutine, explicitly check and rethrow CancellationException to avoid breaking structured concurrency and memory leaks.
    • Use CoroutineExceptionHandler exclusively for uncaught exceptions in root coroutines (launch). Note that it does NOT work for async (where exceptions are deferred until await()).
• Scope Functions: Avoid excessive or nested scope functions (run, also, apply, let). Only use them when they significantly improve readability.
• Serialization: Prefer kotlinx.serialization over Jackson/Gson for multiplatform readiness.
• Networking (Ktor): Prefer Ktor Client for HTTP requests in KMP projects. Use ContentNegotiation with kotlinx.serialization for JSON parsing. Define separate request/response DTOs rather than exposing domain models directly to the network layer.
• Error Handling Boundary: Do NOT leak raw Exceptions to the UI. The Result<T> from a UseCase must be mapped by the ViewModel/Component into a precise, UI-friendly Error State or a One-Time UI Event.
• Compiler & API: Use the K2 Compiler defaults. Enable Explicit API mode in core modules (require explicit visibility and return types) to reduce LLM/IDE inferencing errors.

3. Project Structure

• Prefer a Multi-Module Gradle (Kotlin DSL) setup (build.gradle.kts).
• Use Version Catalogs (gradle/libs.versions.toml) for all dependencies and plugins.
• Separate core (pure Kotlin, no framework dependencies) from framework-specific modules (like app, backend, android, etc.).