Android

Android & Compose Architecture Standards

Use this skill for Android work built with Kotlin, Compose, and related modern app architecture patterns.

1. Jetpack Compose UI Rules

1. Model-View-Intent (MVI) & UDF:
   • Model: Expose a single, immutable ViewState (Data Class) from the ViewModel. Do not expose multiple independent state flows unless strictly isolated.
   • View: A pure function rendering the Model. State flows down.
   • Intent: User actions are routed to the ViewModel as explicit Intents/Events. Events flow up.
2. Screen vs View Approach:
   • <Name>Screen: Handles DI, Navigation3 KMP routing, and connects the ViewModel to the UI.
   • <Name>View(state, onEvent): A completely pure, stateless Composable.
3. Minimize Logic in Compose:
   • Do NOT use remember for business logic. Business logic belongs in the ViewModel.
4. Performance:
   • Assume Strong Skipping Mode is enabled. Do not manually wrap lambdas in remember.
   • Pass modifier chains explicitly: modifier: Modifier = Modifier.
5. Visibility:
   • @Preview Composables MUST be private or restricted visibility.
   • Do NOT make Composable functions public unless intended as an external design system component.

2. Navigation3 KMP (Routing & State)

If using Navigation3 KMP for architecture:

1. ViewModel = Logic + State: The ViewModel handles all business logic, manages the CoroutineScope, and exposes state to the UI.
2. ViewModel Interface:
   • Public functions inside a ViewModel MUST represent user intents/events and generally return Unit.
   • Any function computing values internally should be private.
   • State should be exposed as an immutable StateFlow.
3. Routing (Navigation3 KMP):
   • Treat navigation strictly as state management (part of the ViewModel or Component).
   • Keep navigation execution strictly inside the <Name>Screen wrapper, NOT inside the pure <Name>View.
   • Use strongly typed destinations (Objects or Data Classes).
   • Crucial: Apply polymorphic @Serializable annotations to your destination keys so they serialize correctly across non-JVM platforms (iOS/Web).

3. Dependency Injection (Koin)

If using Koin:

• Prefer single and factory instead of bind with generic provider/singleton blocks for clearer syntax and safety.

4. Project Structure & Layering

Prefer a clear 3-layer layout and keep boundaries strict:

• data/: implementations (local DB, remote APIs, repository impls)
• domain/: pure business logic (models, repository interfaces, use cases)
• ui/: Compose screens, reusable components, theme
• di/: DI wiring only (no business logic)

Rule: UI depends on domain, domain depends on nothing, data depends on domain (interfaces).

5. Coroutines, Flow, and State

1. UI state:
   • Use MutableStateFlow internally and expose StateFlow via asStateFlow().
   • Update state via update { it.copy(...) } to keep changes atomic.

Example (micro):

private val _state = MutableStateFlow(ViewState())
val state: StateFlow<ViewState> = _state.asStateFlow()
_state.update { it.copy(isLoading = true) }

2. Long-running streams:
   • Use flowOn(Dispatchers.IO) for data layer work.
   • Prefer injecting a CoroutineDispatcher for testability.
3. Error handling:
   • Catch at the right boundary (usually data/usecase) and surface a user-safe error state.

6. Result Modeling (Optional)

For operations that can be loading/success/error, prefer a sealed result type over nullable juggling.

Rule: keep it small (Loading, Success(data), Error(exception)), and map/transform explicitly.

7. Testing (Unit-first)

Recommended stack (when it fits the repo):

• kotlinx-coroutines-test for deterministic coroutine tests
• MockK for mocking
• Turbine for testing Flow/StateFlow emissions

Rules:

1. Tests must be deterministic (no real network, no timing races).
2. Prefer injecting dispatchers and using test dispatchers.
3. For flows: assert emission order and terminal states, not implementation details.

Example (micro):

@Test fun emitsLoadingThenData() = runTest {
  // collect state/flow and assert emissions (use Turbine if available)
}

8. Lint + CI (Keep It Boring)

1. Run static checks in CI:
   • detekt (complexity/style)
   • ktlint (formatting)
   • unit tests
2. Keep thresholds explicit (e.g., long method/parameter limits) and fail CI on violations.
3. If the repo uses GitHub Actions, keep Android CI minimal:
   • checkout
   • JDK setup
   • Gradle cache
   • detekt/ktlint
   • unit tests
   • assemble (optional)