Android Kotlin — Architecture Module
Parent kit: ../../SKILL.md · Index: ../../AGENTS.md
Read First
MVI Contract (Mandatory)
@Immutable
data class FeatureUiState(
val items: List<ItemUi> = emptyList(),
val isLoading: Boolean = false,
val error: UiError? = null
)
sealed interface FeatureUiEvent {
data class SearchChanged(val query: String) : FeatureUiEvent
data object Refresh : FeatureUiEvent
data class ItemClicked(val id: String) : FeatureUiEvent
}
sealed interface FeatureUiEffect {
data class ShowMessage(@StringRes val messageRes: Int) : FeatureUiEffect
data class NavigateToDetail(val id: String) : FeatureUiEffect
}
class FeatureViewModel @Inject constructor(
private val repository: FeatureRepository
) : ViewModel() {
private val _state = MutableStateFlow(FeatureUiState())
val state: StateFlow<FeatureUiState> = _state.asStateFlow()
private val _effects = Channel<FeatureUiEffect>(Channel.BUFFERED)
val effects: Flow<FeatureUiEffect> = _effects.receiveAsFlow()
fun onEvent(event: FeatureUiEvent) {
when (event) {
is FeatureUiEvent.SearchChanged -> onSearchChanged(event.query)
FeatureUiEvent.Refresh -> refresh()
is FeatureUiEvent.ItemClicked -> emitNavigate(event.id)
}
}
private fun onSearchChanged(query: String) {
_state.update { it.copy(searchQuery = query) } // ONLY mutation style allowed
}
private fun refresh() {
viewModelScope.launch {
_state.update { it.copy(isLoading = true, error = null) }
repository.sync()
.onSuccess { _state.update { it.copy(isLoading = false) } }
.onFailure { e ->
_state.update { it.copy(isLoading = false, error = e.toUiError()) }
}
}
}
private fun emitNavigate(id: String) {
viewModelScope.launch { _effects.send(FeatureUiEffect.NavigateToDetail(id)) }
}
}
Module Layout (Large Apps)
app/
feature-<name>/
presentation/ # Composables + ViewModel
domain/ # Models, repository interfaces, UseCases
data/ # Room, Ktor, mappers
core/
core-ui/ # Design system
core-domain/
core-testing/
Feature modules must not depend on other feature modules — only core-* and domain contracts.
UseCase Rule
| Use UseCase |
Skip UseCase |
| Logic shared by 2+ ViewModels |
Single-line repo delegate |
| Multi-repo orchestration |
One ViewModel, one repo |
| Needs isolated unit tests |
CRUD with no extra rules |
Anti-Patterns (Architecture)
| Banned |
Fix |
_state.value = newState |
_state.update { it.copy(...) } |
| God ViewModel (>200 lines) |
Split screen or extract UseCases |
| ViewModel in composable params |
Pass onEvent lambdas |
Domain model with @Entity |
Entity in data/, map in repository |
Collecting Flow in init without stateIn |
stateIn(WhileSubscribed(5_000)) |
