Feature Screen Pattern

To maintain clean code and enable Jetpack Compose @Preview, we separate the "Logic" from the "UI".

The Component: Screen

• Responsibility: Pure UI rendering.
• Dependencies: None. It relies on simple data classes, primitives, or State objects.
• State: Stateless (receives state as parameters).
• Events: Exposes lambdas (e.g., onCurrencySelected: (String) -> Unit).
• Benefit: Can be previewed easily in Android Studio with dummy data.

@Composable
fun DefaultCurrencyScreen(
    availableCurrencies: List<Currency>, // Pure data
    selectedCurrencyCode: String?,       // Pure data
    onCurrencySelected: (String) -> Unit // Event
) {
    // UI Implementation...
}

The Wrapper: Feature

• Responsibility: Wiring the logic.
• Dependencies: ViewModel, NavController, ScreenUiProvider (via FeatureScaffold).
• State: Collects StateFlow from the ViewModel.
• Events: Handles navigation, side effects, or calls ViewModel functions.
• Benefit: Isolates dependencies from the rendering logic.

@Composable
fun DefaultCurrencyFeature(
    viewModel: DefaultCurrencyViewModel = koinViewModel()
) {
    // 1. Collect State
    val selectedCurrency by viewModel.selectedCurrencyCode.collectAsState()
    
    // 2. Setup Dependencies
    val navController = LocalRootNavController.current
    val scope = rememberCoroutineScope()

    // 3. Wrap with Scaffold (injects TopBar/FAB based on Route)
    FeatureScaffold(currentRoute = Routes.SETTINGS_DEFAULT_CURRENCY) {
        
        // 4. Render the Screen
        DefaultCurrencyScreen(
            availableCurrencies = viewModel.availableCurrencies,
            selectedCurrencyCode = selectedCurrency,
            onCurrencySelected = { newCurrencyCode ->
                viewModel.onCurrencySelected(newCurrencyCode)
                // Handle navigation side-effect
                scope.launch {
                    delay(UiConstants.NAV_FEEDBACK_DELAY)
                    navController.popBackStack()
                }
            }
        )
    }
}

Rule of Thumb

• Never pass a ViewModel into a Screen.
• Never pass a NavController into a Screen.
• Pass them into the Feature, which orchestrates the Screen.