Android Jetpack Compose Expert

2. State Management Pattern (MVI/MVVM)

Use ViewModel with StateFlow to expose UI state. Avoid exposing MutableStateFlow.

// UI State Definition
data class UserUiState(
    val isLoading: Boolean = false,
    val user: User? = null,
    val error: String? = null
)

// ViewModel
class UserViewModel @Inject constructor(
    private val userRepository: UserRepository
) : ViewModel() {

    private val _uiState = MutableStateFlow(UserUiState())
    val uiState: StateFlow<UserUiState> = _uiState.asStateFlow()

    fun loadUser() {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true) }
            try {
                val user = userRepository.getUser()
                _uiState.update { it.copy(user = user, isLoading = false) }
            } catch (e: Exception) {
                _uiState.update { it.copy(error = e.message, isLoading = false) }
            }
        }
    }
}

3. Creating the Screen Composable

Consume the state in a "Screen" composable and pass data down to stateless components.

@Composable
fun UserScreen(
    viewModel: UserViewModel = hiltViewModel()
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()

    UserContent(
        uiState = uiState,
        onRetry = viewModel::loadUser
    )
}

@Composable
fun UserContent(
    uiState: UserUiState,
    onRetry: () -> Unit
) {
    Scaffold { padding ->
        Box(modifier = Modifier.padding(padding)) {
            when {
                uiState.isLoading -> CircularProgressIndicator()
                uiState.error != null -> ErrorView(uiState.error, onRetry)
                uiState.user != null -> UserProfile(uiState.user)
            }
        }
    }
}

Examples

Example 1: Type-Safe Navigation

Using the new Navigation Compose Type Safety (available in recent versions).

// Define Destinations
@Serializable
object Home

@Serializable
data class Profile(val userId: String)

// Setup NavHost
@Composable
fun AppNavHost(navController: NavHostController) {
    NavHost(navController, startDestination = Home) {
        composable<Home> {
            HomeScreen(onNavigateToProfile = { id ->
                navController.navigate(Profile(userId = id))
            })
        }
        composable<Profile> { backStackEntry ->
            val profile: Profile = backStackEntry.toRoute()
            ProfileScreen(userId = profile.userId)
        }
    }
}

Best Practices

• ✅ Do: Use remember and derivedStateOf to minimize unnecessary calculations during recomposition.
• ✅ Do: Mark data classes used in UI state as @Immutable or @Stable if they contain List or other unstable types to enable smart recomposition skipping.
• ✅ Do: Use LaunchedEffect for one-off side effects (like showing a Snackbar) triggered by state changes.
• ❌ Don't: Perform expensive operations (like sorting a list) directly inside the Composable function body without remember.
• ❌ Don't: Pass ViewModel instances down to child components. Pass only the data (state) and lambda callbacks (events).

Troubleshooting

Problem: Infinite Recomposition loop. Solution: Check if you are creating new object instances (like List or Modifier) inside the composition without remember, or if you are updating state inside the composition phase instead of a side-effect or callback. Use Layout Inspector to debug recomposition counts.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.