Android Native Mobile Engineering & Jetpack Architecture Masterclass

// 1. Immutable UI State Model
sealed interface UserUiState {
    object Loading : UserUiState
    data class Success(val user: User) : UserUiState
    data class Error(val message: String) : UserUiState
}

// 2. ViewModel with Unidirectional Data Flow & StateFlow
@HiltViewModel
class UserProfileViewModel @Inject constructor(
    private val userRepository: UserRepository
) : ViewModel() {

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

    fun loadUserProfile(userId: String) {
        viewModelScope.launch {
            _uiState.value = UserUiState.Loading
            userRepository.getUserFlow(userId)
                .catch { exception ->
                    _uiState.value = UserUiState.Error(exception.localizedMessage ?: "Unknown error")
                }
                .collect { user ->
                    _uiState.value = UserUiState.Success(user)
                }
        }
    }
}

// 3. Declarative Composable Screen with Recomposition Safety
@Composable
fun UserProfileScreen(
    viewModel: UserProfileViewModel = hiltViewModel(),
    onNavigateBack: () -> Unit
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()

    Scaffold(
        topBar = {
            TopAppBar(
                title = { Text("User Profile") },
                navigationIcon = {
                    IconButton(onClick = onNavigateBack) {
                        Icon(Icons.Default.ArrowBack, contentDescription = "Back")
                    }
                }
            )
        }
    ) { paddingValues ->
        Box(modifier = Modifier.fillMaxSize().padding(paddingValues)) {
            when (val state = uiState) {
                is UserUiState.Loading -> CircularProgressIndicator(modifier = Modifier.align(Alignment.Center))
                is UserUiState.Error -> Text(
                    text = state.message,
                    color = MaterialTheme.colorScheme.error,
                    modifier = Modifier.align(Alignment.Center)
                )
                is UserUiState.Success -> {
                    Column(modifier = Modifier.padding(16.dp)) {
                        Text(text = state.user.name, style = MaterialTheme.typography.headlineMedium)
                        Spacer(modifier = Modifier.height(8.dp))
                        Text(text = state.user.email, style = MaterialTheme.typography.bodyLarge)
                    }
                }
            }
        }
    }
}