Android viewmodel

Android ViewModel & State Management

Instructions

Use ViewModel to hold state and business logic. It must outlive configuration changes.

1. UI State (StateFlow)

• What: Represents the persistent state of the UI (e.g., Loading, Success(data), Error).
• Type: StateFlow<UiState>.
• Initialization: Must have an initial value.
• Exposure: Expose as a read-only StateFlow backing a private MutableStateFlow.

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

• Updates: Update state using .update { oldState -> ... } for thread safety.

2. One-Off Events (SharedFlow)

• What: Transient events like "Show Toast", "Navigate to Screen", "Show Snackbar".
• Type: SharedFlow<UiEvent>.
• Configuration: Must use replay = 0 to prevent events from re-triggering on screen rotation.

  private val _uiEvent = MutableSharedFlow<UiEvent>(replay = 0)
  val uiEvent: SharedFlow<UiEvent> = _uiEvent.asSharedFlow()
  

• Sending: Use .emit(event) (suspend) or .tryEmit(event).

3. Collecting in UI

• Compose: Use collectAsStateWithLifecycle() for StateFlow.

  val state by viewModel.uiState.collectAsStateWithLifecycle()
  

  For SharedFlow, use LaunchedEffect with LocalLifecycleOwner.
• Views (XML): Use repeatOnLifecycle(Lifecycle.State.STARTED) within a coroutine.

4. Scope

• Use viewModelScope for all coroutines started by the ViewModel.
• Ideally, specific operations should be delegated to UseCases or Repositories.