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.