I’m trying to ‘return a result’ via Compose - setting a value on the `SavedStateHandle` of the `prev...

# compose

[Tim Malseed]

I’m trying to ‘return a result’ via Compose - setting a value on the SavedStateHandle of the previousBackStackEntry and then popping the backstack.

But, the SavedStateHandle that I want to interact with belongs to the initial screen’s ViewModel, and I sense that this is not the same instance as the SavedStateHandle found on the previousBackStackEntry

I suspect this has something to do with scoping ViewModels, but I’ve tried a few things and I’m out of ideas. Code in thread

[Tim Malseed]

So, the user starts on the Home screen. The Home screen contains a ViewModel, which stores whether the user has entered ‘guest mode’ or not.

@Composable
fun HomeScreen(
    viewModel: HomeViewModel = hiltViewModel()
        ...

@HiltViewModel
class HomeViewModel @Inject constructor(
    savedStateHandle: SavedStateHandle
) : ViewModel() {

    private val guestMode: StateFlow<Boolean> = savedStateHandle.getStateFlow(KEY_GUEST_MODE, false)
    
        ...

Under certain conditions, the user is navigated to the ‘Login’ screen. They can then choose to enter ‘guest’ mode, which attempts to update the SavedStateHandle belonging to the HomeViewModel, and then pop the LoginScreen off the stack, returning the user to the HomeScreen.

The NavHost looks like so:

NavHost(
    ..
) {
    homeGraph(
        onNavigateToLogin = {
            navController.navigate(LoginDestination.route)
        }
    )
    loginGraph(
        navController = navController,
        onEnterGuestMode = {
            navController.previousBackStackEntry?.savedStateHandle?.set(HomeViewModel.KEY_GUEST_MODE, true)
            navController.popBackStack()
        }
    )
}

fun NavGraphBuilder.homeGraph(
    onNavigateToLogin: () -> Unit
) {
    composable(route = HomeDestination.route) {
        HomeScreen(
            onNavigateToLogin = onNavigateToLogin
        )
    }
}

fun NavGraphBuilder.loginGraph(
    navController: NavHostController,
    onEnterGuestMode: () -> Unit
) {
    composable(route = LoginDestination.route) {
        LoginScreen(
            onEnterGuestMode = onEnterGuestMode
        )
    }
...

[Tim Malseed]

---

The question is: How can the onEnterGuestMode callback on the loginGraph access the SavedStateHandle belonging to the HomeViewModel.

Or alternatively, how can the HomeViewModel use the same SavedStateHandle instance associated with the Home destination’s backStackEntry?

[Ian Lake]

Here's the last thread on this: https://kotlinlang.slack.com/archives/CJLTWPH7S/p1638267881279200