<@UJBPFB3SN>, Since Nav3's `NavBackStack` really ...

# compose-android

[Archie]

@Ian Lake,

Since Nav3's NavBackStack really is just a List<T> , I always wonder if it could be part of the Dependency Graph so it could be injected to like a ViewModel . But if I do add it to the graph, same with this implementation here, I now lose the ability to restore the state of the NavBackStack. I just wonder whats the advise here? Should it be part of the DI or should it stack within the compose scope?

[Albert Chang]

> I now lose the ability to restore the state of the NavBackStack.
You don't. You can use something like this to save/restore the back stack.

@Provides
@ActivityScoped
fun provideBackStack(
    @ActivityContext context: Context,
): BackStack {
    val activity = context as ComponentActivity
    val backStack = activity.savedStateRegistry.consumeRestoredStateForKey(KEY)?.let {
        // Restore back stack
    } ?: BackStack()
    activity.savedStateRegistry.registerSavedStateProvider(KEY) {
        SavedState().apply {
            // Save back stack
        }
    }
    return backStack
}

[Archie]

thats interesting.. thanks @Albert Chang

[Archie]

@Albert Chang, wouldn't this scope the instance to the activity? If the BackStack is getting injected inside a view model, would that be a problem? the viewmodel could likely outlast the activity lifecycle, wouldn't the viewmodel be referencing an old instance in cases like configuration change?

[Albert Chang]

Personally I just disable activity recreation for all config changes, but yeah, that can be a problem for apps that don't.
In that case, you can use a view model to hold the back stack:

@Provides
@ActivityScoped
fun provideBackStack(
    @ActivityContext context: Context,
): BackStack {
    val activity = context as ComponentActivity
    return ViewModelProvider.create(
        activity.viewModelStore,
        activity.defaultViewModelProviderFactory,
        activity.defaultViewModelCreationExtras,
    )[BackStackViewModel::class.java].backStack
}

private class BackStackViewModel(savedStateHandle: SavedStateHandle) : ViewModel() {
    val backStack by savedStateHandle.saveable(saver = BackStack.Saver) {
        BackStack()
    }
}

[Archie]

Yeah I thought of that as well, but if I place it inside the ViewModel, its kinda of sketchy on how I would be able to inject it to another ViewModel

[efemoney]

Your dependency graph (ie your component) is what should be in the VM / Retained scope.

[Archie]

@efemoney could you clarify a bit more?