Feel like I'm getting a bit lost in all of the vie...

# compose-android

[Colton Idle]

Feel like I'm getting a bit lost in all of the viewModel() apis.

Would this be the right way to get a NavHost scoped ViewModel when using a NavDrawer?

[Colton Idle]

ModalNavigationDrawer(
    drawerContent = {
      val parentEntry = remember(navBackStackEntry) { 
navController.getBackStackEntry("parentNavigationRoute") 
}
      val parentViewModel: RootAppViewModel = hiltViewModel(parentEntry)
      
      NavDrawerContent(
          currentDestination,
          drawerState,
          buttonClick = { parentViewModel.doSomething() }
      )
    },
    content = {
      ModalBottomSheetLayout(bottomSheetNavigator) {
        NavHost(navController, startDestination = "home") {

[Colton Idle]

Actually... that doesn't seem to work. Must have been running a previous piece of code.

The above crashes on launch with

java.lang.IllegalArgumentException: No destination with route parentNavigationRoute is on the NavController's back stack. The current destination is null

EDIT: Im trying to follow these docs btw: https://developer.android.com/topic/libraries/architecture/viewmodel/viewmodel-apis#compose

[Stylianos Gakis]

You can name the root route in your NavHost, there's a route parameter on it afaik. Then get the backstack entry just as you're doing already.
Then idk about the hilt api, but that probably looks correct

[Colton Idle]

Okay. Updated the name of my navHost to rootNavGraph. Seems like some general race condition. where my navgraph doesn't actually exist yet.

java.lang.IllegalArgumentException: No destination with route rootNavGraph is on the NavController's back stack. The current destination is null

[Stylianos Gakis]

Oh wait, you're doing this outside of the NavHost itself, before it ever gets a chance to initialize itself then?

[Stylianos Gakis]

Or, you got two NavHosts? A bit confused, probably would be interesting to see more of the code.

[Colton Idle]

I am doing it outside the navHost as I have a navDrawer that wraps my root NavHost, and I want a click of an item in NavDrawer to call a navHost's viewModel method.

[Colton Idle]

So. basically i have a button in nav drawer and I want to call a VM method on click.

[Colton Idle]

Current code. Notice 1 NavHost, wrapped with a NavDrawer

@Composable
fun MyApp(
  appStateHolder: AppStateHolder
) {
  val bottomSheetNavigator = rememberBottomSheetNavigator()
  val navController = rememberNavController(bottomSheetNavigator)
  val navBackStackEntry by navController.currentBackStackEntryAsState()
  val currentDestination = navBackStackEntry?.destination

ModalNavigationDrawer(
    drawerContent = {
      val parentEntry = remember(navBackStackEntry) { 
navController.getBackStackEntry("rootNavGraph") 
}
      val parentViewModel: RootAppViewModel = hiltViewModel(parentEntry)
      
      NavDrawerContent(
          currentDestination,
          drawerState,
          buttonClick = { parentViewModel.doSomething() }
      )
    },
    content = {
      ModalBottomSheetLayout(bottomSheetNavigator) {
        NavHost(route = "rootNavGraph", navController = navController, startDestination = "home") {
            composable("home") { backStackEntry -> Demo1Screen() }
...

[Stylianos Gakis]

Right right, which means it may be doing all this too early

[Stylianos Gakis]

Do you need that to be in a ViewModel specifically?
Anything that's outside of the NavHost can just be served with a simple MyAppState, exactly like NiaAppState does here https://github.com/android/nowinandroid/blob/main/app/src/main/java/com/google/samples/apps/nowinandroid/ui/NiaAppState.kt

[Stylianos Gakis]

I do the same in our app and it works super well. Besides, why do you want to scope something to the root of your NavHost? It's the same as scoping it to your activity, aka not doing anything about scoping it. Their lifetime is exactly the same.

[Colton Idle]

I guess I just want a VM that's scoped to the highest "composable" in the app (basically my root app) without the VM/scope being the activity.

[Stylianos Gakis]

What's the difference between those two though?

[Colton Idle]

meaningfully, nothing. but im trying to keep this new project separated from "android" as much as possible. hence i dont want to be scoped to the "activity"

[Colton Idle]

<=== I believe this would be "scoped" to the activity. And id like it to be scoped to my apps root composable instead

fun MyApp(
  appStateHolder: AppStateHolder,
  activityViewModel: RootAppViewModel = hiltViewModel()
) {

[Stylianos Gakis]

"separated from android"
Then what better thing than separating it completely by not even making it an Android VM in the first place?

[Colton Idle]

touche!

[Colton Idle]

thats a good point. i guess im asking for something that doesn't exist? a scope in between activity scope and the NavHost scope.

[Colton Idle]

sorta related. is there an easy way to know what scope/`ViewModelStoreOwner`. my viewModel belongs to?

Like in...

fun MyApp(
  appStateHolder: AppStateHolder,
  activityViewModel: RootAppViewModel = hiltViewModel()
) {

im not sure if its actually scoping it to the Activity or NavGraph?

[Stylianos Gakis]

a scope in between activity scope and the NavHost scope.

Yeah there's nothing between there because those two already are equivalent, they have the same lifecycle.

[Stylianos Gakis]

For that you'd have to look into what hiltViewModel does, I don't use hilt so I can't say with high confidence.
It probably looks at the CompositionLocal which holds a lifecycle owner of some sorts. On your activity that would be your activity.
Inside androidx.navigation destinations that'd be the backstackentry itself. So if you're top level it just is your activity I'd say.
You can look into the sources of what composable("route") {} does and see which locals it sets, and then see which ones hiltViewModel reads, there must be some overlap there.

[Colton Idle]

Yeah there's nothing between there because those two already are equivalent, they have the same lifecycle.

wait. do they really? like. i know they have a similar lifecycle, but for the navHost is it really the activity lifecycle/scope/viewModelStoreOwner? hmm. might be a pain and ping Ian when he's back from vacay 😅

[Colton Idle]

ill try to do some digging through the source. thanks for the conversation stylianos!