How do I get an activity-scoped ViewModel in Compo...

# compose

[Florian]

How do I get an activity-scoped ViewModel in Compose? Do just call (hilt)`viewModel()` in the activity and pass it to my Composable screen?

[Tgo1014]

Just call hiltViewModel() directly in the composable parameter for ViewModel

[Florian]

wait, doesn't that scope the viewmodel to the composable?

[Tgo1014]

No, for now it’s to the activity. If you want you can scope by navigation passing the parameter for it.

[Colton Idle]

I think if you use navigation and hilt then when you call hiltViewModel() it'll be scoped to composable, but if you need to scope to just the activity, you need to pass an argument into hiltViewModel(some arg I don't know off the top of my head)

[Florian]

I think you just need to instantiate it in the activity

[Ian Lake]

hiltViewModel() / viewModel() use LocalViewModelStoreOwner.current - when you're in an Activity/Fragment, that'll be the default. When you're within a NavHost, it'll be that current destination. You'll just need to be explicit if you want something other than the default

val outerOwner = LocalViewModelStoreOwner.current
NavHost(...) {
  composable(...) {
    val outerScopedViewModel: MyViewModel = viewModel(outerOwner)
  }
}

[Ian Lake]

Of course, if you want something scoped to every destination in a NavHost, that's exactly why you can define a route for your root navigation graph (the one that is always present):

NavHost(navController, startDestination = "home", route="root") {
  composable(...) {
    val rootScopedViewModel: MyViewModel = viewModel(navController.getBackStackEntry("root"))
  }
}

[Florian]

@Ian Lake if I want an activity-scoped ViewModel in my composable screen, do I instantiate it in the activity (with hiltViewModel()) and then pass it down through to that Composable?

[Florian]

so in onCreate

[Ian Lake]

There's no reason to do that. All you need is the right owner, literally nothing else matters as the owner sets all of the scoping

[Colton Idle]

Makes sense. Yeah, I think it's just a shift from how I currently think about VMs. My old apps were single activity with nav and fragments.

So all I know is: 1. fragment/screen scope. 2. nav graph scope. 3. and activity scope. so I think it's just learning to break away from that mindset.

[Ian Lake]

Those are indeed the three main types of owners, nothing has changed there

[Colton Idle]

Hm. Okay. then I'm a little lost. This is how I would get an activityVM in my fragments in the past.

private val sharedViewModel: ActivityViewModel by activityViewModels()

How would I do this in the compose world since all of my destinations/screens are in my NavHost?

[Ian Lake]

Did you look at what activityViewModels() is doing? https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:fragment[…]main/java/androidx/fragment/app/FragmentViewModelLazy.kt;l=80

It is just getting the activity and using its ViewModelStore, just like any other ViewModelStoreOwner

[Ian Lake]

Instead of reaching up, Compose wants you to pass down the right values - hence the outerOwner approach I mentioned above

[Colton Idle]

I did not. outerOwner example above makes sense now. Thank you!

[Florian]

@Ian Lake but how do I get the activity to pass it as the owner in the composable?

[Florian]

wouldnt I need to pass the activity down to the composable?

[Florian]

oh I guess the outerOwner approach above is the answer

[Ian Lake]

Yep. It might be the activity or a fragment - whatever is hosting your whole Composable hierarchy

[Florian]

alright, thank you!

[Florian]

@Ian Lake how do I handle nullability of LocalViewModelStoreOwner.current 🤔

[Florian]

I don't get why I wouldn't just instantiate the ViewModel in the place where you call LocalViewModelStoreOwner.current and then pass that ViewModel to the screens

[Florian]

then I don't have to handle nullability

[Ian Lake]

If you're in an Activity/Fragment, you can use `!!`; it'll always be filled in there. Like I said, it doesn't matter at all where you call viewModel() - what matters is using the right owner.

[Florian]

but if I call viewModel() in the activity the owner will be set to that activity, right?

[Florian]

seems pretty straight forward and no !!

[Florian]

wait, I am referring to hiltViewModel()

[Florian]

maybe only hiltViewModel() does that automatic scoping

[Ian Lake]

hiltViewModel() just calls viewModel() with a Hilt provided factory. They both internally do the !! for you and throw if it isn't there (which, like I said, it will always be there in an activity/fragment)

[Florian]

ok, thank you