Any ETA on official navigation/backstack? :sweat_smile:

# compose

[dagomni]

Any ETA on official navigation/backstack? 😅

[Ian Lake]

https://issuetracker.google.com/issues/161472731 is the issue to star for progress updates

[Javier]

@Ian Lake is it available in navigation 2.4 in the androidx snapshot repo?

[Ian Lake]

It is a separate androidx.compose.navigation artifact group at the moment

[Ian Lake]

But yes, there is a snapshot available in pre-dev level of quality

[Javier]

I have a few questions:
• Where can I give feedback about improvements for androidx.dev web? (actually, if I want to find ui/androidx/compose/navigation, I have to click "next page" a few minutes, and in the url I can't put the page number too because it is using a token
• I didn't see samples with args, so I guess they are not supported at this moment

[Ian Lake]

Args are being added in https://android-review.googlesource.com/c/platform/frameworks/support/+/1423769

[Alexander Karkossa]

Is there a way to use a Crossfade with the current implementation of NavHost?

[Ian Lake]

No, that's one of the topics we're going to be working on prior to any release

[Sergey Y.]

Injection into a Composable function? Please no

[Ian Lake]

Navigation in Compose supports ViewModels at the screen level. Navigation doesn't care/control/have anything to do with where you get your ViewModel Factory from, Hilt or not

[Alexander Karkossa]

Hmm ok, maybe I have to check again. Currently he is searching for an empty constructor in my ViewModels. It seems that the HiltViewModelFactory was not used

[Ian Lake]

Sounds like it isn't using your Hilt provided factory at all. Have you tried the instructions in the Hilt guide? https://developer.android.com/training/dependency-injection/hilt-jetpack#viewmodel-navigation

[Alexander Karkossa]

Jup. ViewModel Injection outside of compose works fine. Don’t know if there is a Problem with NavBackStackEntry within the androidx navigation library. The getDefaultViewModelProviderFactory created the SavedStateVieeModelFactory

[Alexander Karkossa]

In NavHost compose the ViewModelStoreOwnerAmbient provides the currentNavBackStackEntry. I thought that would cause these problem

[Javier]

@Ian Lake so if I annotate the activity as EntryPoint, hilt should work with compose ootb (using the ViewModel function inside the composable)?

[Ian Lake]

More official documentation on Hilt+Compose is still to come, but if you use the containing Activity/Fragment's defaultViewModelProviderFactory and Compose's viewModel function, then it'll certainly use Hilt's factory

[Ian Lake]

Assuming your containing Activity/Fragment is correctly set up in Hilt land (i.e., as an EntryPoint)

[Ian Lake]

That's all independent of Navigation

[Alexander Karkossa]

Hmm I did all of this. I will try it tomorrow again.

[Alexander Karkossa]

@Ian Lake Tried it again. 

Test 1 (worked):
val viewModel: MyViewModel by viewModels() in the MainActivity injects the ViewModel with Hilt. 

Test 2 (worked):
val viewModel: MyViewModel = viewModel() within compose

Test 3(crash):
val viewModel: MyViewModel = viewModel() within NavHost.
NewInstanceFactory tries to create an instance. No zero argument constructor.

[Ian Lake]

You did read the docs I linked above right? You have to pass in the factory when in Navigation

[Ian Lake]

Hilt only does things at the Activity or Fragment layer, it has no idea about the Navigation layer, so when you create ViewModels that are scoped at the Navigation layer, you need to be explicit that you want to pull a ViewModelProvider.Factory out from your Activity/Fragment

[Ian Lake]

This is part of what the Hilt team is looking at for best practices for Hilt in Compose

[Alexander Karkossa]

You were right. didn’t read correctly. Worked with using the defaultViewModelProviderFactory from the Activity.

But now it crashes, when i enter a subview second time.

SavedStateProvider with given key is already registered.

Think i am still doing something wrong.

[Ian Lake]

50/50 chance that it isn't your fault too. There's a reason that Navigation and Hilt isn't part of the current alpha release yet - still more to do 😄

[michaelevans]

@Alexander Karkossa did you ever solve this?

[Alexander Karkossa]

Hey,

this was a while ago, so I’ll try to get it together once.

I found a way to use Hilt `ViewModel`s in NavHost, but I haven’t checked for a long time to see if it works without that workaround in the meantime.

class ViewModel(
  private val repository: Repository,
) : ViewModel() {

}

class ViewModelFactory(private val context: Context) : ViewModelProvider.Factory {

  @EntryPoint
  @InstallIn(ApplicationComponent::class)
  interface ViewModelProviderEntryPoint {
    fun repository(): Repository
  }

  @Suppress("UNCHECKED_CAST")
  override fun <T : ViewModel?> create(modelClass: Class<T>): T {
    val provider = EntryPointAccessors.fromApplication(context, ViewModelProviderEntryPoint::class.java)
    return ViewModel(
      repository = provider.repository(),
    ) as T
  }
}

@Composable
fun ComposeView(
  modifier: Modifier = Modifier
) {
  val viewModel: ViewModel = viewModel(factory = ViewModelFactory(AmbientContext.current))
}