I have navigation problem. app has 3 screens 1st -> list of items 2nd -> item details 3rd -&gt...

# compose

[Marko Novakovic]

I have navigation problem. app has 3 screens
1st -> list of items
2nd -> item details
3rd -> gallery of item images
I use compose navigation component version 2.4.0-alpha05 and am navigating from 1st to 2nd screen like this:

navController.navigate("$route/${id.value}")

and routes are defined like so:

@JvmInline
value class Routes(val route: String) {
    companion object {
        val Home = Routes("home")
        val ItemDetails = Routes("item_details/{id}")
        val ItemGallery = Routes("item_gallery")
    }
}

I access that id parameter inside ViewModel

savedStateHandle.get<String>(ARG_ITEM_ID) // ARG_ITEM_ID = "id", same as route arg

when I go from 1st screen, list of items, to 2nd, details, it works fine BUT when I go from details to gallery and press system back button ARG_ITEM_ID is null. it works fine with navigation version 2.4.0-alpha04

[Ian Lake]

So how do you go from details to gallery? The ViewModel of your item details would still exist, so anything you do in init (which would the appropriate place to read the ID from the savedStateHandle ) shouldn't be done again when you return to that previous destination

[Marko Novakovic]

yes am doing it inside init but it crashes. that’s weird. I go from details to gallery with navigate(route_string). when I go back details ViewModel is recreated

[Marko Novakovic]

I’ve set up the logs and it’s hitting init even when going back

[Marko Novakovic]

ViewModel is annotated with @HiltViewModel and created with hiltViewModel() function

[Marko Novakovic]

const val hiltNavigation = "1.0.0-alpha02"

[Ian Lake]

That seems wrong. If you can reproduce it in a sample project, I'd love to take a look if you could attach it to an issue: https://issuetracker.google.com/issues/new?component=409828

[Ian Lake]

That way we can confirm if it is already fixed for alpha06 or get a fix in

[Marko Novakovic]

I will create sample project

[Marko Novakovic]

in my sample project everything works fine 😕. I will do some more digging

[Ian Lake]

Do you have some custom back behavior (i.e., a BackHandler or overriding onBackPressed() in your activity)?

[Marko Novakovic]

no. but am passing ViewModel from the details screen to the gallery

composable(Routes.Gallery.route) {
    val ItemDetails = navController.previousBackStackEntry!!
    val viewModel = hiltViewModel<ItemDetailsViewModel>(itemDetails)
    Gallery(viewModel = viewModel)
}

can this be a problem?

[Marko Novakovic]

I didn’t do that in the sample but it’s in the original code. I will try in sample now

[Marko Novakovic]

yes that’t the problem @Ian Lake

[Marko Novakovic]

am I doing it wrong or it’s a bug?

[Marko Novakovic]

androidx.hilt:hilt-navigation-compose:1.0.0-alpha03
androidx.navigation:navigation-compose:2.4.0-alpha05
com.google.dagger:hilt-android:2.37
com.google.dagger:hilt-android-compiler:2.37

[Ian Lake]

Can you wrap your previousBackStackEntry call in a remember?

[Ian Lake]

Keep in mind that with animations, your destination will continue to be recomposed as it is animating out. That means previousBackStackEntry starts pointing to your Home destination

[Ian Lake]

Which obviously doesn't have any arguments like the ID

[Ian Lake]

I suspect that by remembering the entry, you won't requery it every time you recompose (i.e., every frame of the animation), but just once (when it is correctly your ItemDetail screen)

[Marko Novakovic]

“Which obviously doesn’t have any arguments like the ID”. I understand but it should not create new one when one already exists?

[Marko Novakovic]

with navigation-compose:2.4.0-alpha04 and hilt-navigation-compose:1.0.0-alpha03 and 02 everything works

[Marko Novakovic]

about animations and recomposition. that’s interesting. what about animating in? will it still recompose during animation?

[Marko Novakovic]

wrapping previousBackStackEntry in remember works. it’s not crashing now. thank you very much!

[Marko Novakovic]

is this happening in 2.4.0-alpha05 because it introduced fade as default animation which wasn’t the case in 2.4.0-alpha04?

[Ian Lake]

Correct. In the old world, you'ld only recompose once, then immediately disappear as soon as the NavController's state changed

[Ian Lake]

What is happening in your code is that you have popped the back stack. At that point, currentBackStackEntry is now ItemDetail and previousBackStackEntry becomes Home

[Ian Lake]

Which means you aren't 'recreating' the ViewModel attached to ItemDetail, you're attempting to create a brand new ViewModel scoped to Home

[Ian Lake]

Which is why the remember works - now, you've 'cached' your previousBackStackEntry entry over recompositions so that it always points to ItemDetail

[Ian Lake]

(You'd actually get a similar problem if you navigate from Gallery to somewhere else - your currentBackStackEntry would become your new destination and previousBackStackEntry would become Gallery)

[Marko Novakovic]

I understand. great explanation. thank you very much Ian