Lots more to do after the first alpha though 😅

Hi @Ian Lake, in the current navigation component in android we have

navController.addOnDestinationChangedListener { 
    ...
    // Hide/show shared UI here
    ...
}

We usually do this for switching persistent

AppBar

or any shared UI across screens. Is this still the case for

Compose

or will Compose Navigation Component introduce something different?

Hi @Ian Lake may I ask another question again please. So in this setup:

val navController = rememberNavController()
val currentBackStackEntry = navController.currentBackStackEntryAsState()
Scaffold(
  topBar = {
    // Use whatever logic you want here
    if (currentBackStackEntry?.value?.arguments?.getBoolean("showAppBar") == true) {
      MyCustomerAppBar()
    } else {
      DifferentAppBar()
    }
) {
  NavHost(navController, startDestination = "start") {
    composable("start") {
    }
  }
}

The app bar is shared across screens. Assuming that the app bar have app bar menu and when clicked, depending on the current screen it triggers a different action. In the current way of doing this with fragments, we do something like:

class MyFragment : Fragment() {
    override fun onCreate(context: Context) {
        ....
        setHasOptionsMenu(true)
        ....
    }

    // To set the menu
    override fun onCreateOptionsMenu(menu: Menu) {
        ....
        menuInflater.inflate(R.menu.myMenu, menu)
        ....
    }

    // to Recieve the event when the menu is clicked
    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        // Handler action when menu item is clicked
    }

    ....
}

How should the same thing be handled in compose? Since the AppBar is now in the scope of the current View. The only way I can think of is doing something like this in compose.

val navController = rememberNavController()
val currentBackStackEntry = navController.currentBackStackEntryAsState()

// create a state which I could pass down and listen to when value gets set
val someState = remember { SomeState() }
Scaffold(
  topBar = {
    Button(
       onClick = { someState.value  = SomeValue() }, // Set the value of the state 
       content = { Text("Click ME!")  },
    )
) {
  NavHost(navController, startDestination = "start") {
    composable("start") {
        MyScreen(someState) // Check value of someState inside My Screen to know when the button is clicked.
    }
  }
}

But I feel like this is the wrong way.

@Ian Lake Last question, So I noticed by doing

viewModel()

inside a

composable(...) {}

the

ViewModel

is now scoped to that

composable(...)

instead of the

Activity

. as seen in this code snippet inside

NavHost

...
if (destination is ComposeNavigator.Destination) {
    // while in the scope of the composable, we provide the navBackStackEntry as the
    // ViewModelStoreOwner and LifecycleOwner
    Providers(
         AmbientNavController provides navController,
         ViewModelStoreOwnerAmbient provides currentNavBackStackEntry!!,
         LifecycleOwnerAmbient provides currentNavBackStackEntry!!,
         children = destination.content
    )
}
....

I was wondering If is also possible to scope a

ViewModel

across multiple

composable(...)

just like how

navGraphViewModels(...)

work?