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sealed class RootRouting {
    object LoggedOut: RootRouting()
    data class LoggedIn(val user: User): RootRouting()
}

sealed class LoggedInRouting {
    object News : LoggedInRouting()
    object Gallery : LoggedInRouting()
    object Profile : LoggedInRouting()
}

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interface Gallery {

    sealed class Routing {
        object AlbumList : Routing()
        data class PhotosOfAlbum(val album: Album) : Routing()
        data class FullScreenPhoto(val photo: Photo) : Routing()
    }

    companion object {
        @Composable

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routing = Routing.PhotosOfAlbum(someAlbum)

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@Model
class BackStack<T>(
    defaultElement: T
) {
    private var elements: List<T> = listOf(defaultElement)
    
    fun last(): T =
        elements.last()

    fun push(element: T) {

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@Composable
fun Content() {
-    var routing by +state<Routing> { Routing.AlbumList }
+    var backStack by +state<BackStack<Routing>> { BackStack(Routing.AlbumList) }

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-    when (routing) {
+    when (currentRouting = backStack.last()) {
    // all Content() on the right hand side are @Composable
    is Routing.AlbumList -> AlbumList.Content()
    is Routing.PhotosOfAlbum -> PhotosOfAlbum.Content(currentRouting.album)
    is Routing.FullScreenPhoto -> FullScreenPhoto.Content(currentRouting..photo)
}

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// “going forward”
backStack.push(Routing.PhotosOfAlbum(album))

// “going backwards” -- we’ll want to trigger this from actual back press
backStack.pop()

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class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                Root.Content()
            }
        }
    }

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@Model
object BackPress {
    var triggered = false
}

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class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                Root.Content(Backpress)
            }
        }
    }