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A [back stack of 2 elements]
└── X [...]
      └── Y [...]
            └── Z [...]

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A [back stack of 1 element]
└── B [back stack of 1 element]
      └── C [back stack of 1 element]
            └── D [back stack of 1 element]

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class Entry<T>(val element: T) {
    val children: MutableMap<Any, BackStack<*>> = mutableMapOf()
}

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class ScopedBackPressHandler(
    // nullable, because root level doesn’t have one
    var backStack: BackStack<*>? = null 
) {
    // remainder the same
}

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@Composable
// id is a new parameter we'll use for the key in the map:
fun <T> BackHandler(id: Any, routing: T, children: @Composable() (BackStack<T>) -> Unit) {
    // grab the parent handler, wherever it's up the tree
    val upstream = +ambient(backPressHandler)
    // is there a back stack in the upstream? then let's grab the map of the last entry
    val saveRestorePoint = upstream.backStack?.lastEntry()?.children
    // use existing or fall back to new instance
    val backStack = saveRestorePoint?.get(id) ?: BackStack(routing).also {
        saveRestorePoint?.set(id, it)

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fun replace(element: T) {
    elements = elements.dropLast(1) + Entry(element)
}

fun newRoot(element: T) {
    elements = listOf(Entry(element))
}

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fun pushAndDropNested(element: T) {
    elements.last().children.clear()
    push(element)
}

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/**
 * Based on https://medium.com/swlh/android-mvi-with-jetpack-compose-b0890f5156ac
 * Adapted for 0.1.0-dev04
 */
@Composable
fun <T> observe(data: LiveData<T>): T? {
    var result by state { data.value }
    val observer = remember { Observer<T> { result = it } }

    onCommit(data) {

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sealed class Routing {
    object ChildScreen1 : Routing()
    object ChildScreen2 : Routing()
    data class ChildScreen3(val foo: Bar) : Routing()
}

@Composable
fun MyParentComposable(defaultRouting: Routing) {
  
    BackHandler(defaultRouting) { backStack ->

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@Composable
fun PhotoAlbum() {
    val photos: List<Photo> = emptyList() // TODO

    TableRenderer(cols = 4, cellSpacing = 8.dp, items = photos) { cell ->
        Text(cell.item.photoDescription)
    }
}