backhandler-9.kt

@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)
    }
    // to keep track of children down the tree
    val downstream = +memo { ScopedBackPressHandler(backStack) }

    // ...
    // from here on it's the same as before + a cast in the last line
    // ...

    // our lambda for handling back press: asking all children,
    val handleBackPressHere: () -> Boolean = { downstream.handle() || backStack.pop() }
    // register our handler in the parent handler
    upstream.handlers.add(handleBackPressHere)
    // remove ourselves from the parent handler when this level goes away
    +onDispose { upstream.handlers.remove(handleBackPressHere) }

    // override the ambient so that it points to this level
    // so that we'll become the upstream of some child later:
    backPressHandler.Provider(value = downstream) {
        // add client code Composable, which will have access to back stack
        // we need an ugly cast to make it work though
        children(backStack as BackStack<T>)
    }
}