backhandler-3.kt

@Composable
fun <T> BackHandler(routing: T, children: @Composable() (BackStack<T>) -> Unit) {
    // grab the parent handler, wherever it's up the tree
    val upstream = +ambient(backPressHandler)
    // to keep track of children down the tree
    val downstream = +memo { ScopedBackPressHandler() }
    // let's have a default back stack on this level
    val backStack =  BackStack(routing)
    // 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:
        children(backStack)
    }
}