NavResult.kt

/**
 * Sets the result to be obtained by [NavResultCallback] attached to [targetBackStackEntry]
 */
abstract class NavResultPusher<T>(
    private val targetBackStackEntry: NavBackStackEntry,
    private val key: String
) {
    open fun setResult(result: T) = targetBackStackEntry.savedStateHandle.set(key, result)
}

/**
 * Observes results passed from a nav destination to this [NavBackStackEntry].
 * Delivers the result [ON_RESUME] to handle both fullscreen and dialog origins.
 *
 * Alternative to savedStateHandle.getLiveData which redelivers the old value despite it being cleared
 * and which doesn't handle dialog destinations well
 * (see: https://developer.android.com/guide/navigation/navigation-programmatic#additional_considerations)
 */
abstract class NavResultCallback<T>(
    private val currentBackStackEntry: NavBackStackEntry,
    private val key: String
) {

    private val flow = MutableStateFlow<T?>(null)

    open fun observeResult(): Flow<T?> {

        val observer = LifecycleEventObserver { _, event ->
            if (event == ON_RESUME && currentBackStackEntry.savedStateHandle.contains(key)) {
                flow.tryEmit(currentBackStackEntry.savedStateHandle.get<T>(key)!!)
            }
        }

        return flow
            .onStart { currentBackStackEntry.lifecycle.addObserver(observer) }
            .onCompletion { currentBackStackEntry.lifecycle.removeObserver(observer) }
    }

    fun clearResult() {
        currentBackStackEntry.savedStateHandle.remove<Boolean>(key)
        flow.tryEmit(null)
    }

}

// ===USAGE===
private const val DELETED_ID = "deleted_id"

//this should be injected into the viewmodel of the the Composable started for result, e.g. some DeleteItemScreen
//when the item is deleted and just before popBackStack, the viewmodel should call pusher.setResult(deletedId)
class DeletedIdResultPusher(
   targetBackStackEntry: NavBackStackEntry
) : NavResultPusher<String>(targetBackStackEntry, DELETED_ID)

//this should be injected into the viewmodel of the the Composable that navigates to DeleteItemScreen for result
//it can e.g. callback.observeResult().collectAsState in the composable to show a Snackbar and when the snackbar is dimissed
//it can call callback.clear() to get rid of the result
class DeletedIdResultCallback(
    currentBackStackEntry: NavBackStackEntry
) : NavResultCallback<String>(currentBackStackEntry, DELETED_ID)