This is how I send a result from an edit screen back to the parent screen on the backstack. But I wo...

# compose

[Florian]

This is how I send a result from an edit screen back to the parent screen on the backstack. But I would like to receive this event only once (like it is the case for the old fragment result API). Is there a way to do that?

composable(TodoScreen.TodoList.name) { navBackStackEntry ->
            TodoListScreen(
                [...]
                addEditResult = navBackStackEntry.savedStateHandle.get<Int>(KEY_ADD_EDIT_RESULT)
            )
        }

As far as I see, I will always receive this result on every recomposition (like when the screen is rotated) causing to show feedback snackbars again.

[Ian Lake]

Quoting the docs (https://developer.android.com/guide/navigation/navigation-programmatic#returning_a_result):

If you’d only like to handle a result only once, you must call remove() on the SavedStateHandle to clear the result.

[Florian]

@Ian Lake Thank you. I tried it earlier and didn't work. But I think I know why. Because the backstack entry and the ViewModel have different SavedStateHandles.

[Florian]

I was trying to clear the value from the ViewModel but I guess I have to clear it directly in the Composable.

[Ian Lake]

If you are using navBackStackEntry.savedStateHandle, you need to call remove() on navBackStackEntry.savedStateHandle, yes

[Florian]

alright, thank you very much!

[Florian]

@Ian Lake Like this right?

NavHost(
        navController = navController,
        startDestination = TodoScreen.TodoList.name,
        modifier = modifier
    ) {
        composable(TodoScreen.TodoList.name) { navBackStackEntry ->
            val addEditResult = navBackStackEntry.savedStateHandle.get<Int>(KEY_ADD_EDIT_RESULT)
            navBackStackEntry.savedStateHandle.remove<Int>(KEY_ADD_EDIT_RESULT)
            TodoListScreen(
                onTodoEditClicked = { todo ->
                    navController.navigate("${TodoScreen.AddEditTodo.name}?todoId=${todo.id}")
                },
                onFabAddNewTodoClicked = {
                    navController.navigate(TodoScreen.AddEditTodo.name)
                },
                addEditResult = addEditResult
            )
        }

[Ian Lake]

I'm always a little worried about calling imperative methods (like remove) as part of composition - generally you'd want to handle events through one of the Effects APIs

[Ian Lake]

Like in this case, on recomposition addEditResult will be null again (because you removed it), thus caling your TodoListScreen to be passed null again. That seems to imply that you only care about a single composition happening with a valid result, which feels weird?

[Ian Lake]

You'll definitely be recomposed multiple times as enter animations occur

[Ian Lake]

How are you handling recomposition, events, and state in this case?

[Florian]

in the Composable itself I handle the result in a `DisposableEffect`:

DisposableEffect(addEditResult) {
        if (addEditResult != null) {
            viewModel.onAddEditResult(addEditResult)
        }
        onDispose {}
    }

[Florian]

where else could I remove this value from the SavedStateHandle?

[Ian Lake]

Well, generally you'd remove it after you finish processing it, so after your onAddEditResult returns

[Florian]

but for this I need a reference to the backstackEntry inside the composable

[Florian]

can I just call rememberNavController there?

[Ian Lake]

Which would involve either passing the SavedStateHandle down (so you could call remove directly) or passing in a lambda parameter that would call remove when it fires

[Ian Lake]

TodoListScreen(
  onTodoEditClicked = { todo ->
    navController.navigate("${TodoScreen.AddEditTodo.name}?todoId=${todo.id}")
  },
  onFabAddNewTodoClicked = {
    navController.navigate(TodoScreen.AddEditTodo.name)
  },
  addEditResult = navBackStackEntry.savedStateHandle.get<Int>(KEY_ADD_EDIT_RESULT)
  onAddEditResultProcessed = {
    navBackStackEntry.savedStateHandle.remove(KEY_ADD_EDIT_RESULT)
  }
)

[Florian]

ok, thank you

[Florian]

and on the receiving side I do this?

@Composable
fun TodoListScreen(
    [...]
    addEditResult: Int?,
    onAddEditResultProcessed: () -> Unit
) {
    [...]

    DisposableEffect(addEditResult) {
        if (addEditResult != null) {
            viewModel.onAddEditResult(addEditResult)
            onAddEditResultProcessed()
        }
        onDispose {}
    }

    [...]
}

[Ian Lake]

Yeah

[Florian]

thank you 👌