Picker: Selected item index is not maintained when the list is changed

Minimal repro:

Program.mkProgram
    (fun () -> false, Cmd.ofMsg ())
    (fun () model -> not model, async { do! Async.Sleep 1000 } |> Cmd.ofAsyncMsg)
    (fun model _ -> View.ContentPage(View.Picker((if model then [1; 2] else [1; 2; 3]), selectedIndex = 0)))

The above code changes the list every second. The selected index is kept constant. However, the selected item unselected.

Compare:

Program.mkSimple id (fun () -> id) (fun _ _ -> View.ContentPage(View.Picker([1; 2], selectedIndex = 0)))

When the list is unchanged, the selected index is set successfully.

[TimLariviere]

A property that depends on another one is not something Fabulous really likes. The source of truth needs to be the ViewElement, not some side effect from the underlying framework.

The way I can imagine to fix it is a custom update code for ItemsSource instead of SelectedIndex. If the ItemsSource needs to change, we will need to automatically reapply the SelectedIndex if possible as well.

[TimLariviere]

You’re right. It’s not possible for the moment, the Generator doesn’t support it. Before we’re able to move forward on this issue, we will need to decide how we proceed with the Generator. (might heavily change if we do the framework agnostic view diffing)