UIThread - Dispatcher - await, a more fundamental question

[drei97]

Hi,

in my project I am trying to follow the MVVM pattern. It seems I am lacking some understanding of asynchronous operation, especially about the handling of menu items and buttons.

in myview.axaml I am typically using the following code snippet

<Button Content="Load"
        Name="BtnLoad"
        CommandParameter="Load"
        Click="Button_OnClick"/>

with myview.axaml.cs

private readonly Dictionary<string, Action> _commands = new();

private void Button_OnClick(object? sender, RoutedEventArgs e)
{
   if (sender is Button btn)
       if (btn.CommandParameter is string commandparameter)
            if (_commands.TryGetValue(commandparameter, out var command))
               command();
}

public MyView()
{
    InitializeComponent();
    DataContext = ViewModel = new MyViewViewModel();
    _commands.Add("Load", Load);    
}


private void Load()
{
    ViewModel.Load();
    Dispatcher.UIThread.InvokeAsync(() =>
        DataGrid.ScrollIntoView(ViewModel.SelectedItem, null));
}

and in myviewviewmodel.cs

public void Load()
{
    Data = new ObservableCollection<DocumentsBillEntity>(_bills.Bills.OrderBy(xx => xx.Date));
    SelectedItem = Data.LastOrDefault();
}

How to apply ScrollIntoView?

Calling DataGrid.ScrollIntoView(ViewModel.SelectedItem, null) without Dispatcher is not working properly. Sometimes it is not scrolled. But why is it not working? The call is within the UI thread.

dito with calling Dispatcher.UIThread.Invoke(() => DataGrid.ScrollIntoView(ViewModel.SelectedItem, null));}

Calling InvokeAsync is working, the last entry is scrolled into view.

Is it better/necessary to do await Dispatcher.InvokeAsync()? I think so. Then the event handler OnClick would have to be async. But if so, why is it working without await for 'InvokeAsync()' if `Invoke()' does not?

Joachim

-- EDIT: --
What is the best dispatcher priority in that case?

[kekekeks]

But why is it not working? The call is within the UI thread.

Layout doesn't happen immediately when you change things in your view model. There might be no ScrollViewer materialized yet, controls might still have no size, etc. Scheduling your callback via dispatcher makes sure that UI is ready for interactions like scrolling

[timunie]

Tip

you can also use Dispatcher.Post if you don't need to wait for the result.

[drei97]

Post() seems sufficient

thnx