ListBox control firing SelectionChanged event before updating SelectedItem property

[3aIno]

Describe the bug

When using the ListBox control in an Avalonia MVVM application, it appears that the control is firing the SelectionChanged event before updating the SelectedItem property. This results in the SelectedItem property referencing the old value instead of the newly selected item. This is only happening when using EventTriggerBehavior, and the issue does not occur when handling the event in code-behind.

To Reproduce

1. Add a ListBox control.
2. Bind the Items property of the ListBox to a collection in the ViewModel.
3. Bind the SelectedItem property of the ListBox to an Observable Property in the ViewModel using a TwoWay binding.
4. Use the Interaction.Behaviors library to bind a RelayCommand to the SelectionChanged event of the ListBox control.
5. In the RelayCommand, log the value of the SelectedItem property.
6. Select an item from the ListBox control.

`<ListBox x:Name="ExampleListBox" 
                         Items="{Binding ExampleItems, Mode=OneWay}"
                         SelectionMode="Single" 
                         SelectedItem="{Binding SelectedItem, Mode=TwoWay}">
                    <Interaction.Behaviors>
                        <EventTriggerBehavior SourceObject="ExampleListBox" 
                                              EventName="SelectionChanged">
                            <InvokeCommandAction Command="{Binding SelectedItemChangedCommand}" />
                        </EventTriggerBehavior>
                    </Interaction.Behaviors>
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding}" />
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>`

` public ObservableCollection<string> ExampleItems { get; set; } = new()
    {
        "Test 1",
        "Test 2",
        "Test 3"
    };

    [ObservableProperty] private string _selectedExampleItem;

    [RelayCommand]
    private void SelectedExampleItemChanged()
    {
        Console.WriteLine(SelectedExampleItem);
    }

Expected behavior The the SelectedItem property should be the item in the ListBox control which has just been selected, rather than the item it was prior to the selection change.

Desktop

• Windows
• Version 11 preview 6

Additional context This may be a similar issue to that being reported here.

[workgroupengineering]

I have same Issue this is sample repo AListboxIssue.zip

@3aIno Have you found a workaround?

[workgroupengineering]

Workaround

    [RelayCommand]
    private void SelectedExampleItemChanged()
    {
        Avalonia.Threading.Dispatcher.UIThread.Post( ()=>
                        Console.WriteLine(SelectedExampleItem));
    }

[wieslawsoltes]

It's Avalonia issue

[lnxon]

I just created a pull request that fixes this issue. As wieslawsoltes suggested it was Avalonia control's issue. Please see AvaloniaUI/Avalonia pull request https://github.com/AvaloniaUI/Avalonia/pull/13503