DataGrid ScrollViewer.ScrollChanged isn't fired

[tms320]

I have the following xaml code in my MainWindow:

<TabControl Grid.Row="0" ItemsSource="{Binding Networks}" DisplayMemberBinding="{Binding Name}">
  <TabControl.ContentTemplate>
    <DataTemplate>
      <TabControl ItemsSource="{Binding Nodes}" DisplayMemberBinding="{Binding Title}">
        <TabControl.ContentTemplate>
          <DataTemplate>
            <Grid RowDefinitions="Auto,*">
              <Label Grid.Row="0" Content="{Binding StateText}" HorizontalAlignment="Left"/>
              <Grid Grid.Row="1" ColumnDefinitions="Auto,*">
                <DataGrid Name="ParamsGrid" Grid.Column="0" ItemsSource="{Binding Objects}" ScrollViewer.ScrollChanged="ParamsGrid_ScrollChanged" IsReadOnly="True">
                  <DataGrid.Columns>
                    <DataGridTextColumn Header="💚" IsReadOnly="True" Binding="{Binding RefreshingValueMark}"/>
                    <DataGridTextColumn Header="Address" IsReadOnly="True" Binding="{Binding Address}"/>
                    <DataGridTextColumn Header="Value" IsReadOnly="True" Binding="{Binding Value}"/>
                  </DataGrid.Columns>
                </DataGrid>
              </Grid>
            </Grid>
          </DataTemplate>
        </TabControl.ContentTemplate>
      </TabControl>
    </DataTemplate>
  </TabControl.ContentTemplate>
</TabControl>

As you can see I set ScrollViewer.ScrollChanged="ParamsGrid_ScrollChanged" in DataGrid.
I have handle method in MainWindow class:

public void ParamsGrid_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
    if ((sender as DataGrid)?.DataContext is NodeViewModel nodeViewModel)
    {
        nodeViewModel.ParamsGrid_ScrollChanged(e);
    }
}

But the event handler isn't calling.
What am I doing wrong?
PS: I use Avalonia 11.2.3.

[thevortexcloud]

As a guess, you might need to change the event registration method.

https://docs.avaloniaui.net/docs/get-started/wpf/tunnelling-events
https://docs.avaloniaui.net/docs/concepts/input/routed-events

[msneijders]

DataGrid is not using a ScrollViewer, but two separate ScrollBars, and is not firing ScrollChangedEvent routed events. I think you can only use these 'normal' events:

class DataGrid
{
        public event EventHandler<ScrollEventArgs> HorizontalScroll;
        public event EventHandler<ScrollEventArgs> VerticalScroll;
}

[tms320]

Update:
VerticalScroll event fired if I drag scrollbar thumb. But if I use mouse wheel the event is not fired.
Found issues:
#15141
#3568

[msneijders]

Looks like you are correct. And there seems no way to get scroll events from DataGrid.

Perhaps there is another way to reach your goal. What is your viewmodel using scroll information for?

Or you could make a PR for raising ScrollViewer.ScrollChangedEvents in DataGrid.

[msneijders]

You could also, in an ugly way, to get hold of the internal ScrollBars of the DataGrid, and listen to its ValueChanged event:

using Avalonia.VisualTree;

class MainView : UserControl
{
   DataGrid myDataGrid;

   protected override void OnLoaded(RoutedEventArgs e)
   {
       base.OnLoaded(e);

       var sb = myDataGrid.GetVisualDescendants().OfType<ScrollBar>().ToList(); // or use reflection to get the internal _vscrollBar

       sb.ForEach(s => s.ValueChanged += (s, e) => System.Diagnostics.Debug.WriteLine($"Value changed: {e.NewValue}"));
   }
}

At least, can you then test out what you wanted to do in your viewmodel.

[tms320]

What is your viewmodel using scroll information for?

My model should update data only for items that are visible in data grid.
In WPF I did something like this (in ScrollViewer.ScrollChanged event handler):

var scrollViewer = FindVisualChild<ScrollViewer>(dataGrid);
var firstRow = (int)scrollViewer.VerticalOffset;
var lastRow = (int)scrollViewer.VerticalOffset + (int)scrollViewer.ViewportHeight + 1;

[tms320]

You could also, in an ugly way, to get hold of the internal ScrollBars of the DataGrid, and listen to its ValueChanged event:

This trick helped. Thanks )