Mouse scroll not working in a scroll viewer with a

Question

I am trying to figure out how to get the mouse scroll working on a wpf window with a scrollviewer and a datagrid within it. The WPF and C# code is below

<ScrollViewer HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid Grid.Row="0">

            <Border Name="DataGridBorder" BorderThickness="2"  Margin="1" CornerRadius="4" BorderBrush="#FF080757">
                <dg:DataGrid AutoGenerateColumns="False" Name="ValuesDataGrid" 
                         BorderThickness="0" CanUserResizeColumns="True" FontWeight="Bold" HorizontalScrollBarVisibility="Auto" 
                         CanUserReorderColumns="False" IsReadOnly="True" IsTextSearchEnabled="True" AlternationCount="2"
                         SelectionMode="Extended" GridLinesVisibility="All"                
                         HeadersVisibility="Column" CanUserAddRows="False" CanUserDeleteRows="False" CanUserResizeRows="False" CanUserSortColumns="False"
                         RowDetailsVisibilityMode="Collapsed"  SelectedIndex="0"
                         RowStyle="{StaticResource CognitiDataGridRowStyle}"
                         >

                    <dg:DataGrid.Columns>
                        <dg:DataGridTemplateColumn Header="Title" >
                            <dg:DataGridTemplateColumn.CellTemplate>
                                <DataTemplate>
                                    <StackPanel Orientation="Horizontal" >
                                        <TextBlock HorizontalAlignment="Left" VerticalAlignment="Center" Text="{Binding Path=Name}" FontWeight="Normal"  />
                                    </StackPanel>
                                </DataTemplate>
                            </dg:DataGridTemplateColumn.CellTemplate>
                        </dg:DataGridTemplateColumn>
                    </dg:DataGrid.Columns>
                </dg:DataGrid>
            </Border>
        </Grid>
        <Button Grid.Row="1" Height="90" >hello world</Button>
    </Grid>
</ScrollViewer>

and the C# code is as follows

 public partial class Window1 : Window
  {
     public Window1()
     {
        InitializeComponent();
        initialize();
      }

    public void initialize()
    {
        ObservableCollection<MyObject> testList = new ObservableCollection<MyObject>();

        for (int i = 0; i < 20; i++)
        {
            MyObject my = new MyObject("jack " + i);
            testList.Add(my);
        }

        ValuesDataGrid.ItemsSource = testList;



    }
}

public class MyObject
{
    public string Name { get; set; }



    public MyObject(string name)
    {
        Name = name;
    }
   }

The problem i am facing is that when using the mouse to scroll, it works fine when it is over the button but as soon as i move the mouse pointer over the grid and try to scroll, nothing happens. I am able to move the scrollbar of the scrollviewer directly though. I am still a wpf novice so any help on how to get the mouse scroll to work over the datagrid would be appreciated. I am guessing there should be a pretty easy solution for this but I havent been able to figure it out

Answer 1

To enable touch support you might also want to set ScrollViewer.PanningMode to None on your DataGrid and set the same property to VerticalFirst or other value on your top level ScrollViewer

Example

<ScrollViewer VerticalScrollBarVisibility="Auto" Margin="5" PanningMode="VerticalFirst">
    <DataGrid ScrollViewer.PanningMode="None" ItemsSource="{Binding Items}" />
</ScrollViewer>

Of course, also use the PreviewMouseWheel event as indicated by Don B's answers to fix the original mouse scrolling issue.

private static void ScrollViewer_PreviewMouseWheel(object sender, System.Windows.Input.MouseWheelEventArgs e)
{
    var scrollViewer = (ScrollViewer)sender;
    scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset - e.Delta);
    e.Handled = true;
}

Or, you can simply set the following Attached Property to your ScrollViewer

public class TopMouseScrollPriorityBehavior
{
    public static bool GetTopMouseScrollPriority(ScrollViewer obj)
    {
        return (bool)obj.GetValue(TopMouseScrollPriorityProperty);
    }

    public static void SetTopMouseScrollPriority(ScrollViewer obj, bool value)
    {
        obj.SetValue(TopMouseScrollPriorityProperty, value);
    }

    public static readonly DependencyProperty TopMouseScrollPriorityProperty =
        DependencyProperty.RegisterAttached("TopMouseScrollPriority", typeof(bool), typeof(TopMouseScrollPriorityBehavior), new PropertyMetadata(false, OnPropertyChanged));

    private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var scrollViewer = d as ScrollViewer;
        if (scrollViewer == null)
            throw new InvalidOperationException($"{nameof(TopMouseScrollPriorityBehavior)}.{nameof(TopMouseScrollPriorityProperty)} can only be applied to controls of type {nameof(ScrollViewer)}");
        if (e.NewValue == e.OldValue)
            return;
        if ((bool)e.NewValue)
            scrollViewer.PreviewMouseWheel += ScrollViewer_PreviewMouseWheel;
        else
            scrollViewer.PreviewMouseWheel -= ScrollViewer_PreviewMouseWheel;
    }

    private static void ScrollViewer_PreviewMouseWheel(object sender, System.Windows.Input.MouseWheelEventArgs e)
    {
        var scrollViewer = (ScrollViewer)sender;
        scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset - e.Delta);
        e.Handled = true;
    }
}

Usage

<ScrollViewer b:TopMouseScrollPriorityBehavior.TopMouseScrollPriority="True" VerticalScrollBarVisibility="Auto" Margin="5" PanningMode="VerticalFirst">
    <DataGrid ScrollViewer.PanningMode="None" ItemsSource="{Binding Items}" />
</ScrollViewer>

Where b: is the namespace that contains this behavior

This way your no code-behind is needed and your app is purely MVVM

Answer 2

I tried Don B's solution and it's solves the issue with scrolling over a data grid pretty well for the cases when you don't have other inner scrollable controls.

For the case when the scroll viewer has as children other scrollable controls and a data grid, then that requires that the event shouldn't be marked as handled at the end of the event handler for the main scroll viewer so it could be catched also in the inner scrollable controls, however that has the side effect that when the scroll should occur only on the inner scrollable control, it will also occur on the main scroll viewer.

So I've updated Dave's solution with the difference for how the scroll viewer is found so it won't be necessary to know the name of the scroll viewer:

private void DataGrid_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
{
    ScrollViewer scrollViewer = (((DependencyObject)sender).GetVisualParent<ScrollViewer>());
    scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset - e.Delta);
}

Answer 3

An improvement to Don B's solution is to avoid using ScrollToVerticalOffset.

scv.ScrollToVerticalOffset(scv.VerticalOffset - e.Delta);

VerticalOffset - Delta results in a pretty jarring experience. The ScrollViewer puts a lot of thought into making the movement smoother than that. I think it also scales the delta down based on dpi and other factors...

I found it's better to catch and handle the PreviewMouseWheelEvent and send a MouseWheelEvent to the intended ScrollViewer. My version of Don B's solution looks like this.

private void ScrollViewer_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
{
    var eventArg = new MouseWheelEventArgs(e.MouseDevice, e.Timestamp, e.Delta);
    eventArg.RoutedEvent = UIElement.MouseWheelEvent;
    eventArg.Source = e.Source;

    ScrollViewer scv = (ScrollViewer)sender;
    scv.RaiseEvent(eventArg);
    e.Handled = true;
}