Kirk Ouimet Kirk Ouimet - 12 days ago 5
C# Question

Make ListView.ScrollIntoView Scroll the Item into the Center of the ListView (C#)

ListView.ScrollIntoView(object)
currently finds an object in the
ListView
and scrolls to it. If you are positioned beneath the object you are scrolling to, it scrolls the object to the top row. If you are positioned above, it scrolls it into view at the bottom row.

I'd like to have the item be scrolled right into the center of my list view if it is currently not visible. Is there an easy way to accomplish this?

Answer

It is very easy to do this in WPF with an extension method I wrote. All you have to do to scroll an item to the center of the view is to call a single method.

Suppose you have this XAML:

<ListView x:Name="view" ItemsSource="{Binding Data}" /> 
<ComboBox x:Name="box"  ItemsSource="{Binding Data}"
                        SelectionChanged="ScrollIntoView" /> 

Your ScrollIntoView method will be simply:

private void ScrollIntoView(object sender, SelectionChangedEventArgs e)
{
  view.ScrollToCenterOfView(box.SelectedItem);
} 

Obviously this could be done using a ViewModel as well rather than referencing the controls explicitly.

Following is the implementation. It is very general, handling all the IScrollInfo possibilities. It works with ListBox or any other ItemsControl, and works with any panel including StackPanel, VirtualizingStackPanel, WrapPanel, DockPanel, Canvas, Grid, etc.

Just put this in a .cs file somewhere in your project:

public static class ItemsControlExtensions
{
  public static void ScrollToCenterOfView(this ItemsControl itemsControl, object item)
  {
    // Scroll immediately if possible
    if(!itemsControl.TryScrollToCenterOfView(item))
    {
      // Otherwise wait until everything is loaded, then scroll
      if(itemsControl is ListBox) ((ListBox)itemsControl).ScrollIntoView(item);
      itemsControl.Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new Action(() =>
        {
          itemsControl.TryScrollToCenterOfView(item);
        }));
    }
  }

  private static bool TryScrollToCenterOfView(this ItemsControl itemsControl, object item)
  {
    // Find the container
    var container = itemsControl.ItemContainerGenerator.ContainerFromItem(item) as UIElement;
    if(container==null) return false;

    // Find the ScrollContentPresenter
    ScrollContentPresenter presenter = null;
    for(Visual vis = container; vis!=null && vis!=itemsControl; vis = VisualTreeHelper.GetParent(vis) as Visual)
      if((presenter = vis as ScrollContentPresenter)!=null)
        break;
    if(presenter==null) return false;

    // Find the IScrollInfo
    var scrollInfo = 
        !presenter.CanContentScroll ? presenter :
        presenter.Content as IScrollInfo ??
        FirstVisualChild(presenter.Content as ItemsPresenter) as IScrollInfo ??
        presenter;

    // Compute the center point of the container relative to the scrollInfo
    Size size = container.RenderSize;
    Point center = container.TransformToAncestor((Visual)scrollInfo).Transform(new Point(size.Width/2, size.Height/2));
    center.Y += scrollInfo.VerticalOffset;
    center.X += scrollInfo.HorizontalOffset;

    // Adjust for logical scrolling
    if(scrollInfo is StackPanel || scrollInfo is VirtualizingStackPanel)
    {
      double logicalCenter = itemsControl.ItemContainerGenerator.IndexFromContainer(container) + 0.5;
      Orientation orientation = scrollInfo is StackPanel ? ((StackPanel)scrollInfo).Orientation : ((VirtualizingStackPanel)scrollInfo).Orientation;
      if(orientation==Orientation.Horizontal)
        center.X = logicalCenter;
      else
        center.Y = logicalCenter;
    }

    // Scroll the center of the container to the center of the viewport
    if(scrollInfo.CanVerticallyScroll) scrollInfo.SetVerticalOffset(CenteringOffset(center.Y, scrollInfo.ViewportHeight, scrollInfo.ExtentHeight));
    if(scrollInfo.CanHorizontallyScroll) scrollInfo.SetHorizontalOffset(CenteringOffset(center.X, scrollInfo.ViewportWidth, scrollInfo.ExtentWidth));
    return true;
  }

  private static double CenteringOffset(double center, double viewport, double extent)
  {
    return Math.Min(extent - viewport, Math.Max(0, center - viewport/2));
  }
  private static DependencyObject FirstVisualChild(Visual visual)
  {
    if(visual==null) return null;
    if(VisualTreeHelper.GetChildrenCount(visual)==0) return null;
    return VisualTreeHelper.GetChild(visual, 0);
  }
}