WPF - Animate ListBox.ScrollViewer.HorizontalOffset?

Question

I have a collection of Visuals in a ListBox. I need to find the XPosition of an element inside it and then animate the HorizontalOffset of the ListBox's ScrollViewer. Essentially I want to created an animated ScrollIntoView method.

This gives me a couple of problems. Firstly, how can I get a reference to the ListBoxs scrollviewer? Secondly, how can i get the relative XPosition or HozintalOfffset of an arbitrary element in the ListBox?

I'm not reponding to any input on the ListBox itself so I can't use Mouse related properties.

Answer 1

I don't think you will be able to use a WPF storyboard for the animation because storyboards animate WPF dependency properties. You will need to call ScrollViewer.SetHorizontalOffset(double) to scroll.

You could try creating a custom dependency property that calls SetHorizontalOffset in the OnDependencyPropertyChanged() function. Then you could animate this property.

public static readonly DependencyProperty ScrollOffsetProperty =
   DependencyProperty.Register("ScrollOffset", typeof(double), typeof(YOUR_TYPE),
   new FrameworkPropertyMetadata(0.0, new PropertyChangedCallback(OnScrollOffsetChanged)));


public double ScrollOffset
{
   get { return (double)GetValue(ScrollOffsetProperty); }
   set { SetValue(ScrollOffsetProperty, value); }
}

private static void OnScrollOffsetChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
   YOUR_TYPE myObj = obj as YOUR_TYPE;

   if (myObj != null)
      myObj.SCROLL_VIEWER.ScrollToHorizontalOffset(myObj.ScrollOffset);
}

To get the scroll viewer you can use the VisualTreeHelper to search the visual children of the ListBox. Save a reference to the ScrollViewer because you will need it later. Try this:

public static childItem FindVisualChild<childItem>(DependencyObject obj)
   where childItem : DependencyObject
{
   // Search immediate children first (breadth-first)
   for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
   {
      DependencyObject child = VisualTreeHelper.GetChild(obj, i);

      if (child != null && child is childItem)
         return (childItem)child;

      else
      {
         childItem childOfChild = FindVisualChild<childItem>(child);

         if (childOfChild != null)
            return childOfChild;
      }
   }

   return null;
}

This function returns the first visual child of the parameter type. Call FindVisualChild<ScrollViewer>(ListBox) to get the ScrollViewer.

Finally, try using UIElement.TranslatePoint(Point, UIElement) to get the X position of the item. Call this function on the item, pass in 0,0 for the point, and pass in the ScrollViewer.

Hope this helps.

Answer 2

Hi I tried using this method to implement smooth scrolling and I get the error: " Cannot animate the 'ScrollOffset' property on 'System.Windows.Media.MatrixTransform' because the object is sealed or frozen. "

Any ideas? Below is the code I'm using to animate...

        DoubleAnimation moveAnimation = new DoubleAnimation();
        moveAnimation.From = scrollViewer.VerticalOffset;
        if (scrollViewer.VerticalOffset >=100)
            moveAnimation.To = scrollViewer.VerticalOffset - 100;
        else
            moveAnimation.To = 0;
        moveAnimation.Duration = new Duration(new TimeSpan(0, 0, 0, 0, 500));
        this.RenderTransform.BeginAnimation(ActionScreen.ScrollOffsetProperty, moveAnimation);