Adorner in ScrollViewer

gistfile1.cs

		void AddLayer(ScrollViewer scrollViewer)
		{
			scrollViewer.ApplyTemplate();
			ScrollBar scrollBar = (ScrollBar)scrollViewer.Template.FindName("PART_VerticalScrollBar", scrollViewer);
			scrollBar.ApplyTemplate();
			Track track = (Track)scrollBar.Template.FindName("PART_Track", scrollBar);
			var grid = VisualTreeHelper.GetParent(track) as Grid;
			if (grid != null) {
				var myElement = new Border();
				Grid.SetColumn(myElement, Grid.GetColumn(track));
				Grid.SetRow(myElement, Grid.GetRow(track));
				Grid.SetColumnSpan(myElement, Grid.GetColumnSpan(track));
				Grid.SetRowSpan(myElement, Grid.GetRowSpan(track));
				Panel.SetZIndex(track, 1);
				//myElement.Background = new SolidColorBrush(Color.FromArgb(128, 255, 0, 0));
				myElement.Child = new Border {
					Background = Brushes.Red,
					Margin = new Thickness(3, 0, 3, 0),
					Height = 2
				};
				myElement.IsHitTestVisible = false;
				grid.Children.Add(myElement);
				
				Thumb thumb = null;
				for (int i = 0; i < VisualTreeHelper.GetChildrenCount(track); i++) {
					thumb = VisualTreeHelper.GetChild(track, i) as Thumb;
					if (thumb != null)
						break;
				}
				
				if (thumb != null) {
					thumb.Opacity = 0.7;
					thumb.MouseEnter += delegate {
						thumb.BeginAnimation(Track.OpacityProperty, new DoubleAnimation(1, new Duration(TimeSpan.FromSeconds(0.3)), FillBehavior.HoldEnd));
					};
					thumb.MouseLeave += delegate {
						thumb.BeginAnimation(Track.OpacityProperty, new DoubleAnimation(0.7, new Duration(TimeSpan.FromSeconds(0.3)), FillBehavior.HoldEnd));
					};
				}
				
				AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(track);
				if (adornerLayer != null) {
					adornerLayer.Add(new TriangleRendererAdorner(track));
				}
			}
		}
		
		class TriangleRendererAdorner : Adorner
		{
			public TriangleRendererAdorner(UIElement adornedElement) : base(adornedElement)
			{
			}
			
			protected override void OnRender(DrawingContext drawingContext)
			{
				const double triangleSize = 6.5;
				var streamGeometry = new StreamGeometry();
				using (var ctx = streamGeometry.Open()) {
					ctx.BeginFigure(new Point(0, triangleSize / 2), true, true);
					ctx.LineTo(new Point(0, -triangleSize / 2), true, false);
					ctx.LineTo(new Point(triangleSize * 0.866, 0), true, false);
				}
				streamGeometry.Freeze();
				drawingContext.PushTransform(new TranslateTransform(1, 20));
				drawingContext.DrawGeometry(Brushes.Red, null, streamGeometry);
				drawingContext.Pop();
				drawingContext.PushTransform(new TranslateTransform(1, 35));
				drawingContext.DrawGeometry(Brushes.Blue, null, streamGeometry);
				drawingContext.Pop();
			}
		}