private static void strokeThickness_Changed(object sender,
                                                    DependencyPropertyChangedEventArgs e)
        {
            SampleViewer sViewer = (SampleViewer)sender;

            sViewer.OnStrokeThicknessChanged((double)e.OldValue, (double)e.NewValue);
        }
        private static void selectedShape_Changed(object sender,
                                                  DependencyPropertyChangedEventArgs e)
        {
            SampleViewer sViewer = (SampleViewer)sender;

            sViewer.OnSelectedShapeChanged((Shape)e.OldValue, (Shape)e.NewValue);
        }