WPF Diagram Designer: Part 1

using System.Windows.Controls.Primitives;
using System.Windows.Controls;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media;
using System;

namespace DragResizeRotateItems
{
    public class RotateThumb : Thumb
    {
        double initialAngle;
        private Vector startVector;
        private Point centerPoint;

        private ContentControl designerItem;
        private ContentControl DesignerItem
        {
            get
            {
                if (designerItem == null)
                {
                    designerItem = this.DataContext as ContentControl;
                }
                return designerItem;
            }
        }

        public RotateThumb()
        {
            base.DragDelta += new DragDeltaEventHandler(RotateThumb_DragDelta);
            base.DragStarted += new DragStartedEventHandler(RotateThumb_DragStarted);
        }

        void RotateThumb_DragStarted(object sender, DragStartedEventArgs e)
        {
            Canvas canvas = VisualTreeHelper.GetParent(DesignerItem) as Canvas;
            if (DesignerItem != null && canvas != null)
            {
                // the RenderTransformOrigin property of DesignerItem defines
                // transformation center relative to its bounds
                centerPoint = DesignerItem.TranslatePoint(
                    new Point(DesignerItem.Width * DesignerItem.RenderTransformOrigin.X,
                              DesignerItem.Height * DesignerItem.RenderTransformOrigin.Y),
                    canvas);

                // calculate startVector, that is the vector from centerPoint to startPoint
                Point startPoint = Mouse.GetPosition(canvas);
                startVector = Point.Subtract(startPoint, centerPoint);

                // check if the DesignerItem already has a RotateTransform set ...
                RotateTransform rotateTransform = DesignerItem.RenderTransform as RotateTransform;
                if (rotateTransform == null)
                {
                    // if not we create one with zero angle 
                    DesignerItem.RenderTransform = new RotateTransform(0);
                    initialAngle = 0;
                }
                else
                    initialAngle = rotateTransform.Angle;
            }
        }

        void RotateThumb_DragDelta(object sender, DragDeltaEventArgs e)
        {
            Canvas canvas = VisualTreeHelper.GetParent(DesignerItem) as Canvas;

            if (DesignerItem != null && canvas != null)
            {
                // calculate deltaVector, that is the vector from centerPoint to current mouse position                
                Point currentPoint = Mouse.GetPosition(canvas);
                Vector deltaVector = Point.Subtract(currentPoint, centerPoint);

                //calculate the angle between startVector and dragVector
                double angle = Vector.AngleBetween(startVector, deltaVector);

                // and update the transformation
                RotateTransform rotateTransform = DesignerItem.RenderTransform as RotateTransform;
                rotateTransform.Angle = initialAngle + Math.Round(angle, 0);
            }
        }
    }
}