using System;
  using System.Windows.Controls;
  using System.Windows.Controls.Primitives;
  using System.Windows.Media;
  
  namespace RoboforkApp
  {
      public class MoveThumb : Thumb
      {
          private DesignerItem designerItem;
          private DesignerCanvas designerCanvas;
  
          public MoveThumb()
          {
              DragStarted += new DragStartedEventHandler(this.MoveThumb_DragStarted);
              DragDelta += new DragDeltaEventHandler(this.MoveThumb_DragDelta);
          }
  
          private void MoveThumb_DragStarted(object sender, DragStartedEventArgs e)
          {
              this.designerItem = DataContext as DesignerItem;
  
              if (this.designerItem != null)
              {
                  this.designerCanvas = VisualTreeHelper.GetParent(this.designerItem) as DesignerCanvas;
              }
          }
  
          private void MoveThumb_DragDelta(object sender, DragDeltaEventArgs e)
          {
              if (this.designerItem != null && this.designerCanvas != null && this.designerItem.IsSelected)
              {
                  double minLeft = double.MaxValue;
                  double minTop = double.MaxValue;
  
                  foreach (DesignerItem item in this.designerCanvas.SelectedItems)
                  {
                      minLeft = Math.Min(Canvas.GetLeft(item), minLeft);
                      minTop = Math.Min(Canvas.GetTop(item), minTop);
                  }
  
                  double deltaHorizontal = Math.Max(-minLeft, e.HorizontalChange);
                  double deltaVertical = Math.Max(-minTop, e.VerticalChange);
  
                  foreach (DesignerItem item in this.designerCanvas.SelectedItems)
                  {
                      Canvas.SetLeft(item, Canvas.GetLeft(item) + deltaHorizontal);
                      Canvas.SetTop(item, Canvas.GetTop(item) + deltaVertical);
                  }
  
                  this.designerCanvas.InvalidateMeasure();
                  e.Handled = true;
              }
          }
      }
  }