Drag & Drop needs a math fix

Drag & Drop needs a math fix

I have a drag/drop behavior for my RichTextBox in Silverlight 4, but i'm having a small snapping problem.

I would like the user to be able to "grab" any border of the RichTextBox and drag/drop it. The RichTextBox should drag relative to where the user "grabbed" the RichTextBox. But instead, as soon as the user begins the drag, the RichTextBox "snaps" to the middle of the mouse position instead of its position staying relative to the mouse position.

So if I grab the bottom right corner, as in the following ...

https://skydrive.live.com/redir?resid=DCC93DD825EF3F43!658

it "snaps" to the middle of the mouse position on the start of the dragging, as in the following ...

https://skydrive.live.com/redir?resid=DCC93DD825EF3F43!659

Here's my dragging code. I'm assuming my math is wrong (in the MouseMove event) ???

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public class CustomRichTextBox : RichTextBox {     private bool isDragging = false;      protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)     {         base.OnMouseLeftButtonDown(e);          bool isDragAllowed = false;         Point pt = e.GetPosition(this);          if (pt.Y >= 0 && pt.Y <= this.BorderThickness.Top)             // Allow dragging if the mouse is down on the top border             isDragAllowed = true;         else if (pt.Y >= (this.RenderSize.Height - this.BorderThickness.Bottom) && pt.Y <= this.RenderSize.Height)             // Allow dragging if the mouse is down on the bottom border             isDragAllowed = true;         else if (pt.X >= 0 && pt.X <= this.BorderThickness.Left)             // Allow dragging if the mouse is down on the left border             isDragAllowed = true;         else if (pt.X >= (this.RenderSize.Width - this.BorderThickness.Right) && pt.X <= this.RenderSize.Width)             // Allow dragging if the mouse is down on the right border             isDragAllowed = true;          if (!isDragAllowed)             return;          this.CaptureMouse();         isDragging = true;     }      protected override void OnMouseMove(MouseEventArgs e)     {         base.OnMouseMove(e);          if (isDragging)         {             CustomRichTextBox elem = this;             CompositeTransform ct = (CompositeTransform)elem.RenderTransform;             UIElement parent = (UIElement)elem.Parent;              ct.TranslateX = e.GetPosition(parent).X - (parent.RenderSize.Width / 2);             ct.TranslateY = e.GetPosition(parent).Y - (parent.RenderSize.Height / 2);         }     }      protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)     {         base.OnMouseLeftButtonUp(e);          isDragging = false;         this.ReleaseMouseCapture();     } } 

Answers & Comments...

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