Use the OnStartDock event handler to implement special processing when the user starts a drag-and-dock operation by dragging the control. 

The OnStartDock event handler can create a TDragDockObjectEx object for the DragObject parameter to specify the appearance of the dragging rectangle and how the dragged control interacts with potential docking sites. If you return TDragDockObjectEx as the drag object, there is no need to call the Free method for the DragObject when dragging is over. If you use TDragDockObject, your application is responsible for freeing the drag object. 

If the OnStartDock event handler sets the DragObject parameter to nil (Delphi) or NULL (C++), a TDragDockObject object is automatically created.

TDesignerObjTree Properties