private void enableDrag() {
	class EditPartDragSourceListener implements TransferDragSourceListener {
		@Override
		public void dragStart(DragSourceEvent event) {
		}

		// Dragging the ID of the model element is fine.
		@Override
		public void dragSetData(DragSourceEvent event) {
			EditPart ep = getSelectedEditPart();
			if (ep != null) {
				ModelElement me = getModelElement(ep); 
				final String transferData = getIdFromModelElement(me);
				event.data = transferData;
			}
		}
		
		@Override
		public void dragFinished(DragSourceEvent event) {
		}
		
		@Override
		public Transfer getTransfer() {
			return TextTransfer.getInstance();
		}
	}
	
	DragSource source = new DragSource(getDiagramGraphicalViewer().getControl(), DND.DROP_MOVE | DND.DROP_COPY);
	source.setTransfer(new Transfer[] { TextTransfer.getInstance() });
	getDiagramGraphicalViewer().addDragSourceListener(new EditPartDragSourceListener());
}

getDiagramGraphicalViewer().addDragSourceListener(new TransferDragSourceListener() {...});