import sys, dia

def center_objects (objs) :
	r = objs[0].bounding_box
	cx = (r.right + r.left) / 2
	cy = (r.bottom + r.top) / 2
	for o in objs[1:] :
		r = o.bounding_box
		(x, y) = o.properties["obj_pos"].value
		dx = (r.right + r.left) / 2 - cx
		dy = (r.bottom + r.top) / 2 - cy
		o.move (x - dx, y - dy)

def dia_objects_center_cb (data, flags) :
	grp = data.get_sorted_selected()
	if (len(grp) > 1) :
		center_objects (grp)
		data.update_extents ()
	dia.active_display().diagram.add_update_all()
	
dia.register_callback ("Center Objects", 
                       "<Display>/Objects/Center", 
                       dia_objects_center_cb)