implement method regist unregist fig model event fig node model element fig regist listen event fire owner exampl fig fig regist event fire oper attribut owner explan origin purpos method given issu function us model chang function certain case imper listen updat remov remov element listen method call old owner owner equal listen owner safe ignor need updat listen relat element relat element replac param owner owner mean listen remov param old owner previou owner mean listen set updat listen object old owner object owner old owner owner old owner remov element listen old owner owner add element listen owner 