public function on($event_name, $listener, $data = null)
    {
        if ($this->isSingle()) {
            BaseObserver::on($event_name, $listener, $data);
        }
    }
public function on($event_name, $listener, $data = null)
    {
        if ($this->isSingle()) {
            BaseObserver::on($event_name, $listener, $data);
        }
    }
public function on ( $ event_name , $ listener , $ data = null ) { if ( $ this -> isSingle ( ) ) { BaseObserver :: on ( $ event_name , $ listener , $ data ) ; } }
Register Event Listener
Register Event Listener