public function on($event, $callback)
        {
            if (!is_callable($callback)) {
                throw new Exception('Socketio::on() type callback must be callable.');
            }

            if (!isset($this->callbacks[$event])) {
                $this->callbacks[$event] = array();
            }

            // @TODO Handle case where callback is a string
            if (in_array($callback, $this->callbacks[$event])) {
                $this->stdout('debug', 'Skip existing callback');
                return;
            }

            $this->callbacks[$event][] = $callback;
        }