how can I register a Callback when using ros::spinOnce ?

edit

Hi all I would like to control if I subscribe or unsubscribe to a topic via a key press. I Can see that the subscribtion works but the registered callback is never called when I use ros::spinOnce(). When I change to ros::spin() the callbacks works but not the key control because ros::spin leaves the main loop and doesn't come back unless the node isn't shutdown. Is there a way to control the subscriber status?

Here is my code:

using namespace sensor_msgs;
using namespace message_filters;
typedef sync_policies::ApproximateTime<sensor_msgs::Image, sensor_msgs::Image> MySyncPolicy;



image_transport::SubscriberFilter *webcam_sub = NULL;
image_transport::SubscriberFilter *rgb_sub = NULL;
bool subscribtion; 

void subscribe(ros::NodeHandle nh, image_transport::ImageTransport it){

      webcam_sub = new image_transport::SubscriberFilter (it, "/panda_cam/image_raw", 200);
      rgb_sub = new image_transport::SubscriberFilter (it, "/camera/rgb/image_color", 200);
      Synchronizer<MySyncPolicy> sync(MySyncPolicy(1000), *webcam_sub, *rgb_sub);
      sync.registerCallback(boost::bind(&sync_callback, _1, _2));

}

void unsubscribe(){

      webcam_sub->unsubscribe();
      rgb_sub->unsubscribe();

}



void sync_callback(const ImageConstPtr&  rgb_image,const ImageConstPtr&  webcam_image){

      ROS_INFO("CALLBACK");

}    

int main(int argc, char **argv){

      ros::init(argc, argv, "subscriber");
      ros::NodeHandle nh;
      image_transport::ImageTransport it (nh);
      ros::Rate rate(100);


      while(ros::ok()){
         rate.sleep();
         int key = kbhit();
           if (key == 122 && subscribtion == false){
             subscribtion = true;
             subscribe(nh,it);
             ROS_INFO("Ready to recieve messages");                                                    
          }
          else if (key == 122 && subscribtion == true){
             subscribtion = false;
             unsubscribe();
             ROS_INFO("unsibscribed to topics");                
          }

         ros::spinOnce();
      }
    return 0;
}