Doubt in callbacks.

edit

As per my understanding, in a single threaded spinning the Subscriber waits for seeing a message and after seeing, puts it in the callback queue and simultaneously the ros::spin processes the callbacks sequentially in the queue. For example if only a Kinect callback(CBK) is there, then the queueing will be as follows: CBK1 || CBK2 || CBK3 || ... and so on. And these will be processed one at a time by ros::spin(). Am I right?

Provided that the above understanding is right, if we have multiple callbacks. For example one callback for a kinect data(CBK) and one for a laser data(CBL) and since the messages from laser are received faster than kinect, we would have a queueing somewhat similar to : CBL1 ||CBL2 ||CBL3 ||CBK1 ||CBL4 ||CBL5 ||CBL6 ||CBK2 ||CBL7 ||CBL8 ||CBL9 ||CBK3 ||CBL10 || ... and so on. Does ros::spin(), process this as well in the same manner as above?

Can someone help me debug why I am not able to use the customized queues? The following are my two callbacks:

  MainThread::MainThread(ros::NodeHandle& nh) : nh(nh)
  {
      this->sub_person_data = this->nh.subscribe("/persondetector/person_data", 1,   &MainThread::personDetectCallback, this);
      this->sub_laser_person_data = this->nh.subscribe("/laserpersondetector/person_data", 1, &MainThread::personDetectLaserCallback, this);
  }

Callback Definition:

   void MainThread::personDetectCallback(persondetector_kinect::PersonData data)
   {
       if(kinect_data_cb_queue.isEnabled())
       {
           std::cout<< "Kinect Data received "<<std::endl;
           kinect_data_cb_queue.callAvailable();
           this->personData_ = data;
           if(kinect_data_cb_queue.isEmpty())
              std::cout<<"The kinect data queue is empty"<<std::endl;
           else
              std::cout<<"The kinect data queue is used now"<<std::endl;
        }
   }

   void MainThread::personDetectLaserCallback(persondetector_kinect::PersonData data)
   {
      nh.setCallbackQueue(&laser_data_cb_queue);
      if(laser_data_cb_queue.isEnabled())
      {
         std::cout<< "Laser Data received "<<std::endl;
         this->laserPersonData_ = data;
         laser_data_cb_queue.callAvailable();
      }
   }

Now, in the main I make use of the multi-threaded spinner:

 int main (int argc, char** argv)
 {
     // Initialize ROS
     ros::init (argc, argv, "simple_tracker");
     ros::NodeHandle nh("~");
     // Declaration of kinect and laser data call back queue
     ros::CallbackQueue kinect_data_cb_queue;
     ros::CallbackQueue laser_data_cb_queue;
     simpletracker::MainThread main(nh);
     nh.setCallbackQueue(&(main.kinect_data_cb_queue));
/*ros::MultiThreadedSpinner laser_callback_queue_spinner(2);
  laser_callback_queue_spinner.spin(&(main.laser_data_cb_queue));*/

/*ros::AsyncSpinner kinect_callback_queue_spinner(0, &(main.kinect_data_cb_queue));
       kinect_callback_queue_spinner.start();*/

      main.work();
      ros::MultiThreadedSpinner kinect_callback_queue_spinner(1);
      kinect_callback_queue_spinner.spin(&(main.kinect_data_cb_queue));
      ros::spin();
      return 0;
}

Even if the rosbag is running and the messages are available, it says "The kinect data queue is empty"