Confusion about AsyncSpinner

edit

Hi, I am confused about the behavior of AsyncSpinner.

My understanding is that AsyncSpinner would allow a node to process several callbacks in parallel. I am trying to test this, but I do not see this behavior. I made two simple nodes to test this. A subscriber node will print a lot of output in the callback - enough to take a long time to finish. The node will receive another msg before finishing the first callback. When using ros::spin(), the callbacks will be entirely sequential. With AsyncSpinner, they should occur in parallel once the 2nd msg gets published.

Subscriber:

#include <iostream>
#include "ros/ros.h"
#include <std_msgs/String.h>

int count=0;
void myCallback(const std_msgs::String& str)
{
  count++;
  ros::Duration d(1);
  printf("\nSubscriber %i is going to sleep\n", count);
  d.sleep();
  printf("\nSubscriber %i is waking up\n", count);   
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "subscriber");
  ros::NodeHandle handle;

  ros::Subscriber sub = handle.subscribe("chatter", 1000, myCallback); 

  ros::AsyncSpinner spinner(4);
  spinner.start();
  ros::waitForShutdown();

  return 0;
}

I want to publish the second message before the subscribing node can finish the first callback to make them happen in parallel. So I publish 2 msgs at 0.5 seconds apart. 0.5 seconds is not enough time for the subscribing node to print 25,000 lines to the console.

Publisher:

#include <iostream>
#include "ros/ros.h"
#include <std_msgs/String.h>

int main(int argc, char** argv)
{
  ros::init(argc, argv, "publisher");
  ros::NodeHandle handle;

  ros::Publisher pub = handle.advertise<std_msgs::String>("chatter", 1000);

  std_msgs::String s;
  ros::Duration d(0.25);
  for(int i=0;i<2;i++)
  {
    pub.publish(s);
    d.sleep();
  }
  return 0;
}

Edit: I changed the subscriber node to simply sleep for a duration as Jackie suggested. I would expect my output to be:

Subscriber 1 is going to sleep

Subscriber 2 is going to sleep

Subscriber 1 is waking up

Subscriber 2 is waking up

However, my output is always:

Subscriber 1 is going to sleep

Subscriber 1 is waking up

Subscriber 2 is going to sleep

Subscriber 2 is waking up

Note that the sleep time is 1 second and the publish time is 0.25 seconds. That ensures that the second msg will always be published while the first subscriber sleeps.

If anyone can help clear up my confusion about the purpose of AsyncSpinner, please do. I do not understand why I am not seeing my expected output.