Using CallbackQueue

edit

Hi,

I also had the same doubts. After you asked the question, I decided to find out.

Here is some working code that should make things very clear.

If someone can assist me in making this a tutorial, it will be great.

#include <ros/ros.h>
#include <ros/callback_queue.h>
#include <ros/callback_queue_interface.h>

#include <std_msgs/Empty.h>

void callback_1(const std_msgs::EmptyConstPtr& _msg)
{
    ROS_INFO("Called callback_1\n");
}

void callback_2(const std_msgs::EmptyConstPtr& _msg)
{
    ROS_INFO("Called callback_2\n");
}

int main(int argn, char* args[])
{
    ros::init(argn, args, "callback_q_subscriber");
    ros::NodeHandle nh_1;
    ros::NodeHandle nh_2;

    ros::CallbackQueue queue_1, queue_2;

    nh_1.setCallbackQueue(&queue_1);
    nh_2.setCallbackQueue(&queue_2);

    ros::Subscriber s_1 = nh_1.subscribe("/topic_1", 1, callback_1);
    ros::Subscriber s_2 = nh_2.subscribe("/topic_2", 1, callback_2);

    pthread_t id_1, id_2;

    int i = 20;
    while(i--)
    {
        queue_1.callOne(ros::WallDuration(1.0)); // can also be callAvailable()
    }

    i = 20;
    while(i--)
    {
        queue_2.callOne(ros::WallDuration(1.0)); // can also be callAvailable()
    }

    ROS_INFO("Hurray!");

    return 0;   
}

STEP 1:

Create a catkin package called callback_queue_test or what have you.

catkin_create_pkg callback_queue_test roscpp std_msgs

STEP 2:

a. Open favorite editor and paste the code inside it. Save it as simple.cpp.
b. Edit CMakeLists.txt to add an executable called simple_node. Add the target_link_libraries() for it.

STEP 3:

Open 4 terminals. Run the following in each in the same order.

a. roscore
b. rostopic pub -r 10 /topic_1 std_msgs/Empty
c. rostopic pub -r 10 /topic_2 std_msgs/Empty
d. rosrun callback_queue_test simple_node

That's it.

Have a nice day!