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6. Customized Topic Messages and Usage

6.1 Customized Topic Messages​

Switch to ~/catkin_ws/src/learning_topic and create a new folder named msg under the topic feature package directory to store custom topic messages.

6.1.1 Define msg Files​

Switch to the msg directory and create a new blank .msg file. Here we use Information.msg as an example:

string company
string city

6.1.2 Add Feature Package Dependencies in package.xml​

Add the following dependencies:

<build_depend>message_generation</build_depend>
<exec_depend>message_runtime</exec_depend>

6.1.3 Add Compilation Options in CMakeLists.txt​

Add the following:

add_message_files(
  FILES
  Information.msg
)

generate_messages(
  DEPENDENCIES
  std_msgs
)

6.1.4 Compile and Generate Language Files​

cd ~/catkin_ws
catkin_make

6.1.5 C++ Language Implementation​

Switch to ~/catkin_ws/src/learning_topic/src and create two files:

  • Information_publisher.cpp
  • Information_subscriber.cpp

Information Publisher​

#include <ros/ros.h>
#include "learning_topic/Information.h"

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

    ros::Publisher info_pub = n.advertise<learning_topic::Information>("/company_info", 10);
    ros::Rate loop_rate(1);

    while (ros::ok())
    {
        learning_topic::Information info_msg;
        info_msg.company = "HemiHex";
        info_msg.city = "Toronto";

        info_pub.publish(info_msg);

        ROS_INFO("Information: company: %s  city: %s",
                 info_msg.company.c_str(), info_msg.city.c_str());

        loop_rate.sleep();
    }

    return 0;
}

Information Subscriber​

#include <ros/ros.h>
#include "learning_topic/Information.h"

void CompanyInfoCallback(const learning_topic::Information::ConstPtr& msg)
{
    ROS_INFO("This is: %s in %s", msg->company.c_str(), msg->city.c_str());
}

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

    ros::Subscriber info_sub = n.subscribe("/company_info", 10, CompanyInfoCallback);

    ros::spin();
    return 0;
}

Modify CMakeLists.txt​

Add the following:

add_executable(Information_publisher src/Information_publisher.cpp)
target_link_libraries(Information_publisher ${catkin_LIBRARIES})
add_dependencies(Information_publisher ${PROJECT_NAME}_generate_messages_cpp)

add_executable(Information_subscriber src/Information_subscriber.cpp)
target_link_libraries(Information_subscriber ${catkin_LIBRARIES})
add_dependencies(Information_subscriber ${PROJECT_NAME}_generate_messages_cpp)

Core Header Import​

#include "learning_topic/Information.h"

This header is auto-generated from the custom .msg file and enables both publisher and subscriber to use the custom message type.