ROS Learning Notes

ROS Learning Notes 03

• How to create a simple subscriber

    # include <ros/ros.h> 
    # include <geometry_msgs/Pose.h>
  
    void poseCallback(const turtlesim::Pose::ConstPtr& msg) // turtlesim::Pose is the message type, ConstPtr is the message pointer type
    {
        // callback function must be quick, otherwise it will block the thread
        ROS_INFO("Turtle pose: x: %0.6f, y: %0.6f", msg->x, msg->y);
    }
  
    int main(){
        // initialize the ros node: ros::init(argc, argv, "my_node_name");
        ros::init(argc, argv, "pose_subscriber");
  
        // create a node handle: ros::NodeHandle n;
        ros::NodeHandle n;
  
        // create a subscriber, subscribe the topic "/turtle1/pose", and call the callback function "poseCallback" when receiving a message, 10 is the queue size for the message
        ros::Subscriber pose_sub = n.subscribe("/turtle1/pose", 10, poseCallback);
  
        // ros::spin() will enter a loop, pumping callbacks. With this version, all callbacks will be called from within this thread (the main one). ros::spin() will exit when Ctrl-C is pressed, or the node is shutdown by the master.
        ros::spin();
  
        return 0;
    }
  

    #!/usr/bin/env python
    # -*- coding: utf-8 -*- 
  
    # First line: Shebang line used to specify the interpreter to be used to run the script. Here is python interpreter, which is located at /usr/bin/env.
  
    # Second line: The encoding declaration. It is used to declare the encoding used in the script.
  
    import rospy
    from turtlesim.msg import Pose
  
    def poseCallback(msg):
        rospy.loginfo("Turtle pose: x: %0.6f, y: %0.6f", msg.x, msg.y)
  
    def pose_subscriber():
        rospy.init_node('pose_subscriber', anonymous=True)
        rospy.Subscriber("/turtle1/pose", Pose, poseCallback)
        rospy.spin()
  
    if __name__ == '__main__':
        pose_subscriber()
  

• Set up the CMakelists.txt

  add_executable(pose_subscriber src/pose_subscriber.cpp)
  target_link_libraries(pose_subscriber ${catkin_LIBRARIES})
  

• Compile and run
  • cpp

    cd ~/rosleaning
    catkin_make
    source devel/setup.bash
    
    roscore
    rosrun turtlesim turtlesim_node
    rosrun topic_learning pose_subscriber
    

  • python: Python ros scripts must be in work_space/src/topic_name/scripts folder

    cd ~/rosleaning
    roscore
    rosrun turtlesim turtlesim_node
    rosrun topic_learning pose_subscriber.py # no need to compile, but must make py file executable