ROS-Subscriber

Study note for ROS subscriber.

Model of Topic

• Publisher(turtlesim)
• • –[Message(geometry_msgs::Twist)]–>
• Topic (/turtle1/pose)
• • –[Message(geometry_msgs::Twist)]–>
• Subscriber(Pose Listener)

Programming realization

How to realize a subscriber

• initialize ROS node
• subscribe required topic
• loop wait the msgs from topic, after receiving msgs, enter callback function
• implement msgs processing in the callback function

Code

Also we put this node into function package learning_topic
Create velocity_publisher.cpp file in learning_topic/src:

#include <ros/ros.h>
#include <turtlesim/Pose.h>

void poseCallback(const turtlesim::Pose::ConstPtr& msg)
{
    ROS_INFO("Turtle pose: x:%0.6f, y:%0.6f, ",msg->x,msg->y);
}

int main(int argc, char *argv[])
{
    ros::init(argc, argv, "pose_subscriber");
    ros::NodeHandle n;
    ros::Subscriber pose_sub = n.subscribe("/turtle1/pose",10,poseCallback）
    //queue also first in first thrown
    //loop waiting for callback function, keep the endless loop
    ros::spin();
    return 0;
}

Note

• Callback function must be sufficient, or newer data will be blocked

Configure the compile run of subscriber

Steps same as publisher
Add two lines to learning\_topic/CMakeLists.txt (in build part):

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

Compile

$ cd ~/ROS_study/catkin_ws
$ catkin_make
$ source ./devel/setup.zsh
$ roscore
$ rosrun turtlesim turtlesim_node
$ rosrun learning_topic velocity_publisher
$ rosrun learning_topic pose_subscriber

The output of pose_subscriber:

...
[ INFO] [1650184199.757606156]: Turtle pose: x:3.100077, y:7.501662,
[ INFO] [1650184199.773327854]: Turtle pose: x:3.101826, y:7.493855,
[ INFO] [1650184199.790252785]: Turtle pose: x:3.103601, y:7.486054,
...