5.Subscribers

5.1 Subscribers

The subscriber receives the data published by the publisher and then enters its callback function, where the received data is processed. The core content is the callback function, which is available for each topic subscribed to by subscribers.

5.2Create a subscriber

5.2.1 Creation steps

1. Initialize ROS node

2. Create handle
3. Subscribe to the required
4. topicsWait for topic messages in a loop, and upon receiving the message, enter the callback function
5. Complete message processing in the callback function.

5.2.2. C++Language Implementation

1. In the "Publisher" tutorial, create a new C++file under the src folder of the created feature pack and name it turtle_ pose_ subscriber.cpp
2. Copy and paste the program code below into the title_ pose_ In the subscriber.cpp file

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

void turtle_poseCallback(const turtlesim::Pose::ConstPtr& msg){
   
    ROS_INFO("Turtle pose: x:%0.3f, y:%0.3f", msg->x, msg->y);
}

int main(int argc, char **argv){
    
    ros::init(argc, argv, "turtle_pose_subscriber");

    ros::NodeHandle n;

  
    ros::Subscriber pose_sub = n.subscribe("/turtle1/pose", 10, turtle_poseCallback);
   
    ros::spin(); // Loop waiting callback function

    return 0;
}

3. Program flowchart, which can be viewed according to 5.2.1 content

4. Configure in CMakelist.txt, under the build area, add the following content

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

5. Compiling code under workspace directory

xxxxxxxxxx
cd ~/catkin_ws
catkin_make
source devel/setup.bash

6. Run program

• Run score

xxxxxxxxxx
roscore

• Running the Little Turtle Node

xxxxxxxxxx
rosrun turtlesim turtlesim_node

• Run subscription and continuously receive posture data sent by the little turtle

xxxxxxxxxx
rosrun learning_topic turtle_pose_subscriber

7. Operation screenshot

8. Program operation instructions

• After running the little turtle's node, the little turtle will continuously send its own pose information, and the topic name sent is/turtle1/pose
• And turnle_ pose_ After the subscriber runs, it will receive the data message sent by the turtle and print out this information in the callback function

5.2.3. Python Language Implementation

1. In the feature pack directory, create a new folder called scripts, and then create a new Python file (with the suffix. py) under the scripts folder, named 'turn'_ pose_ subscriber.py
2. Copy and paste the program code below into the title_ pose_ In the subscriber.py file

x
#!/usr/bin/env python
# -*- coding: utf-8 -*-

import rospy
from turtlesim.msg import Pose

def poseCallback(msg):
    rospy.loginfo("Turtle pose: x:%0.3f, y:%0.3f", msg.x, msg.y)

def turtle_pose_subscriber():
    
    rospy.init_node('turtle_pose_subscriber', anonymous=True)# ROS node initialization

    
    rospy.Subscriber("/turtle1/pose", Pose, poseCallback)

    
    rospy.spin()# Loop waiting callback function

if __name__ == '__main__':
    turtle_pose_subscriber()

3. Process Flow Chart

4.run a program

• Run score

xxxxxxxxxx
roscore

• Running the Little Turtle Node

xxxxxxxxxx
rosrun turtlesim turtlesim_node

• Run subscription and continuously receive posture data sent by the little turtle

xxxxxxxxxx
rosrun learning_topic turtle_pose_subscriber

5.Refer to 5.2.2 for operational effects and program instructions.