18、 ROS topic name setting

1. ROS Topic name remapping

1.1 brief introduction

stay ROS in , Sometimes the topic names are duplicated , Lead to information release , Reception mismatch ; Sometimes nodes with different topic names need to communicate with each other . At this time , We need to use the method of topic name remapping to solve the problem

1.2 Implementation method

stay ROS Operating system , Provides ros-noetic-teleop-twist-keyboard Keyboard control package , Topic name is cmd_vel, stay turtlesim Function pack , Its keyboard control topic is /turtle1/cmd_vel, Through topic remapping , To realize the communication between the two

1.2.1 rosrun Set topic remapping

• Format ： rosrun Function pack The node name Topic name := New node roll call

1.2.1.1 Scheme 1

• The goal is ： /cmd_vel Mapping to /turtle1/cmd_vel
• Code implementation ：

  bash1: Start the tortoise node 
  rosrun turtlesim turtlesim_node 
  
  bash2： Start keyboard control node , And map 
  rosrun teleop_twist_keyboard teleop_twist_keyboard.py /cmd_vel:=/turtle1/cmd_vel
  

1.2.1.2 Option two

• The goal is ：/turtle1/cmd_vel Mapping to /cmd_vel
• Code implementation ：

  bash1: Start keyboard control node 
  rosrun teleop_twist_keyboard teleop_twist_keyboard.py 
  
  bash2： Start the tortoise node , And carry out topic mapping 
  rosrun turtlesim turtlesim_node /turtle1/cmd_vel:=/cmd_vel
  

1.2.2 launch File set topic remapping

• Format ：

  <node pkg="" type="" name="">
      <remap from=" The original topic " to=" New topics "/>
  </node>
  

1.2.2.1 Scheme 1

• The goal is ： take teleop_twist_keyboard The topic of the node is set to /turtle1/cmd_vel
• Code implementation ：

  <launch>
      <node pkg="turtlesim" type="turtlesim_node" name="t1" />
      <node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="t2">
          <remap from="/cmd_vel" to="/turtle1/cmd_vel" />
      </node>
  </launch>
  

1.2.2.2 Option two

• The goal is ： take turtlesim The topic name of the node is set to /cmd_vel
• Code implementation

  <launch>
      <node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="t1" />
      <node pkg="turtlesim" type="turtlesim_node" name="t2">
          <remap from="/turtle1/cmd_vel" to="/cmd_vel" />
      </node>
  </launch>
  

1.2.3 Code and set the topic name

• The name and namespace of the topic , The name of the node is related , According to the relationship , Topic names can be roughly divided into three categories （ It can be understood that the prefixes added before the topic are different ）：
  • overall situation ： Refer to the whole ROS System , Level with namespace
  • relative ： Reference namespace , Level with node name
  • private ： Reference node name , Is the child of the node

1.2.3.1 C++ Realization

1. Global topics

• Format ： has / start
• Code implementation ：

  #include"ros/ros.h"
  #include"std_msgs/String.h"
  
  int main(int argc,char *argv[]){
         
      ros::init(argc,argv,"topic_name");
      ros::NodeHandle nh;
      // Global topics ： Add  / 
      ros::Publisher pub = nh.advertise<std_msgs::String>("/chtter",1000);
      // Set your own namespace 
      ros::Publisher pub1 = nh.advertise<std_msgs::String>("/xxx/chtter",1000);
  
      while (ros::ok())
      {
        
      
      }
      return 0;
  }
  

• Running results ：

  bash1:  Start node 
  rosrun rename_01 topic_name
  
  bash2： Check the list of topics 
  rostopic list
  /chtter    // Global topics 
  /rosout
  /rosout_agg
  /xxx/chtter     // Added its own namespace 
  

2. Relative topics

• Format ： Not / The first topic
• Code implementation ：

  #include"ros/ros.h"
  #include"std_msgs/String.h"
  
  int main(int argc,char *argv[]){
         
      ros::init(argc,argv,"topic_name");
      ros::NodeHandle nh;
      // Global topics ： Add  / 
      ros::Publisher pub = nh.advertise<std_msgs::String>("chatter",1000);
      // Set your own namespace 
      ros::Publisher pub1 = nh.advertise<std_msgs::String>("yyy/chatter",1000);
  
      while (ros::ok())
      {
        
      
      }
      return 0;
  }
  

• Running results ：

  bash1: Start node , And add the namespace 
  rosrun rename_01 topic_name __ns:=xxx
  
  bash2: Check the topic 
  rostopic list
  /rosout
  /rosout_agg
  /xxx/chatter
  /xxx/yyy/chatter
  

3. Private topics

• Format ： ros::NodeHandle nh(“~”);
• Code implementation ：

  #include"ros/ros.h"
  #include"std_msgs/String.h"
  
  int main(int argc,char *argv[]){
         
      ros::init(argc,argv,"topic_name");
      // Private topics , And NH Use a combination of , Add after ~
      ros::NodeHandle nh("~"); 
      ros::Publisher pub = nh.advertise<std_msgs::String>("chatter",1000);
      // If the  /  start , Is the overall topic 
      ros::Publisher pub1 = nh.advertise<std_msgs::String>("/chatter",1000);
  
      while (ros::ok())
      {
        
      
      }
      return 0;
  }
  

• Running results ：

  bash1:  Start node 
  rosrun rename_01 topic_name __ns:=xxx
  
  bash2： View the list of nodes 
  rostopic list
  /chatter
  /rosout
  /rosout_agg
  /xxx/topic_name/chatter
  

1.2.3.2 Python Realization

• Code implementation ：

  #! /usr/bin/env python
  
  from ast import Str
  import rospy
  from std_msgs.msg import String
  
  if __name__ == "__main__":
      rospy.init_node("topic_name_p")
      # 1. overall situation 
      pub1 = rospy.Publisher("/chatter1",String,queue_size=10)
      #  Set your own namespace , Not subject to  __ns:=xxx  Influence 
      pub2 = rospy.Publisher("/zzz/chatter2",String,queue_size=10)
  
  
      # 2. relative 
      pub3 = rospy.Publisher("chatter3",String,queue_size=10)
      #  Set your own namespace 
      pub4 = rospy.Publisher("yyy/chatter4",String,queue_size=10)
  
  
      # 3. private 
      pub5 = rospy.Publisher("~chatter5",String,queue_size=10)
      #  Set your own namespace 
      pub6 = rospy.Publisher("~ns/chatter6",String,queue_size=10)
      while not rospy.is_shutdown():
          pass
  

• Running results ：

  bash1: Start node 
  rosrun rename_01 topic_name_p.py __ns:=xxx
  
  bash2: View the list of nodes 
  rostopic list
  /chatter1
  /rosout
  /rosout_agg
  /xxx/chatter3
  /xxx/topic_name_p/chatter5
  /xxx/topic_name_p/ns/chatter6
  /xxx/yyy/chatter4
  /zzz/chatter2