Welcome to Raspberry Pi 4B repository

9. Custom service messages and usage

9.1 Custom service message

Switch to the ~/catkin_ws/src/learning_server function package directory, and then create a new folder named srv to store custom service messages.

9.1.1 define srv file

Switch to the srv directory, create a new blank srv file, and use srv as the suffix to indicate that it is a srv file. Here we take IntPlus.srv as an example, and copy the following code into the srv file just created.

​x
uint8   a 
uint8   b 

--- 
uint8  result 

The composition of the srv file is described here, which is divided into upper and lower parts by the symbol ---, the upper part indicates the request and the lower part is the response.

9.1.2 Add function package dependencies in package.xml

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

9.1.3 Add compile options in CMakeLists.txt

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Add message_generation to find_package 
add_service_files(FILES IntPlus.srv) 
generate_messages(DEPENDENCIES std_msgs) 

9.1.4 Compile and generate language-related files

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cd ~/catkin_ws 
catkin_make 

9.1.5 C++ language implementation

1. switch to ~/catkin_ws/src/learning_server/src, create two new cpp files, named IntPlus_server.cpp and IntPlus_client.cpp, and copy the following codes into them respectively,

IntPlus_server.cpp

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/** 
 * This routine will execute /Two_Int_Plus service, service data type learning_service::IntPlus 
 */ 
 
#include <ros/ros.h> 
#include "learning_server/IntPlus.h" 

// service callback function, input parameter req, output parameter res 
bool  IntPlusCallback(learning_server::IntPlus::Request   & req, 
                    learning_server::IntPlus::Response  & res) 
{ 
    
    ROS_INFO("number 1 is:%d,number 2 is:%d ", req.a, req.b); // Display request data 

    res.result  =  req.a  +  req.b; // The feedback result is the sum of the two numbers 

    return  res.result; 
} 

int  main(int  argc, char  ** argv) 
{ 
   
    ros::init(argc, argv, "IntPlus_server"); // ROS node initialization 
    
    ros::NodeHandle  n; // create node handle 

    // Create a server and register the callback function IntPlusCallback 
    ros::ServiceServer  Int_Plus_service  =  n.advertiseService("/Two_Int_Plus", IntPlusCallback); 

    // loop waiting for the callback function 
    ROS_INFO("Ready to caculate."); 
    ros::spin(); 

    return  0; 
} 

IntPlus_client.cpp

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/* 
 This routine will request /Two_Int_Plus service, service data type learning_service::IntPlus 
Add and sum two integers 
 */ 

#include <ros/ros.h> 
#include "learning_server/IntPlus.h" 
#include <iostream> 
using  namespace  std; 
int  main(int  argc, char**  argv) 
{ 
    int  i, k; 
    cin >> i; 
    cin >> k; 

    ros::init(argc, argv, "IntPlus_client"); // initialize the ROS node 

    
    ros::NodeHandle  node; // create node handle 

    // After discovering the /Two_Int_Plus service, create a service client 
    ros::service::waitForService("/Two_Int_Plus"); 
    ros::ServiceClient  IntPlus_client  =  node.serviceClient < learning_server::IntPlus >("/Two_Int_Plus"); 

    // Initialize the request data for learning_service::IntPlus 
    learning_server::IntPlus  srv; 
    srv.request.a  =  i; 
    srv.request.b   =  k; 

    ROS_INFO("Call service to plus %d and %d", srv.request.a, srv.request.b); // request service call 

    IntPlus_client.call(srv); 

    // Display the result of the service call 
    ROS_INFO("Show the result : %d", srv.response.result); // show the result of the service call 

    return  0; 
} 

2. modify the CMakeLists.txt file

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

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

3. the core part

The implementation process here is the same as before, the main difference is the introduction of header files and the use of custom service files:

The import header file is

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#include "learning_server/IntPlus.h" 

The front learning_server is the function package name, and the latter IntPlus.h is the header file name generated by the srv file just created

Using a custom service file is

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client： 
learning_server::IntPlus  srv; 
srv.request.a  =  i; 
srv.request.b   =  k; 
#i, k is the addend input by the terminal 
ros::ServiceClient  IntPlus_client  =  node.serviceClient < learning_server::IntPlus >("/Two_Int_Plus"); 
IntPlus_client.call(srv); 
server： 
ros::ServiceServer  Int_Plus_service  =  n.advertiseService("/Two_Int_Plus", IntPlusCallback); 
bool  IntPlusCallback(learning_server::IntPlus::Request   & req, 
                    learning_server::IntPlus::Response  & res) 

4. run the program

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roscore 
rosrun  learning_server  IntPlus_client 
rosrun  learning_server  IntPlus_server 

5. run the screenshot

6. program description

After running IntPlus_server, you will be prompted to prepare for calculation; after running IntPlus_client, the terminal will input two integer numbers, then IntPlus_server will calculate the result, return it to IntPlus_client, and then print the result.

9.1.6 Python language implementation

1. switch to ~/catkin_ws/src/learning_server/script, create two new py files, named IntPlus_server.py and IntPlus_client.py, and copy the following codes into them respectively,

IntPlus_server.py

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

from  learning_server.srv  import  IntPlus, IntPlusResponse 

 def IntPlusCallback(req): 
    
    rospy.loginfo("Ints: a:%db:%d", req.a, req.b) # show request data 
    
    return  IntPlusResponse(req.a + req.b) # Feedback data 

 def IntPlus_server(): 
    
    rospy.init_node ' IntPlus_server(') # ROS node initialization 

    # Create a server and register the callback function IntPlusCallback 
    s  =  rospy.Service('/Two_Int_Plus', IntPlus, IntPlusCallback) 

    
    print  "Ready to caculate two ints." # Loop waiting for the callback function 

    rospy.spin() 

if  __name__  ==  "__main__" : 
    IntPlus_server() 

IntPlus_client.py

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# !/usr/bin/env python 
# -*- coding: utf-8 -*- 
import  sys 
import  rospy 
from  learning_server.srv  import  IntPlus, IntPlusRequest 

 def Plus_client(): 
    # ROS node initialization 
    rospy.init_node('IntPlus_client') 

    rospy.wait_for_service('/Two_Int_Plus') 
    try : 
        Plus_client  =  rospy.ServiceProxy('/Two_Int_Plus', IntPlus) 

        response = Plus_client(22, 20) # Request service call, enter request data 

        return  response.result 
    except  rospy.ServiceException, e : 
        print  "failed to call service : %s" % e 

if  __name__  ==  "__main__" : 
    #Service call and display the call result 
    print  "Show two_int_plus result : %s"  %(Plus_client()) 

2. the core part

Here is mainly to explain how to import the custom service message module and use it:

import

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server : 
from  learning_server.srv  import  IntPlus, IntPlusResponse 
client : 
from  learning_server.srv  import  IntPlus, IntPlusRequest 

use

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server： 
s  =  rospy.Service('/Two_Int_Plus', IntPlus, IntPlusCallback) 
return  IntPlusResponse(req.a + req.b) # Feedback data 
client： 
response = Plus_client(12, 20) # Request service call, enter request data 
return  response.result 

3. run the program

Before running the program, first add executable permissions to the py file

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sudo chmod a+x IntPlus_server.py 
sudo chmod a+x IntPlus_client.py 

run the program

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roscore 
rosrun learning_server IntPlus_client.py 
rosrun learning_server IntPlus_server.py 

4. program operation instructions

What is inconsistent with the C++ version here is that the addend here is set in the program(12 and 20) so after the service is started, the result can be returned immediately.