Introduction to ROS (4): ROS communication programming (service programming)

Reference: "ROS Robot Development Practice"
Note: continue to implement communication programming under the function package learning_communication under the catkin_ws workspace

1. Service Communication Model

Second, service programming implementation

Note: The addition listener releases two numbers to the talker, and the talker adds it to the listener

1. Service programming process:

1. Create a server;
2. Create a client;
3. Add compilation options;
4. Run an executable file;

2. Custom service file srv

a. Create the srv directory under learning_communication and the AddTwoInts.srv file under the srv directory

int64 a
int64 b
---
int64 sum

b. Add function package dependency in package.xml

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

c. Add compilation options in CMakeLists.txt

find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  std_msgs
  message_generation
  )

catkin_package(
#  INCLUDE_DIRS include
#  LIBRARIES learning_communication
  CATKIN_DEPENDS roscpp rospy std_msgs message_runtime
#  DEPENDS system_lib
)

add_service_files(FILES AddTwoInts.srv)
generate_messages(DEPENDENCIES std_msgs)

d. Compilation
Generate an executable file under the catkin_ws workspace:

catkin_make

3. Create a server

a. Create the server.cpp file in the following directory:

b. The server.cpp file code is as follows:

/*
AddTwoInts Server
*/

#include "ros/ros.h"
#include "learning_communication/AddTwoInts.h"

//service回调函数，输入参数req，输出参数res
bool add(learning_communication::AddTwoInts::Request  &req,
         learning_communication::AddTwoInts::Response &res)
{
  //将输入参数中的请求数据相加，结果放到应答变量中
  res.sum = req.a + req.b;
  ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
  ROS_INFO("sending back response: [%ld]", (long int)res.sum);

  return true;
}

int main(int argc, char **argv)
{
  //ROS节点初始化
  ros::init(argc, argv, "add_two_ints_server");
  
  //创建节点句柄
  ros::NodeHandle n;

  //创建一个名为add_two_ints的server，注册回调函数add()
  ros::ServiceServer service = n.advertiseService("add_two_ints", add);

  //循环等待回调函数
  ROS_INFO("Ready to add two ints.");
  ros::spin();

  return 0;
}

c. Implementation logic
(1) Initialize the ROS node;
(2) Create a Server instance;
(3) Wait for the service request circularly and enter the callback function;
(4) Complete the service function processing in the callback function and feed back the response data;

4. Create a client

a. Create the client.cpp file in the following directory:

b. The client.cpp file code is as follows:

/*
AddTwoInts Client
*/

#include <cstdlib>
#include "ros/ros.h"
#include "learning_communication/AddTwoInts.h"


int main(int argc, char **argv)
{
  //Ros节点初始化
  ros::init(argc, argv, "add_two_ints_client");
  
  //从终端命令行获取两个加数
  if (argc != 3)
  {
    ROS_INFO("usage: add_two_ints_client X Y");
    return 1;
  }

  //创建节点句柄
  ros::NodeHandle n;

  //创建一个client，请求add_two_int service，service消息类型是learning_communication::AddTwoInts
  ros::ServiceClient client = n.serviceClient<learning_communication::AddTwoInts>("add_two_ints");

  //创建learning_communication::AddTwoInts类型的service消息
  learning_communication::AddTwoInts srv;
  srv.request.a = atoll(argv[1]);
  srv.request.b = atoll(argv[2]);

  //发布service请求，等待加法运算的应答结果
  if (client.call(srv))
  {
    ROS_INFO("Sum: %ld", (long int)srv.response.sum);
  }
  else
  {
    ROS_ERROR("Failed to call service add_two_ints");
    return 1;
  }

  return 0;
}

c. Implementation logic
(1) Initialize ROS node;
(2) Create Client instance;
(3) Publish service request data;
(4) Wait for the response result after Server processing;

5. Add compilation options

Compile method

Compile the code in the CMakeLists.txt file:

(1) Set the code to be compiled and the executable file generated;
(2) Set the link library;
(3) Set the dependency;

Specific operation

Open the CMakeLists.txt file in the learning_communication function package and add the following code:

add_executable(server src/server.cpp)
target_link_libraries(server ${catkin_LIBRARIES})
add_dependencies(server ${PROJRCT_NAME}_gencpp)

add_executable(client src/client.cpp)
target_link_libraries(client ${catkin_LIBRARIES})
add_dependencies(client ${PROJRCT_NAME}_gencpp)

Generate

Generate an executable file under the catkin_ws workspace:

catkin_make

6. Run the executable file

Note that every time you open a new terminal to add environment variables: (if you need to manually add)

source ~/test/catkin_ws/devel/setup.bash

a. Please
open the terminal of ROS Master :

roscore

b. Start the server and
open a new terminal:

rosrun learning_communication server

c. Start the client and
open the terminal:

rosrun learning_communication client 3 4