【ROS】消息通讯——服务器/客户端的来龙去脉

1. 创建功能包：

cd ~/learning_ws/src
catkin_create_pkg ros_tutorials_service message_generation std_msgs roscpp //创建功能包和依赖

2. 修改功能包说明文件package.xml：

<?xml version="1.0"?>
<package format="2">
  <name>ros_tutorials_service</name>
  <version>0.0.0</version>
  <description>The ros_tutorials_service package</description>
  <author email="12345678@163.com">jjjj</author>
  <maintainer email="12345678@163.com">jjjj</maintainer>
  <license>Apache License 2.0</license>
  <url type="website">https://jjjj.blog</url>
  
  <buildtool_depend>catkin</buildtool_depend>
  <build_depend>message_generation</build_depend>
  <build_depend>roscpp</build_depend>
  <build_depend>std_msgs</build_depend>
  <build_export_depend>roscpp</build_export_depend>
  <build_export_depend>std_msgs</build_export_depend>
  <exec_depend>roscpp</exec_depend>
  <exec_depend>std_msgs</exec_depend>

  <export></export>
</package>

3. CMake编译配置文件CMakeLists.txt：

cmake_minimum_required(VERSION 2.8.3)
project(ros_tutorials_service)

find_package(catkin REQUIRED COMPONENTS
  message_generation
  roscpp
  std_msgs
)
#添加服务文件即请求和响应信息
add_service_files(
   FILES
   SrvTutorial.srv
)

generate_messages(
   DEPENDENCIES
   std_msgs
)

catkin_package(
  LIBRARIES ros_tutorials_service
  CATKIN_DEPENDS roscpp std_msgs
)

include_directories(
  ${catkin_INCLUDE_DIRS}
)

add_executable(service_server src/service_server.cpp)
add_dependencies(service_server ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
target_link_libraries(service_server ${catkin_LIBRARIES})

add_executable(service_client src/service_client.cpp)
add_dependencies(service_client ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
target_link_libraries(service_client ${catkin_LIBRARIES})

4. 添加服务文件：

与CMakeLists.txt对应：add_service_files( FILES SrvTutorial.srv)

内容如下：—用于分割请求和响应，—上面为请求，下面为响应：

int64 a
int64 b
---
int64 result

5. 创建服务器节点：

与CMakeLists.txt对应：add_executable(service_server src/service_server.cpp)

内容如下：

#include <ros/ros.h>
#include <ros_tutorials_service/SrvTutorial.h>//服务头文件
//如果收到客户端的请求，将执行以下处理，服务请求为req,服务响应为res
bool calculation(ros_tutorials_service::SrvTutorial::Request &req,
                ros_tutorials_service::SrvTutorial::Response &res)
{
	//收到请求时，将两数之和保存到服务响应值中
	res.result = req.a+req.b;
	//显示a,b的值及结果
	ROS_INFO("request:x=%ld,y=%ld",(long int)req.a,(long int)req.b);
	ROS_INFO("sending back response:%ld",(long int)res.result);
	return true;
}

int main(int argc,char **argv)
{
	ros::init(argc,argv,"service_server");//声明服务器节点名称
	ros::NodeHandle nh;
	//声明服务器，创建一个使用ros_tutorials_service功能包SrvTutorial服务文件的服务器
	//ros_tutorial_service_server,服务名称是ros_tutorial_srv,有服务请求，执行calculation函数
	ros::ServiceServer ros_tutorials_service_server = nh.advertiseService("ros_tutorial_srv", calculation);
	
	ROS_INFO("ready srv server!");
	ros::spin();//等待客户端的（服务）请求
	
	return 0;
}

6. 创建客户端节点：

与CMakeLists.txt对应：add_executable(service_client src/service_client.cpp)

内容如下：

#include <ros/ros.h>
#include <ros_tutorials_service/SrvTutorial.h>
#include <cstdlib>

int main(int argc,char **argv)
{
	ros::init(argc,argv,"service_client");//声明客户端节点名称
	if(argc!=3)//输入值错误提示
	{
	  ROS_INFO("cmd:rosrun ros_tutorials_service service_client arg0 arg1");
	  ROS_INFO("arg0:double number,arg1:double number");
	  return 1;
	}
	ros::NodeHandle nh;
	//声明客户端，创建一个使用ros_tutorials_service功能包SrvTutorial服务文件的客户端
	//ros_tutorial_service_client,服务名称是ros_tutorial_srv,
	ros::ServiceClient ros_tutorials_service_client = nh.serviceClient<ros_tutorials_service::SrvTutorial>("ros_tutorial_srv");
	//声明要使用SrvTutorial服务文件名为srv的服务
	ros_tutorials_service::SrvTutorial srv;
	srv.request.a = atoll(argv[1]);
	srv.request.b = atoll(argv[2]);
	//请求服务，并显示服务器响应的结果
	if(ros_tutorials_service_client.call(srv))
	{
		ROS_INFO("send srv,srv.Request.a and b:%ld,%ld",(long int)srv.request.a,(long int)srv.request.b);
		ROS_INFO("sreceive srv,srv.Response.result:%ld",(long int)srv.response.result);
	}
	else
	{
		ROS_ERROR("Failed to call service ros_tutorial_srv");
		return 1;
	}
	return 0;
}

7. 运行效果：

• 如果没有输入a,b的值，会有如下提示：
  
• 输入正确会有结果反馈（输入之间要有空格）：
  

同时看到，服务和客户端之间的请求是一次触发，只有客户端有正确的请求才会触发服务器响应，响应完后就会自动断开，不同于上一节的话题消息，只要建立后就一直会发送。

8. 节点关系图：

• rqt_graph，由于服务是一次性的连接，不好采集建立时的关系图，只显示服务器，客户端请求完收到响应后自动断开不能看到：
  
• 利用rqt插件看一下它们的效果。在Experssion输入a,b的值，点击call：