理解ROS：创建并使用自己的msg和srv

1. 创建msg和srv

在包的src同一级创建msg和srv文件夹，并在里面创建自己的消息和服务。如：
my_msg.msg

Header header
string child_frame_id
geometry_msgs/PoseWithCovariance pose
geometry_msgs/TwistWithCovariance twist

my_srv.srv
注意：虚线以上为请求(request)，虚线下面为相应(response)

int64 a
int64 b
---
int64 sum

2. 配置msg和srv

• CmakeList.txt配置

find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  message_generation
)
add_message_files(FILES my_msg.msg)
add_service_files(FILES my_srv.srv)
generate_messages(DEPENDENCIES std_msgs)
catkin_package()
include_directories(include ${catkin_INCLUDE_DIRS})

• package.xml配置

实际上，如果在创建包的时候加上message_generation，就没必要添加下面的配置了，同理cmakeList.txt也可以不用加message_generation。在这里我还是加上了，以避免忘记加。

  <build_depend>message_generation</build_depend>
  <build_export_depend>message_generation</build_export_depend>
  <exec_depend>message_generation</exec_depend>

3. 使用service

普通消息和自己创建的消息使用情况一样，在发布器和订阅器已经使用过消息，这里不再赘述。下面开始讲服务。
直接贴代码和自己的注释：

• service端

#include "ros/ros.h"
#include "test_service/my_srv.h"//创建srv的头文件

bool add(test_service::my_srv::Request  &req, test_service::my_srv::Response &res)//求和服务
{
  res.sum = req.a + req.b;//求和服务，相应的值存入response里面
  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::init(argc, argv, "node_client");
  ros::NodeHandle n;

  ros::ServiceServer service = n.advertiseService("add_two_ints", add);//建立service，并在ROS内发布出来，这里发布的是求和
  ROS_INFO("Ready to add two ints.");
  ros::spin();

  return 0;
}

• client端

#include "ros/ros.h"
#include "test_service/my_srv.h"
#include <cstdlib>

int main(int argc, char **argv)
{
  ros::init(argc, argv, "node_server");
  if (argc != 3)
  {
    ROS_INFO("usage: add_two_ints_client X Y");
    return 1;
  }

  ros::NodeHandle n;
  ros::ServiceClient client = n.serviceClient<test_service::my_srv>("add_two_ints");//创建client
  test_service::my_srv srv;
  srv.request.a = atoll(argv[1]);//利用终端传入两个参数
  srv.request.b = atoll(argv[2]);
  if (client.call(srv))//调用service，调用成功返回true
  {
    ROS_INFO("Sum: %ld", (long int)srv.response.sum);
  }
  else
  {
    ROS_ERROR("Failed to call service add_two_ints");
    return 1;
  }

  return 0;
}

• 配置添加

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

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