SLAM从入门到精通（服务通信）

前面我们谈到了消息通信，但是这种通信是单向的。也就是一个人发送消息，一个人接收消息，仅此而已。在ros当中，还有一种通信，称之为服务通信。它和单项通信最大的区别就是，我们把数据发送给对方，还能得到对方的反馈结果。这在很多场合下都是非常适合的。

1、创建srv文件

和消息传递一样，服务通信也需要创建脚本文件。首先在package目录下面创建一个srv文件加，里面添加PassWord.srv文件，内容如下，

int64 password
---
bool result

内容是比较简单的。这个时候cd到workspace顶层，输入catkin_make，就可以生成对应的头文件，当然CMakeLists.txt也是需要修改的。

2、修改CMakeLists.txt


cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)

## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS message_generation roscpp rospy std_msgs genmsg)
add_message_files(FILES Student.msg)
add_service_files(FILES PassWord.srv)

## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)

## Declare a catkin package
catkin_package()

## Build talker and listener
include_directories(include ${catkin_INCLUDE_DIRS})

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

通过make编译以及查找，我们发现生成的.h文件都被放到了devel目录下了，

feixiaoxing@feixiaoxing-VirtualBox:~/Desktop/catkin_ws$ find . -name "*.h"
./devel/include/beginner_tutorials/PassWordResponse.h
./devel/include/beginner_tutorials/PassWordRequest.h
./devel/include/beginner_tutorials/Student.h
./devel/include/beginner_tutorials/PassWord.h

3、重写talker.cpp文件


#include "ros/ros.h"
#include "beginner_tutorials/PassWord.h"
#include <cstdlib>

using namespace std;

int main(int argc, char** argv)
{
	ros::init(argc, argv, "node_client");
	if(argc < 2)
	{
		cout << "Error param, please run eg: rosrun beginner_tutorials talker 123456" << endl;
		return 1;
	}

	ros::NodeHandle nh;
	ros::ServiceClient client = nh.serviceClient<beginner_tutorials::PassWord>("pswserver", 100);
	beginner_tutorials::PassWord srv;

	srv.request.password = atoi(argv[1]);
	if(client.call(srv))
	{
		ROS_INFO("client connect success!");
		if(srv.response.result)
		{
			ROS_INFO("Welcom, correct password!");
		}
		else
		{
			ROS_INFO("Sorry, password error");
		}
	}
	else
	{
		ROS_INFO("client connect fail");
	}
	return 0;
}

代码中唯一需要注意的就是client.call这个用法。

4、重写listener.cpp文件


#include "ros/ros.h"
#include "beginner_tutorials/PassWord.h"

bool serverCallback(beginner_tutorials::PassWord::Request& req,
		beginner_tutorials::PassWord::Response& res)
{
	res.result = (req.password == 123456) ? true :false;
	return true;
}

int main(int argc, char* argv[])
{
	ros::init(argc, argv, "server_node");
	ros::NodeHandle nh;
	ros::ServiceServer serv = nh.advertiseService("pswserver", &serverCallback);
	ros::spin();
	return 0;
}

这里的回调函数注意一下，也就是serverCallback的用法。

5、启动和测试

准备了节点之后就可以开始编译。编译的方法也比较简单，就是输入catkin_make。确认没有问题之后，就是启动roscore和source ~/devel/setup.sh。依次打开listener和talker，

rosrun beginner_tutorials listener

不出意外，打开talker之后，就可以看到这样的打印信息，

feixiaoxing@feixiaoxing-VirtualBox:~/Desktop/catkin_ws$ rosrun beginner_tutorials talker 123456
[ INFO] [1695354474.608237290]: client connect success!
[ INFO] [1695354474.608742554]: Welcom, correct password!