SLAM从入门到精通（消息传递）

前面我们只是编写了一个publisher节点，以及一个subscribe节点。有了这两个节点，它们之间就可以通信了。在实际生产中，我们除了简单的通信之外，要传递的数据可能还有很多。这个时候，我们就要构建一个消息体。这个消息体可以是各种脚本，但需要同时能被publisher和subscribe访问到。幸好，ros给我们提供了这样的一种形式。

1、构建消息体

在package下面创建一个msg文件夹。文件夹中保存一个Student.msg文件，它的内容如下所示，

string name
float64 height
float64 weight

这个文件其实只是一个脚本。通过在workspace顶层输入catkin_make，就可以生成对应的头文件Student.h。注意，头文件保存的位置位于./devel/include/beginner_tutorials/Student.h。编译之前也要修改下CMakelists.txt文件，如下面2所示。

2、修改CMakelists.txt文件


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)

## Declare ROS messages and services
#add_message_files(FILES Num.msg)
#add_service_files(FILES AddTwoInts.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)

3、重新构建talker.cpp文件


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

using namespace std;

int main(int argc, char* argv[])
{
	ros::init(argc, argv, "node_MyMsgPub");
	if(argc!=4)
	{
		cout << "Error command parameter!\n" \
			"Please run command eg:\n" \
			"rosrun begin_tutorials talker LiLei 180 160" << endl;
		return 1;
	}

	ros::NodeHandle nh;
	
	ros::Publisher MyMsgPub = nh.advertise<beginner_tutorials::Student>("MyMsg", 100);
	beginner_tutorials::Student sdtMsg;
	sdtMsg.name = argv[1];
	sdtMsg.height = atof(argv[2]);
	sdtMsg.weight = atof(argv[3]);
	
	ros::Rate rate(10);
	while(ros::ok())
	{
		MyMsgPub.publish(sdtMsg);
		rate.sleep();
	}

	return 0;
}

代码大部分和之前差不多，只要注意beginner_tutorials::Student这个结构体即可。

4、重新构建listener.cpp文件

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

void MyMsgCallback(const beginner_tutorials::Student& sdtInfo)
{
	ROS_INFO("Thestudent information is:\n"
		"name:%s--height:%f--weight:%f\n",
		sdtInfo.name.c_str(),
		sdtInfo.height,
		sdtInfo.weight);
}

int main(int argc, char* argv[])
{
	ros::init(argc, argv, "node_MyMsgSub");

	ros::NodeHandle nh;
	ros::Subscriber MyMsgSub = nh.subscribe("MyMsg", 1000, &MyMsgCallback);
	ros::spin();

	return 0;
}

和talker.cpp一样，代码中只需要关注下beginner_tutorials::Student这个结构体就行。

5、顶层编译

cd到workspace的顶层，直接输入catkin_make即可。

6、准备运行talker和listener节点

在运行talker和listener节点之前，有两件事情要做。第一步，打开roscore；第二步，source ./dev/setup.sh文件。最后，分别打开talker和listenner即可。

rosrun beginner_tutorials talker LiLei 180 160
rosrun beginner_tutorials listener

没什么意外的化，你会看到这样的打印。如果能看到这些打印，基本上说明我们已经学会使用了ros的消息功能。

[ INFO] [1695351213.590856004]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351213.692688848]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351213.791460924]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351213.891075380]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351213.990753652]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351214.090438037]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351214.190497066]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351214.291007141]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351214.390375087]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000

[ INFO] [1695351214.490385779]: Thestudent information is:
name:LiLei--height:180.000000--weight:160.000000