1. 编写发布器节点
主要步骤如下：
- 包含一系列头文件；
- 初始化ROS；
- 创建句柄；
- 用句柄的advertise函数( 用法：advertise<类型>(“主题”，发布序列大小) )得到一个ros::Publisher对象，相当于是在Master上完成了发布器的注册。之后可利用Publisher的成员函数publish()在Topic上发布信息；
- 创建ros::Rate对象指定自循环的频率
- 进入循环（终止条件为ctrl-c操作）；
- 根据消息对象的数据成员来生成消息；
- 用之前得到的Publisher的成员函数publish()(用法：publish(消息对象))来发布消息；
- 调用回调函数；
- 休眠来控制发布信息的频率；
- 退出循环。
可以看出在发布器的实现过程中，没有涉及到回调函数，因为在发布器和订阅器中，订阅器属于一直等待的一方（可与服务端和客户端做对比），回调函数在订阅器中编写。
C++代码（talker.cpp文件）：
#include "ros/ros.h"
#include "std_msgs/String.h"
#include <sstream>
/**
*/
int main(int argc, char **argv)
{
/**
* The ros::init() function needs to see argc and argv so that it can perform
* any ROS arguments and name remapping that were provided at the command line. For programmatic
* remappings you can use a different version of init() which takes remappings
* directly, but for most command-line programs, passing argc and argv is the easiest
* way to do it. The third argument to init() is the name of the node.
*
* You must call one of the versions of ros::init() before using any other
* part of the ROS system.
*/
ros::init(argc, argv, "talker");
/**
* NodeHandle is the main access point to communications with the ROS system.
* The first NodeHandle constructed will fully initialize this node, and the last
* NodeHandle destructed will close down the node.
*/
ros::NodeHandle n;
/**
* The advertise() function is how you tell ROS that you want to
* publish on a given topic name. This invokes a call to the ROS
* master node, which keeps a registry of who is publishing and who
* is subscribing. After this advertise() call is made, the master
* node will notify anyone who is trying to subscribe to this topic name,
* and they will in turn negotiate a peer-to-peer connection with this
* node. advertise() returns a Publisher object which allows you to
* publish messages on that topic through a call to publish(). Once
* all copies of the returned Publisher object are destroyed, the topic
* will be automatically unadvertised.
*
* The second parameter to advertise() is the size of the message queue
* used for publishing messages. If messages are published more quickly
* than we can send them, the number here specifies how many messages to
* buffer up before throwing some away.
*/
ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);
ros::Rate loop_rate(10);
/**
* A count of how many messages we have sent. This is used to create
* a unique string for each message.
*/
int count = 0;
while (ros::ok())
{
/**
* This is a message object. You stuff it with data, and then publish it.
*/
std_msgs::String msg;
std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();
ROS_INFO("%s", msg.data.c_str());
/**
* The publish() function is how you send messages. The parameter
* is the message object. The type of this object must agree with the type
* given as a template parameter to the advertise<>() call, as was done
* in the constructor above.
*/
chatter_pub.publish(msg);
ros::spinOnce();
loop_rate.sleep();
++count;
}
return 0;
}
}
2.编写订阅器节点
主要步骤如下：
- 包含一系列头文件；
- 编写回调函数（当有消息到达订阅器所订阅的Topic时，会执行回调函数），回调函数的参数是一个智能指针，指向消息对象（注意参数内是用的&），可利用此指针执行一些操作；
- 编写主函数；
- 初始化ROS；
- 创建句柄；
- 用句柄的subscribe函数( 用法：subscribe(“主题”, 发布序列大小, 回调函数名) ，订阅器是不用知道消息的类型的)得到一个ros::Subscribe对象，相当于在Master上完成了订阅器的注册。知道了主题，就能得到到达主题的消息，有了消息就可以以指向消息的指针为参数调用回调函数，以实现一些操作；
- 用ros::spin()进入自循环，以尽可能快的速度调用回调函数；
可以看出在订阅器的实现过程中，涉及到了回调函数，因为在发布器和订阅器中，订阅器属于一直等待的一方（可与服务端和客户端做对比），回调函数在订阅器中编写。
C++代码（listener.cpp文件）：
#include "ros/ros.h"
#include "std_msgs/String.h"
/**
* This tutorial demonstrates simple receipt of messages over the ROS system.
*/
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
ROS_INFO("I heard: [%s]", msg->data.c_str());
}
int main(int argc, char **argv)
{
/**
* The ros::init() function needs to see argc and argv so that it can perform
* any ROS arguments and name remapping that were provided at the command line. For programmatic
* remappings you can use a different version of init() which takes remappings
* directly, but for most command-line programs, passing argc and argv is the easiest
* way to do it. The third argument to init() is the name of the node.
*
* You must call one of the versions of ros::init() before using any other
* part of the ROS system.
*/
ros::init(argc, argv, "listener");
/**
* NodeHandle is the main access point to communications with the ROS system.
* The first NodeHandle constructed will fully initialize this node, and the last
* NodeHandle destructed will close down the node.
*/
ros::NodeHandle n;
/**
* The subscribe() call is how you tell ROS that you want to receive messages
* on a given topic. This invokes a call to the ROS
* master node, which keeps a registry of who is publishing and who
* is subscribing. Messages are passed to a callback function, here
* called chatterCallback. subscribe() returns a Subscriber object that you
* must hold on to until you want to unsubscribe. When all copies of the Subscriber
* object go out of scope, this callback will automatically be unsubscribed from
* this topic.
*
* The second parameter to the subscribe() function is the size of the message
* queue. If messages are arriving faster than they are being processed, this
* is the number of messages that will be buffered up before beginning to throw
* away the oldest ones.
*/
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
/**
* ros::spin() will enter a loop, pumping callbacks. With this version, all
* callbacks will be called from within this thread (the main one). ros::spin()
* will exit when Ctrl-C is pressed, or the node is shutdown by the master.
*/
ros::spin();
return 0;
}
3.编译节点
在CMakeLists.txt中增加几行，变为：
cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)
## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)
## 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)
最后用catkin_make编译工作空间即可。