ROS消息发布器和订阅器的编写

1. 编写发布器节点

主要步骤如下：

1. 包含一系列头文件；
2. 初始化ROS；
3. 创建句柄；
4. 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；
5. 创建ros::Rate对象指定自循环的频率
6. 进入循环（终止条件为ctrl-c操作）；
7. 根据消息对象的数据成员来生成消息；
8. 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；
9. 调用回调函数；
10. 休眠来控制发布信息的频率；
11. 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 

/**
 */
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：

1. 包含一系列头文件；
2. 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；
3. 编写主函数；
4. 初始化ROS；
5. 创建句柄；
6. 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；
7. 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。