1 寫一個釋出（Publisher）節點

節點（node）是連線到ROS網路中可執行的基本單元。我們在這建立一個釋出者—“talker”節點，這個節點持續對外發布訊息。

首先我們要把目錄切換到我們的beginner_tutorials工程包中

$ cd ~/catkin_ws/src/beginner_tutorials  

因為我們已經編譯過這個工程包了，所以會在beginner_tutorials資料夾下看到CmakeList.txt、package.xml檔案和include、src這兩個目錄。接下來進入src子目錄
在src目錄中建立一個talker.cpp檔案，裡面的內容如下：

#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**寫一個訂閱（Subscriber）節點**
還是在src目錄下，建立一個listener.cpp檔案。內容如下：

    #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 編譯建立的節點

在編譯我們建立的節點之前，我們還需要編輯Cmakelist.txt檔案（注意：是beginner_tutorials專案包下的CMakelist檔案），告訴編輯器我們需要編輯什麼檔案，需要什麼依賴。

    $ gedit CMakeLists.txt 

在檔案末尾新增如下語句：

 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) 

將目錄切換到工作區目錄，並執行catkin_make執行命令：

    $ cd ~/catkin_ws  
    $ catkin_make  

至此，程式已經建立完成，而接下來我們要檢查一下我們建立的程式是否正確。
測試程式的正確性
首先，我們得要啟動ROS核心程式
roscore.
在使用我們的程式之前，需要先把程式註冊

    $ cd ~/catkin_ws  
    $ source ./devel/setup.bash 

執行talker節點：

$ rosrun beginner_tutorials talker   

出現如下圖所示：

這就表示釋出（Publisher）節點已經正確的運行了。
接下來執行listener節點：

    $ rosrun beginner_tutorials listener  

這說明訂閱節點（listener）已經成功的接收到了釋出節點（talker）釋出的資訊。至此，整個程式結束！