ros如何編寫Publisher and Subscriber

publisher編寫

• “節點”是用於連接到網絡的ROS的可執行的ROS術語。在這裏，我們將創建一個發佈者（“說話者”），這將不斷地廣播消息的節點。

// %Tag(ROS_HEADER)%
#include "ros/ros.h"  //引入ros的頭文件
// %Tag(MSG_HEADER)%
#include "std_msgs/String.h"  //引入消息類型的頭文件

#include <sstream>  //stringstream完成內存string的IO

/**
 * This tutorial demonstrates simple sending of messages over the ROS system.
 */
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.
   */
   
//使用任何其他前必須先調用ROS::的init()
// %Tag(INIT)%
  ros::init(argc, argv, "talker");
// %EndTag(INIT)%

  /**
   * 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.
   */
// %Tag(NODEHANDLE)%
  ros::NodeHandle n;  
// %EndTag(NODEHANDLE)%

  /**
   * 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.
   */
// %Tag(PUBLISHER)%
  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000); //創建發佈者對象
// %EndTag(PUBLISHER)%

// %Tag(LOOP_RATE)%
  ros::Rate loop_rate(10);  //向構造函數傳入參數，frequency 	The desired rate to run at in Hz 
// %EndTag(LOOP_RATE)%

  /**
   * A count of how many messages we have sent. This is used to create
   * a unique string for each message.
   *我們已發送多少消息的計數。這用於創建每個消息的唯一字符串
   */
// %Tag(ROS_OK)%
  int count = 0;
  while (ros::ok())
  {
// %EndTag(ROS_OK)%
    /**
     * This is a message object. You stuff it with data, and then publish it.
     *這是一個消息對象。 您用數據填充它，然後發佈它
     */
// %Tag(FILL_MESSAGE)%
    std_msgs::String msg;  //創建消息對象

    std::stringstream ss;
    ss << "hello world " << count;
    msg.data = ss.str();
// %EndTag(FILL_MESSAGE)%

// %Tag(ROSCONSOLE)%
    ROS_INFO("%s", msg.data.c_str());
// %EndTag(ROSCONSOLE)%

    /**
     * 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.
     */
// %Tag(PUBLISH)%
    chatter_pub.publish(msg);  //發佈消息
// %EndTag(PUBLISH)%

// %Tag(SPINONCE)%
    ros::spinOnce();
// %EndTag(SPINONCE)%

// %Tag(RATE_SLEEP)%
    loop_rate.sleep();
// %EndTag(RATE_SLEEP)%
    ++count;
  }


  return 0;
}
// %EndTag(FULLTEXT)%

• 程序點撥：

1. ros::init(argc, argv, "talker");
   對於編程的重新映射，你可以使用不同版本的init（），接受直接重新映射，但對於大多數命令行程序，傳遞argc和argv是做到這一點最簡單的方法。
2. ros::NodeHandle n;
   NodeHandle建設將全面初始化這個節點，最後NodeHandle破壞並關閉該節點。
3. ros::Rate loop_rate(10);
   向構造函數傳入參數，frequency The desired rate to run at in Hz
4. ros::ok()
   用來檢查關機的各種狀態
5. ros::spinOnce();
   ROS消息回調處理函數，在調用後還可以繼續執行之後的程序,若程序裏添加了訂閱的函數，則必須要有ros::spinOnce();。

Subscriber編寫

// %Tag(FULLTEXT)%
#include "ros/ros.h"
#include "std_msgs/String.h"

/**
 * This tutorial demonstrates simple receipt of messages over the ROS system.
 */
// %Tag(CALLBACK)%
void chatterCallback(const std_msgs::String::ConstPtr& msg)  //回調函數
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());  //打印出訂閱的消息
}
// %EndTag(CALLBACK)%

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.
   */
// %Tag(SUBSCRIBER)%
  ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
// %EndTag(SUBSCRIBER)%

  /**
   * 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.
   */
// %Tag(SPIN)%
  ros::spin();
// %EndTag(SPIN)%

  return 0;
}
// %EndTag(FULLTEXT)%

• 程序解讀：

1. 注意要編寫回調函數，處理處理訂閱的信息