ROS的初步学习（五）--自己写一个简单的发布（Publisher）、订阅(Subscriber)程序

1 写一个发布（Publisher）节点

节点（node）是连接到ROS网络中可执行的基本单元。我们在这创建一个发布者—“talker”节点，这个节点持续对外发布消息。

首先我们要把目录切换到我们的beginner_tutorials工程包中

$ cd ~/catkin_ws/src/beginner_tutorials  

因为我们已经编译过这个工程包了，所以会在beginner_tutorials文件夹下看到CmakeList.txt、package.xml文件和include、src这两个目录。接下来进入src子目录
在src目录中创建一个talker.cpp文件，里面的内容如下：

#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**写一个订阅（Subscriber）节点**
还是在src目录下，创建一个listener.cpp文件。内容如下：

    #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 编译创建的节点

在编译我们创建的节点之前，我们还需要编辑Cmakelist.txt文件（注意：是beginner_tutorials项目包下的CMakelist文件），告诉编辑器我们需要编辑什么文件，需要什么依赖。

    $ gedit CMakeLists.txt 

在文件末尾添加如下语句：

 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) 

将目录切换到工作区目录，并执行catkin_make运行命令：

    $ cd ~/catkin_ws      $ catkin_make  

至此，程序已经创建完成，而接下来我们要检查一下我们创建的程序是否正确。
测试程序的正确性
首先，我们得要启动ROS核心程序
roscore.
在使用我们的程序之前，需要先把程序注册

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

运行talker节点：

$ rosrun beginner_tutorials talker   

出现如下图所示：

这就表示发布（Publisher）节点已经正确的运行了。
接下来运行listener节点：

    $ rosrun beginner_tutorials listener  

这说明订阅节点（listener）已经成功的接收到了发布节点（talker）发布的信息。至此，整个程序结束！