ROS主题发布订阅

     节点是一个可执行程序，它连接到了ROS的网络系统中。我们将会创建一个发布者，也就是说话者节点，它将会持续的广播一个信息。

     改变目录到之前所建立的那个包下：

 

cd ~/catkin_ws/src/beginner_tutorials  

 

     在beginner_tutorials包下面建立一个src文件夹：

mkdir -p ~/catkin_ws/src/beginner_tutorials/src  

     创建文件src/talker.cpp：

gedit src/talker.cpp   

 

     将下面的内容复制进去：

    #include "ros/ros.h"  
    #include "std_msgs/String.h"  
      
    #include <sstream>  
      
    /**  
     * 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.  
       */  
      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;  
    }  

 

 

     解释一下代码：

#include "ros/ros.h"  

 

ros/ros.h包括了使用ROS系统最基本的头文件。

 

#include "std_msgs/String.h"

这条代码包括了std_msgs/String消息，它存在于std_msgs包中。这是有std_msgs中的String.msg文件自动产生的。

ros::init(argc, argv, "talker");  

 

初始化ROS，它允许ROS通过命令行重新命名，现在还不太重要。这里也是我们确切说明节点名字的地方，在运行的系统中，节点的名字必须唯一

 

ros::NodeHandle n;  

为处理的节点创建了一个句柄，第一个创建的节点句柄将会初始化这个节点，最后一个销毁的节点将会释放节点所使用的所有资源。

 

ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);  

 

告诉主机，我们将会在一个名字为chatter的话题上发布一个std_msgs/String类型的消息，这就使得主机告诉了所有订阅了chatter 话题的节点，我们将在这个话题上发布数据。第二个参数是发布队列的大小，它的作用是缓冲。当我们发布消息很快的时候，它将能缓冲1000条信息。如果慢了 的话就会覆盖前面的信息。

NodeHandle::advertise()将会返回ros::Publisher对象，该对象有两个作用，首先是它包括一个publish()方法可以在制定的话题上发布消息，其次，当超出范围之外的时候就会自动的处理。

 

ros::Rate loop_rate(10);  

 

一个ros::Rate对象允许你制定循环的频率。它将会记录从上次调用Rate::sleep()到现在为止的时间，并且休眠正确的时间。在这个例子中，设置的频率为10hz。

 

    int count = 0;  
    while (ros::ok())  
    {  

 

默认情况下，roscpp将会安装一个SIGINT监听，它使当Ctrl-C按下时，ros::ok()将会返回false。

ros::ok()在以下几种情况下也会返回false：（1）按下Ctrl-C时（2）我们被一个同名同姓的节点从网络中踢出（3）ros::shutdown()被应用程序的另一部分调用（4）所有的ros::NodeHandles都被销毁了。一旦ros::ok()返回false，所有的ROS调用都会失败。

 

    std_msgs::String msg;  
      
    std::stringstream ss;  
    ss << "hello world " << count;  
    msg.data = ss.str();  

 

我们使用message-adapted类在ROS中广播信息，这个类一般是从msg文件中产生的。我们现在使用的是标准的字符串消息，它只有一个data数据成员，当然更复杂的消息也是可以的。

chatter_pub.publish(msg);

现在我们向话题chatter发布消息。

ROS_INFO("%s", msg.data.c_str());  

 

ROS_INFO是cout和printf的替代品。

ros::spinOnce();  

 

在这个简单的程序中调用ros::spinOnce();是不必要的，因为我们没有收到任何的回调信息。然而如果你为这个应用程序添加一个订阅者，并且在这里没有调用ros::spinOnce()，你的回调函数将不会被调用。所以这是一个良好的风格。

 

loop_rate.sleep();  

 

休眠一下，使程序满足前面所设置的10hz的要求。      下面总结一下创建一个发布者节点的步骤：（1）初始化ROS系统（2）告诉主机我们将要在chatter话题上发布std_msgs/String类型的消息（3）循环每秒发送10次消息。

 

 

打开一个终端，进入到beginner_tutorials包下面：

cd ~/catkin_ws/src/beginner_tutorials  

 

     编辑文件src/listener.cpp：

 

gedit src/listener.cpp  

 

     将下面的代码复制到文件中：

 

 #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;  
    }  

 

 

 

 

 

 

 

    void chatterCallback(const std_msgs::String::ConstPtr& msg)  
    {  
      ROS_INFO("I heard: [%s]", msg->data.c_str());  
    }  

 

当一个消息到达chatter话题时，这个回调函数将会被调用。

    ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);  

 

订阅chatter话题，当一个新的消息到达时，ROS将会调用chatterCallback()函数。第二个参数是对列的长度，如果我们处理消息的速度不够快，会将收到的消息缓冲下来，一共可以缓冲1000条消息，满1000之后，后面的到达的消息将会覆盖前面的消息。

 

NodeHandle::subscribe()将会返回一个ros::Subscriber类型的对象，当订阅对象被销毁以后，它将会自动从chatter话题上撤销。

 

    ros::spin();  

 

ros::spin()进入了一个循环，可以尽快的调用消息的回调函数。不要担心，如果它没有什么事情可做时，它也不会浪费太多的CPU。当ros::ok()返回false时，ros::spin()将会退出。这就意味着，当ros::shutdown()被调用，或按下CTRL+C等情况，都可以退出。下面总结一下写一个订阅者的步骤：（1）初始化ROS系统（2）订阅chatter话题（3）Spin，等待消息的到来（4）当一个消息到达时，chatterCallback()函数被调用。

 

     下面看一下如何构建节点。这时候你的CMakeLists.txt看起来应该是下面这个样子，包括前面所做的修改，注释部分可以除去：

 

    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(DIRECTORY msg FILES Num.msg)  
    add_service_files(DIRECTORY srv FILES AddTwoInts.srv)  
      
    ## Generate added messages and services  
    generate_messages(DEPENDENCIES std_msgs)  
      
    ## Declare a catkin package  
    catkin_package()  

 

 

     将下面几行代码添加到CMakeLists.txt的最后。最终你的CMakeLists.txt文件看起来样该是下面这个样子：

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)

这将会创建两个可执行文件，talker和listener。它们将会产生在~/catkin_ws/devel/lib/share/<package name>目录下，下面开始构建，在你的工作空间根目录下输入：

catkin_make