ROS学习（十二）—— 编写简单的消息发布器和订阅器（C++）

一、创建发布器节点

1 节点功能：

　　不断的在ROS网络中广播消息

2 创建节点

（1）打开工作空间目录

cd ~/catkin_ws/src/beginner_tutorials

创建一个发布器节点("talker")，它将不断的在ROS网络中广播消息。
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（2）创建src文件夹

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

（3）创建talkler.cpp文件，幷附上代码

#include "ros/ros.h"              //      ros/ros.h是一个实用的头文件，它引用了ROS系统中大部分常用的头文件，使用它会使得编程很简便。
#include "std_msgs/String.h"      //      这引用了std_msgs/String 消息, 它存放在std_msgs package里，是由String.msg文件自动生成的头文件。
                                  //  需要更详细的消息定义,参考msg页面. 

#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");   // 初始化ROS。它允许ROS通过命令行进行名称重映射——目前，这不是重点。
                                     // 同样，我们也在这里指定我们节点的名称——必须唯一。

  /**
   * 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.

    为这个进程的节点创建一个句柄。第一个创建的NodeHandle会为节点进行初始化，最后一个销毁的会清理节点使用的所有资源。
   */

  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.
   */

  /* 1、告诉master我们将要在chatter topic上发布一个std_msgs/String的消息。这样master就会告诉所有订阅了chatter topic的节点，将要有数据发布。
     2、第二个参数是发布序列的大小。在这样的情况下，如果我们发布的消息太快，缓冲区中的消息在大于1000个的时候就会开始丢弃先前发布的消息。
     3、NodeHandle::advertise() 返回一个 ros::Publisher对象,它有两个作用: 1) 它有一个publish()成员函数可以让你在topic上发布消息；
                                                                     2) 如果消息类型不对,它会拒绝发布。
  */
  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", );

  //ros::Rate对象可以允许你指定自循环的频率。它会追踪记录自上一次调用Rate::sleep()后时间的流逝，并休眠直到一个频率周期的时间。
  ros::Rate loop_rate();

  /**
   * A count of how many messages we have sent. This is used to create
   * a unique string for each message.
   */
  int count = ;

  /* roscpp会默认安装一个SIGINT句柄，它负责处理Ctrl-C键盘操作——使得ros::ok()返回FALSE。
         ros::ok()返回false，如果下列条件之一发生：
              1）SIGINT接收到(Ctrl-C)
              2）被另一同名节点踢出ROS网络
              3）ros::shutdown()被程序的另一部分调用
              4） 所有的ros::NodeHandles都已经被销毁
  */

  while (ros::ok())
  {
    /**
     * This is a message object. You stuff it with data, and then publish it.
     */
    //我们使用一个由msg file文件产生的‘消息自适应’类在ROS网络中广播消息。现在我们使用标准的String消息，它只有一个数据成员"data"。当然你也可以发布更复杂的消息类型。
    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);    //现在我们已经向所有连接到chatter topic的节点发送了消息

    ros::spinOnce();             //在这个例子中并不是一定要调用ros::spinOnce()，因为我们不接受回调。
                                 //然而，如果你想拓展这个程序，却又没有在这调用ros::spinOnce()，你的回调函数就永远也不会被调用。所以，在这里最好还是加上这一语句。 

    loop_rate.sleep();           //这条语句是调用ros::Rate对象来休眠一段时间以使得发布频率为10hz。 

    ++count;
  }

  return ;
}

二、编写订阅器节点

在src文件中，继续写入listener.cpp文件，幷附上代码

#include "ros/ros.h"
#include "std_msgs/String.h"

/**
 * This tutorial demonstrates simple receipt of messages over the ROS system.
 * 这是一个回调函数，当消息到达chatter topic的时候就会被调用。消息是以 boost shared_ptr指针的形式传输，这就意味着你可以存储它而又不需要复制数据
 */
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.
   */

   //告诉master我们要订阅chatter topic上的消息。当有消息到达topic时，ROS就会调用chatterCallback()函数。
   //第二个参数是队列大小，以防我们处理消息的速度不够快，在缓存了1000个消息后，再有新的消息到来就将开始丢弃先前接收的消息。
   //NodeHandle::subscribe()返回ros::Subscriber对象,你必须让它处于活动状态直到你不再想订阅该消息。当这个对象销毁时，它将自动退订消息。
   //有各种不同的NodeHandle::subscribe()函数，允许你指定类的成员函数，甚至是Boost.Function对象可以调用的任何数据类型。roscpp overview 提供了更为详尽的信息。 

   ros::Subscriber sub = n.subscribe("chatter", , 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()进入自循环，可以尽可能快的调用消息回调函数。如果没有消息到达，它不会占用很多CPU，所以不用担心。
  //一旦ros::ok()返回FALSE，ros::spin()就会立刻跳出自循环。
  //   1)这有可能是ros::shutdown()被调用，
  //   2)或者是用户按下了Ctrl-C，使得master告诉节点要shutdown。也有可能是节点被人为的关闭。

  ros::spin();

  return ;
}

三、编译节点

1、在beginner_tutorials 包中的CMakeLists.txt 的末尾加入

## 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)

　　这会生成两个可执行文件, talker 和 listener, 默认存储到devel space目录,具体是在~/catkin_ws/devel/lib/<package name>中.

这会生成两个可执行文件, talker 和 listener, 默认存储到devel space目录,具体是在~/catkin_ws/devel/lib/中.
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2、编译

# In your catkin workspace
catkin_make  

四、测试消息发布器和订阅器

1、运行roscore

2、加入到ROS工作空间中

# In your catkin workspace
cd ~/catkin_ws
source ./devel/setup.bash

3、运行talker节点

rosrun beginner_tutorials talker

rosrun beginner_tutorials listener

创建一个发布器节点("talker")，它将不断的在ROS网络中广播消息。
-->