Ros学习——C++发布器publisher和订阅器subscriber

1.编写发布器

• 初始化 ROS 系统

• 在 ROS 网络内广播我们将要在 chatter 话题上发布 std_msgs/String 类型的消息

• 以每秒 10 次的频率在 chatter 上发布消息

在 beginner_tutorials package 里创建 src/talker.cpp 文件：

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

#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.可指定节点的名称。节点的名称必须唯一(名称内不能包含 / 等符号)

  /**
   * 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;　　//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 NodeHandle 则会释放该节点所占用的所有资源
  /**
   * 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", );//告诉master 我们将要在 chatter（话题名） 上发布 std_msgs/String 消息类型的消息。
　　　　　　　　　　　　　//如果我们发布的消息的频率太高，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。
　　　　　　　　　　　　　//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::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 = ;
  while (ros::ok())　　　　　　//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());//ROS_INFO 和其他类似的函数可以用来代替 printf/cout 等函数。

    /**
     * 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);//向所有订阅 chatter 话题的节点发送消息。

    ros::spinOnce();　　　　　　//ros::spinOnce()这一语句，否则你的回调函数就永远也不会被调用

    loop_rate.sleep();　　　　//调用 ros::Rate 对象来休眠一段时间以使得发布频率为 10Hz。
    ++count;
  }

  return ;
}

2.编写订阅器

　　

• 初始化ROS系统

• 订阅 chatter 话题

• 进入自循环，等待消息的到达

• 当消息到达，调用 chatterCallback() 函数

在 beginner_tutorials package 目录下创建 src/listener.cpp 文件：

#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)//是一个回调函数，当接收到 chatter 话题的时候就会被调用。
{
  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", , chatterCallback);

//告诉 master 我们要订阅 chatter 话题上的消息。当有消息发布到这个话题时，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，当缓存达到 1000 条消息后，再有新的消息到来就将开始丢弃先前接收的消息。

/** * 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();//ros::spin() 进入自循环，可以尽可能快的调用消息回调函数。

return ; 
}

3.编译

• 1.确认CMakeLists.txt文件

cmake_minimum_required(VERSION 2.8.)
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(

• 2.在 CMakeLists.txt 文件末尾加入几条语句:

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)

• 1-2会生成两个可执行文件, talker 和 listener, 默认存储到 devel space 目录下,具体在~/catkin_ws/devel/lib/<package name> 中.
• 3.如果在 *Groovy* 版本下，你可以使用下边的这个变量来添加对所有必须的文件依赖:

add_dependencies(talker ${catkin_EXPORTED_TARGETS})

• 4.运行 catkin_make

4.测试结果：

5.问题：

1.当你在package.xml中，添加完run_depend后，编译出错，显示The manifest must not cotain the following  tags:run_depend。

原因：软件包格式分两种，第一种的四种依赖关系分别是：

<buildtool_depend>

<build_depend>

<run_depend>

<test_depend>

第二种的依赖关系变成：

<buildtool_depend>

<build_depend>

<build_export_depend>

<exec_depend>

<test_depend>

<doc_depend>

解决：当你run_depend出错时，有可能是你的软件包格式是第二种，所以要把run_depend改成exec_depend

2.invoking "make cmake_check_build_system" failed

仔细检查：

cmake_minimum_required(VERSION 2.8.)
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()　　#尤其要注意，里面为空否则报上面的错