ROS基础（三）

十、编写简单的消息发布器和订阅器 (C++)

（以beginner_tutorials为名的包为例）

编写发布器节点

1.cd到src文件的相应包内

$ cd ~/catkin_ws/src/beginner_tutorials

2.在 beginner_tutorials package 路径下创建一个src文件夹：

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

（这个文件夹将会用来放置 beginner_tutorials package 的所有源代码）

3.在 beginner_tutorials package 里创建 src/talker.cpp 文件，并将如下代码粘贴到文件内：

＃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;
 }

代码注释版：

 #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。

 #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。

 #include <sstream>
 int main(int argc, char **argv)
 {
 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,在这里，我们也可以指定节点的名称——运行过程中，节点的名称必须唯一。这里的名称必须是一个 base name ，也就是说，名称内不能包含 / 等符号。

 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。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", 1000);

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。这样 master 就会告诉所有订阅了 chatter 话题的节点，将要有数据发布。第二个参数是发布序列的大小。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。

 ros::Rate loop_rate(10);

单位Hz。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。

 while (ros::ok())

roscpp 会默认生成一个 SIGINT 句柄，它负责处理 Ctrl-C 键盘操作——使得 ros::ok() 返回 false。

	/＊＊
	＊一旦 ros::ok() 返回 false, 所有的 ROS 调用都会失效。
	＊如果下列条件之一发生，ros::ok() 返回false：
	＊SIGINT 被触发 (Ctrl-C)
	＊被另一同名节点踢出 ROS 网络	
	＊ros::shutdown() 被程序的另一部分调用
	＊节点中的所有 ros::NodeHandles 都已经被销毁
	＊＊/ 

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

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。

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。（ROS控制台输出库提供http://wiki.ros.org/rosconsole）

chatter_pub.publish(msg);

向所有订阅 chatter 话题的节点发送消息。

ros::spinOnce();

如果你的程序里包含其他回调函数，最好在这里加上 ros::spinOnce()这一语句，否则你的回调函数就永远也不会被调用了。

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.

编写订阅器节点

在 beginner_tutorials package 目录下创建 src/listener.cpp 文件，并粘贴如下代码：

#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;
  }

代码注释版：

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

这是一个回调函数，当接收到 chatter 话题的时候就会被调用。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。

ros::Subscriber sub = n.subscribe("chatter", 1000, 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。
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 chatter 话题的消息。
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。

ros::spin();

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。这有可能是 ros::shutdown() 被调用，或者是用户按下了 Ctrl-C，使得 master 告诉节点要终止运行。也有可能是节点被人为关闭的。

总结:
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

编译节点

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_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})

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

为可执行文件添加对生成的消息文件的依赖：

add_dependencies(talker beginner_tutorials_generate_messages_cpp)
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

修改后完整的 CMakeLists.txt ：

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)

然后在catkin工作空间中执行catkin_make

十一、写一个简单的消息发布器和订阅器 (Python)

编写发布器节点

1.cd到src文件的相应包内

$ cd ~/catkin_ws/src/beginner_tutorials

2.创建一个名为scripts的文件来储存Python代码

$ mkdir scripts
$ cd scripts

3.获取talker.py的示例代码并增加权限

$ wget https://raw.github.com/ros/ros_tutorials/kinetic-devel/rospy_tutorials/001_talker_listener/talker.py
$ chmod +x talker.py

4.查看 talker.py

$ rosed beginner_tutorials talker.py

代码解释：

＃!/usr/bin/env python

每个Python ROS节点都有这个声明，确保该Python代码能执行。

import rospy
from std_msgs.msg import String

引入相应模块，rospy为ROS节点所必须，引入String则可以发布String的信息。

pub = rospy.Publisher('chatter', String, queue_size=10)

发布到话题chatter上，信息类型为String，queue_size为buffer长度，推荐10或者更多

rospy.init_node('talker', anonymous=True)

初始化，告诉rospy节点的名称，这里的名称必须是一个base name，名称内不能包含 / 等符号。
anonymous=True通过将随机数添加到名称的结尾，确保节点具有唯一的名称。

rate = rospy.Rate(10)

10Hz的传输速率，即1秒10次

while not rospy.is_shutdown():
hello_str = "hello world %s" % rospy.get_time()

先判断执行标志位，然后编写要发送的字符串

rospy.loginfo(hello_str)

三个作用：把信息打印打屏幕，写入节点的日志文件，输出到rosout方便debug

pub.publish(hello_str)

发布信息

rate.sleep()

延时以达到rospy.Rate的发送速率

if __name__ == '__main__':
	try:
		talker()
	except rospy.ROSInterruptException:
		pass

中断报错，防止sleep()后继续执行程序

编写订阅器节点

$ roscd beginner_tutorials/scripts/
$ wget https://raw.github.com/ros/ros_tutorials/kinetic-devel/rospy_tutorials/001_talker_listener/listener.py

获取示例代码

$ chmod +x listener.py

添加权限

代码解释：

rospy.spin()

防止节点退出直到它被关闭（与roscpp不同，这不影响回调）

编译节点：

$ cd　/catkin_ws
$ catkin_make

十二、测试发布器和订阅器

1.确保roscore可用，并运行：

$ roscore

2.如果使用catkin，确保你在调用catkin_make后，在运行你自己的程序前，已经source了catkin工作空间下的setup.sh文件：

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

3.运行发布者"talker"

$ rosrun beginner_tutorials talker      (C++)
$ rosrun beginner_tutorials talker.py   (Python) 

你将看到如下的输出信息:

[INFO] [WallTime: 1314931831.774057] hello world 1314931831.77
[INFO] [WallTime: 1314931832.775497] hello world 1314931832.77
[INFO] [WallTime: 1314931833.778937] hello world 1314931833.78
[INFO] [WallTime: 1314931834.782059] hello world 1314931834.78
[INFO] [WallTime: 1314931835.784853] hello world 1314931835.78
[INFO] [WallTime: 1314931836.788106] hello world 1314931836.79

发布器节点已经启动运行。现在需要一个订阅器节点来接受发布的消息。

4.运行订阅者"listener"

$ rosrun beginner_tutorials listener     (C++)
$ rosrun beginner_tutorials listener.py  (Python)

你将会看到如下的输出信息:

[INFO] [WallTime: 1314931969.258941] /listener_17657_1314931968795I heard hello world 1314931969.26
[INFO] [WallTime: 1314931970.262246] /listener_17657_1314931968795I heard hello world 1314931970.26
[INFO] [WallTime: 1314931971.266348] /listener_17657_1314931968795I heard hello world 1314931971.26
[INFO] [WallTime: 1314931972.270429] /listener_17657_1314931968795I heard hello world 1314931972.27
[INFO] [WallTime: 1314931973.274382] /listener_17657_1314931968795I heard hello world 1314931973.27
[INFO] [WallTime: 1314931974.277694] /listener_17657_1314931968795I heard hello world 1314931974.28
[INFO] [WallTime: 1314931975.283708] /listener_17657_1314931968795I heard hello world 1314931975.28

你已经测试完了发布器和订阅器