机器人操作系统ROS Indigo 入门学习（12）——用C++语言写一个简单的发布者和订阅者

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这个教程将会包含怎样用C++去写一个发布者和订阅者．

1.1写一个发布者Node

“Node”是连接在ROS网络中一个可执行单元的术语．这里我们创建一个会不断广播messages的发布者（“talker”）node.

改变目录到你之前创建的工作空间的beginner_tutorials package中：

cd ~/catkin_ws/src/beginner_tutorials

1.1.1代码

在beginner_tutorials packag目录中创建一个srv目录：

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

这个目录会包含所有beginner_tutorials package中的源文件．

在beginner_tutorials package中创建一个src/talker.cpp文件．并且把下面的代码粘贴上去＂：

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/talker/talker.cpp

#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;

}

1.1.2代码解释

现在我们分解代码．

#include “ros/ros.h”

ros/ros.h是一个非常方便的头文件它包含了最常用的ROS系统部分所必须的一些头文件．

#include “std_msgs/String.h”

这里包含了std_msgs package中的std_msgs/String message ．这个头文件自动的从String.msg.file中产生．更多关于message的信息，请查看msg page．

Ros::init(argc,argv,”talker”);

初始化ROS.这个允许ROS通过命令行重新映射名字 –现在不重要.同样可以用来指定node的名字．在系统中Nodes的名字必须是唯一的．

名字必须是一个基本的名字(base name)，比如，不能有/在里面．

Ros::NodesHandle n;

为这个node创建一个handle．创建的第一个NodeHandle用来初始化node,最后一个销毁的NodeHandle会清除所有node占有的资源．

ros::Publisher chatter_pub = n.advertise<std_msgs::

String>("chatter", 1000);

告诉master我们将要在chatter topic中要发布一个std_msgs::String类型的message，这就会让master告诉所有的nodes听取chatter这个topic，在这个tpoic上在我们将要发布数据．第二个参数是发布队列的大小．这样的话如果我们发布的太快，在开始丢弃之前的message前，它会最大缓冲是1000个messages．

NodeHandle::advertise()返回一个ros::Publisher的对象，它有两个作用：（１）它允许你发布message到它创建的topic上的publish()（２）当它超出范围时，会自动解除广播．

ros::Rate loop_rate(10);

ros::Rate对象会指定一个你想循环的频率．它会跟踪距离上次调用Rate::sleep(),有多长时间了，并且休眠正确长度的时间．

这里我们设置为10hz:

int count =0;

while(ros::ok())

{}

默认roscpp会安装一个SIGINT信号处理函数提供对ctrl+c的处理，ctrl+c会导致ros::ok()返回错误．

ros::ok()会返回错误如果：

接受到SIGINT(ctrl+c)

我们通过用另一个有同样名字的node网络。

ros::shutdown()被应用的另一部分调用。

所有的 ros::NodeHandles都被摧毁了。

一旦ros::ok()返回错误，所有的ROS调用都会失败．

87 std_msgs::String msg;

89 std::stringstream ss;

90 ss << "hello world " << count;

91 msg.data = ss.str();

我们使用适应message的类在ROS上广播了一个message，通常从一个msg文件产生出来。其他复杂的数据类型也是是可以的，但是现在我们准备用标准的String message，它有一个成员：”data”。

chatter_pub.publish(msg);

现在实际上我们在向任何一个连接上的人广播这个message.

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

ROS_INFO和它的友元类都是用来替代printf/cout的。更多信息请看rosconsole documentation

ros::spinOnce();

对于这个程序调用ros::splinOnce()不是必要的，因为我们不会接受到任何回叫信号。然而，如果你打算为这个应用添加一个订阅，并且没有调用ros::splinOnce(),你绝不会得到回叫信号，所以还是添加的好。

loop_rate.sleep();

现在使用ros::Rate对象去空耗掉剩下的时间以满足10hz的发布速度。

这里是步骤的简要描叙：

初始化ROS系统

广告给master我们将要发布std_msgs/Stringmessage到chatter topic上

在发布message的时候循环以满足10次每秒钟

1.2写一个订阅者 Node

1.2.1代码

在beginner_tutorials package中src目录下创建listener.cpp文件，并且把下面的代码粘贴进去：

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/listener/listener.cpp

#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;

}

1.2.2代码解释

现在，我们把代码打断成一段段的，忽略上面已经分析过的部分

34 void chatterCallback(const std_msgs::String::ConstPtr& msg)

35 {

36 ROS_INFO("I heard: [%s]", msg->data.c_str());

37 }

当一个新的message抵达chatter topic时这个回调函数会被调用．这个message以boost shared_ptr,的形式传递，这意味着你可以储存它，而不用担心它会在被删除，并且无需拷贝底层的数据．

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

在master启动的前提下订阅chatter topic，ROS会调用chatter Callback()函数只要一个新的message到达时．第二个参数是队列的大小，假设我们没有足够的能力去使发送message足够的快．这样的话，如果队列达到1000个messages,随着新的message的到来，我们会开始丢掉旧的message.

NodeHandle::subscribe()会返回一个ros::Subscriber 对象，你必须坚持这个订阅对象直到你想取消订阅．当订阅对象被摧毁时，它会自动取消订阅chatter topic.

这里有不同版本的NodeHandle::subscribe()函数允许你指定一个类的成员函数，或者甚至任何被BoostFunction对象调用的东西，roscpp overview包含更多的信息．

ros::spin();

ros::spin()进入了一个循环，调用message回调尽可能的快．即使这样，但不用担心，如果没有什么要做就不会占用许多CPU资源，ros::spin()会退出一旦ros::ok()返回错误，这意味着ros::shutdown()被调用了，不是被默认的Ctrl+c处理函数，然后master告诉我们去关机，就是被手动调用．

还有其他调用回调函数的方法，但是这里我们不关心它．roscpp_tutorials package中有一些关于这个的应用演示。roscpp overview　也包含更多的信息．

这里再一次简要的概括以上内容：

初始化ROS系统

订阅chatter topic

Spin,等待message到来

当一个message到来时，chatterCallback()函数被调用

1.3编译代码

在之前的教程中你用catkin_create_pkg去创建一个package.xml和一个CMakeLists.txt文件．

产生的这个CMakeLists.txt文件看起来应该像这样（保留了在Creating Msgs and Srvs中的修改和除去没有用的注释和例子）：

https://raw.github.com/ros/catkin_tutorials/master/create_package_modified/catkin_ws/src/beginner_tutorials/CMakeLists.txt

1 cmake_minimum_required(VERSION 2.8.3)

2 project(beginner_tutorials)

4 ## Find catkin and any catkin packages

5 find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)

7 ## Declare ROS messages and services

8 add_message_files(DIRECTORY msg FILES Num.msg)

9 add_service_files(DIRECTORY srv FILES AddTwoInts.srv)

11 ## Generate added messages and services

12 generate_messages(DEPENDENCIES std_msgs)

14 ## Declare a catkin package

15 catkin_package()

不要担心修改被注释掉的例子，只需要添加下面几行到你的CMakeLists.txt文件就好了：

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)

最后的CMakeLists.txt文件看起来应该像这样：

https://raw.github.com/ros/catkin_tutorials/master/create_package_pubsub/catkin_ws/src/beginner_tutorials/CMakeLists.txt

1 cmake_minimum_required(VERSION 2.8.3)

2 project(beginner_tutorials)

4 ## Find catkin and any catkin packages

5 find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)

7 ## Declare ROS messages and services

8 add_message_files(FILES Num.msg)

9 add_service_files(FILES AddTwoInts.srv)

11 ## Generate added messages and services

12 generate_messages(DEPENDENCIES std_msgs)

14 ## Declare a catkin package

15 catkin_package()

17 ## Build talker and listener

18 include_directories(include ${catkin_INCLUDE_DIRS})

20 add_executable(talker src/talker.cpp)

21 target_link_libraries(talker ${catkin_LIBRARIES})

22 add_dependencies(talker beginner_tutorials_generate_messages_cpp)

24 add_executable(listener src/listener.cpp)

25 target_link_libraries(listener ${catkin_LIBRARIES})

26 add_dependencies(listener beginner_tutorials_generate_messages_cpp)

这会创建两个可执行的文件，talker和listener,默认是在的devel的package目录中，默认是在~/catkin_ws/devel/lib/share/<package name>.

注意你应该为可执行目标添加依赖到message generation 目标：

add_dependencies(talker beginner_tutorials_generate_messages_cpp)

这就可以保证package的message header在使用之前可以生成，如果你使用你工作空间的其他package产生的messages,你也需要为它们单独生成的目标添加依赖。

因为catkin平行编译所有的工程。如果是“Groovy”你可以用下面的变量去依靠所有必须的目标：

add_dependencies(talker ${catkin_EXPORTED_TARGETS})

如果你可以使用rosrun去调用他们，可以直接调用它们。他们不是放在'<prefix>/bin' 中因为在安装package到你的系统时会破坏PATH.如果你希望你的可执行文件安装的时候在PATH上，你可以建立一个安装目标，查看：catkin/CMakeLists.txt

catkin/CMakeLists.txt的更多详细的描叙请看：catkin/CMakeLists.txt

现在运行：

$ catkin_make

注意：如果你在添加一个新的pkg,也许需要告诉catkin去强制编译通过—force-cmake选项。参阅catkin/Tutorials/using_a_workspace#With_catkin_make.

原文地址：http://blog.csdn.net/bobsweetie/article/details/43638835

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