本文介绍了如何使用C++和Python在ROS环境中实现消息发布与订阅的功能。通过编写发布器和订阅器节点，实现了数据的传输与接收。文章详细解析了代码实现及编译过程。
编写简单的消息发布器和订阅器 (C++)
Description: 本教程将介绍如何用 C++ 编写发布器节点和订阅器节点。Tutorial Level: BEGINNER
Next Tutorial: 测试消息发布器和订阅器
编写发布器节点
『节点』(Node) 是指 ROS 网络中可执行文件。接下来，我们将会创建一个发布器节点("talker")，它将不断的在 ROS 网络中广播消息。
切换到之前创建的 beginner_tutorials package 路径下：
cd ~/catkin_ws/src/beginner_tutorials
源代码
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。
在 beginner_tutorials package 里创建 src/talker.cpp 文件，并将如下代码粘贴到文件内：
https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/talker/talker.cpp
27 #include "ros/ros.h"
28 #include "std_msgs/String.h"
29
30 #include <sstream>
31
32 /**
33 * This tutorial demonstrates simple sending of messages over the ROS system.
34 */
35 int main(int argc, char **argv)
36 {
37 /**
38 * The ros::init() function needs to see argc and argv so that it can perform
39 * any ROS arguments and name remapping that were provided at the command line. For programmatic
40 * remappings you can use a different version of init() which takes remappings
41 * directly, but for most command-line programs, passing argc and argv is the easiest
42 * way to do it. The third argument to init() is the name of the node.
43 *
44 * You must call one of the versions of ros::init() before using any other
45 * part of the ROS system.
46 */
47 ros::init(argc, argv, "talker");
48
49 /**
50 * NodeHandle is the main access point to communications with the ROS system.
51 * The first NodeHandle constructed will fully initialize this node, and the last
52 * NodeHandle destructed will close down the node.
53 */
54 ros::NodeHandle n;
55
56 /**
57 * The advertise() function is how you tell ROS that you want to
58 * publish on a given topic name. This invokes a call to the ROS
59 * master node, which keeps a registry of who is publishing and who
60 * is subscribing. After this advertise() call is made, the master
61 * node will notify anyone who is trying to subscribe to this topic name,
62 * and they will in turn negotiate a peer-to-peer connection with this
63 * node. advertise() returns a Publisher object which allows you to
64 * publish messages on that topic through a call to publish(). Once
65 * all copies of the returned Publisher object are destroyed, the topic
66 * will be automatically unadvertised.
67 *
68 * The second parameter to advertise() is the size of the message queue
69 * used for publishing messages. If messages are published more quickly
70 * than we can send them, the number here specifies how many messages to
71 * buffer up before throwing some away.
72 */
73 ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);
74
75 ros::Rate loop_rate(10);
76
77 /**
78 * A count of how many messages we have sent. This is used to create
79 * a unique string for each message.
80 */
81 int count = 0;
82 while (ros::ok())
83 {
84 /**
85 * This is a message object. You stuff it with data, and then publish it.
86 */
87 std_msgs::String msg;
88
89 std::stringstream ss;
90 ss << "hello world " << count;
91 msg.data = ss.str();
92
93 ROS_INFO("%s", msg.data.c_str());
94
95 /**
96 * The publish() function is how you send messages. The parameter
97 * is the message object. The type of this object must agree with the type
98 * given as a template parameter to the advertise<>() call, as was done
99 * in the constructor above.
100 */
101 chatter_pub.publish(msg);
102
103 ros::spinOnce();
104
105 loop_rate.sleep();
106 ++count;
107 }
108
109
110 return 0;
111 }
代码说明
现在，我们来分段解释代码。
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。
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。需要关于消息的定义，可以参考 msg 页面。
初始化 ROS 。它允许 ROS 通过命令行进行名称重映射——然而这并不是现在讨论的重点。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。
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。
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。
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::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。
在这个例子中，我们让它以 10Hz 的频率运行。
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。
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
一旦 ros::ok() 返回 false, 所有的 ROS 调用都会失效。
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。
这里，我们向所有订阅 chatter 话题的节点发送消息。
ROS_INFO 和其他类似的函数可以用来代替 printf/cout 等函数。具体可以参考 rosconsole documentation，以获得更多信息。
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。
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。
对上边的内容进行一下总结：
- 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
接下来我们要编写一个节点来接收这个消息。
编写订阅器节点
源代码
在 beginner_tutorials package 目录下创建 src/listener.cpp 文件，并粘贴如下代码：
https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/listener/listener.cpp
28 #include "ros/ros.h"
29 #include "std_msgs/String.h"
30
31 /**
32 * This tutorial demonstrates simple receipt of messages over the ROS system.
33 */
34 void chatterCallback(const std_msgs::String::ConstPtr& msg)
35 {
36 ROS_INFO("I heard: [%s]", msg->data.c_str());
37 }
38
39 int main(int argc, char **argv)
40 {
41 /**
42 * The ros::init() function needs to see argc and argv so that it can perform
43 * any ROS arguments and name remapping that were provided at the command line. For programmatic
44 * remappings you can use a different version of init() which takes remappings
45 * directly, but for most command-line programs, passing argc and argv is the easiest
46 * way to do it. The third argument to init() is the name of the node.
47 *
48 * You must call one of the versions of ros::init() before using any other
49 * part of the ROS system.
50 */
51 ros::init(argc, argv, "listener");
52
53 /**
54 * NodeHandle is the main access point to communications with the ROS system.
55 * The first NodeHandle constructed will fully initialize this node, and the last
56 * NodeHandle destructed will close down the node.
57 */
58 ros::NodeHandle n;
59
60 /**
61 * The subscribe() call is how you tell ROS that you want to receive messages
62 * on a given topic. This invokes a call to the ROS
63 * master node, which keeps a registry of who is publishing and who
64 * is subscribing. Messages are passed to a callback function, here
65 * called chatterCallback. subscribe() returns a Subscriber object that you
66 * must hold on to until you want to unsubscribe. When all copies of the Subscriber
67 * object go out of scope, this callback will automatically be unsubscribed from
68 * this topic.
69 *
70 * The second parameter to the subscribe() function is the size of the message
71 * queue. If messages are arriving faster than they are being processed, this
72 * is the number of messages that will be buffered up before beginning to throw
73 * away the oldest ones.
74 */
75 ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
76
77 /**
78 * ros::spin() will enter a loop, pumping callbacks. With this version, all
79 * callbacks will be called from within this thread (the main one). ros::spin()
80 * will exit when Ctrl-C is pressed, or the node is shutdown by the master.
81 */
82 ros::spin();
83
84 return 0;
85 }
代码说明
下面我们将逐条解释代码，当然，之前解释过的代码就不再赘述了。
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。
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。roscpp overview 提供了更为详尽的信息。
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。
还有其他的方法进行回调，但在这里我们不涉及。想要了解，可以参考 roscpp_tutorials package 里的一些 demo 应用。需要更为详尽的信息，可以参考 roscpp overview。
下边，我们来总结一下:
- 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
之前教程中使用 catkin_create_pkg 创建了 package.xml 和 CMakeLists.txt 文件。
生成的 CMakeLists.txt 看起来应该是这样(在 Creating Msgs and Srvs 教程中的修改和未被使用的注释和例子都被移除了):
1 cmake_minimum_required(VERSION 2.8.3)
2 project(beginner_tutorials)
3
4 ## Find catkin and any catkin packages
5 find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)
6
7 ## Declare ROS messages and services
8 add_message_files(DIRECTORY msg FILES Num.msg)
9 add_service_files(DIRECTORY srv FILES AddTwoInts.srv)
10
11 ## Generate added messages and services
12 generate_messages(DEPENDENCIES std_msgs)
13
14 ## Declare a catkin package
15 catkin_package()
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})
结果，CMakeLists.txt 文件看起来大概是这样:
1 cmake_minimum_required(VERSION 2.8.3)
2 project(beginner_tutorials)
3
4 ## Find catkin and any catkin packages
5 find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)
6
7 ## Declare ROS messages and services
8 add_message_files(FILES Num.msg)
9 add_service_files(FILES AddTwoInts.srv)
10
11 ## Generate added messages and services
12 generate_messages(DEPENDENCIES std_msgs)
13
14 ## Declare a catkin package
15 catkin_package()
16
17 ## Build talker and listener
18 include_directories(include ${catkin_INCLUDE_DIRS})
19
20 add_executable(talker src/talker.cpp)
21 target_link_libraries(talker ${catkin_LIBRARIES})
22 add_dependencies(talker beginner_tutorials_generate_messages_cpp)
23
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.
现在要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)
这样就可以确保自定义消息的头文件在被使用之前已经被生成。因为 catkin 把所有的 package 并行的编译，所以如果你要使用其他 catkin 工作空间中其他 package 的消息，你同样也需要添加对他们各自生成的消息文件的依赖。当然，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})
你可以直接调用可执行文件，也可以使用 rosrun 来调用他们。他们不会被安装到 <prefix>/bin 路径下，因为那样会改变系统的 PATH 环境变量。如果你确定要将可执行文件安装到该路径下，你需要设置安装位置，请参考 catkin/CMakeLists.txt
如果需要关于 CMakeLists.txt 更详细的信息，请参考 catkin/CMakeLists.txt
现在运行 catkin_make：
# In your catkin workspace $ catkin_make
注意：如果你是添加了新的 package，你需要通过 --force-cmake 选项告诉 catkin 进行强制编译。参考 catkin/Tutorials/using_a_workspace#With_catkin_make。
既然已经编写好了发布器和订阅器，下面让我们来测试消息发布器和订阅器。
写一个简单的消息发布器和订阅器 (Python)
Description: 本教程将通过Python编写一个发布器节点和订阅器节点。Tutorial Level: BEGINNER
Next Tutorial: 执行你的消息发布器和订阅器
目录
Writing the Publisher Node
"Node" is the ROS term for an executable that is connected to the ROS network. Here we'll create the publisher ("talker") node which will continually broadcast a message.
Change directory into the beginner_tutorials package, you created in the earlier tutorial, creating a package:
$ roscd beginner_tutorials
The Code
First lets create a 'scripts' folder to store our Python scripts in:
$ mkdir scripts $ cd scripts
Then download the example script talker.py to your new scripts directory and make it executable:
$ wget https://raw.github.com/ros/ros_tutorials/kinetic-devel/rospy_tutorials/001_talker_listener/talker.py $ chmod +x talker.py
You can view and edit the file with $ rosed beginner_tutorials talker.py or just look below.
1 #!/usr/bin/env python
2 # license removed for brevity
3 import rospy
4 from std_msgs.msg import String
5
6 def talker():
7 pub = rospy.Publisher('chatter', String, queue_size=10)
8 rospy.init_node('talker', anonymous=True)
9 rate = rospy.Rate(10) # 10hz
10 while not rospy.is_shutdown():
11 hello_str = "hello world %s" % rospy.get_time()
12 rospy.loginfo(hello_str)
13 pub.publish(hello_str)
14 rate.sleep()
15
16 if __name__ == '__main__':
17 try:
18 talker()
19 except rospy.ROSInterruptException:
20 pass
The Code Explained
Now, let's break the code down.
Every Python ROS Node will have this declaration at the top. The first line makes sure your script is executed as a Python script.
You need to import rospy if you are writing a ROS Node. The std_msgs.msg import is so that we can reuse the std_msgs/String message type (a simple string container) for publishing.
This section of code defines the talker's interface to the rest of ROS. pub = rospy.Publisher("chatter", String, queue_size=10) declares that your node is publishing to the chatter topic using the message type String. String here is actually the class std_msgs.msg.String. The queue_size argument is New in ROS hydro and limits the amount of queued messages if any subscriber is not receiving the them fast enough. In older ROS distributions just omit the argument.
The next line, rospy.init_node(NAME), is very important as it tells rospy the name of your node -- until rospy has this information, it cannot start communicating with the ROS Master. In this case, your node will take on the name talker. NOTE: the name must be a base name, i.e. it cannot contain any slashes "/".
This line creates a Rate object rate. With the help of its method sleep(), it offers a convenient way for looping at the desired rate. With its argument of 10, we should expect to go through the loop 10 times per second (as long as our processing time does not exceed 1/10th of a second!)
This loop is a fairly standard rospy construct: checking the rospy.is_shutdown() flag and then doing work. You have to check is_shutdown() to check if your program should exit (e.g. if there is a Ctrl-C or otherwise). In this case, the "work" is a call to pub.publish(hello_str) that publishes a string to our chatter topic. The loop calls rate.sleep(), which sleeps just long enough to maintain the desired rate through the loop.
(You may also run across rospy.sleep() which is similar to time.sleep() except that it works with simulated time as well (see Clock).)
This loop also calls rospy.loginfo(str), which performs triple-duty: the messages get printed to screen, it gets written to the Node's log file, and it gets written to rosout. rosout is a handy for debugging: you can pull up messages using rqt_consoleinstead of having to find the console window with your Node's output.
std_msgs.msg.String is a very simple message type, so you may be wondering what it looks like to publish more complicated types. The general rule of thumb is that constructor args are in the same order as in the .msg file. You can also pass in no arguments and initialize the fields directly, e.g.
msg = String() msg.data = str
or you can initialize some of the fields and leave the rest with default values:
String(data=str)
You may be wondering about the last little bit:
In addition to the standard Python __main__ check, this catches a rospy.ROSInterruptException exception, which can be thrown by rospy.sleep() and rospy.Rate.sleep() methods when Ctrl-C is pressed or your Node is otherwise shutdown. The reason this exception is raised is so that you don't accidentally continue executing code after the sleep().
Now we need to write a node to receive the messages.
Writing the Subscriber Node
The Code
Download the listener.py file into your scripts directory:
$ roscd beginner_tutorials/scripts/ $ wget https://raw.github.com/ros/ros_tutorials/kinetic-devel/rospy_tutorials/001_talker_listener/listener.py
The file contents look close to:
1 #!/usr/bin/env python
2 import rospy
3 from std_msgs.msg import String
4
5 def callback(data):
6 rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)
7
8 def listener():
9
10 # In ROS, nodes are uniquely named. If two nodes with the same
11 # node are launched, the previous one is kicked off. The
12 # anonymous=True flag means that rospy will choose a unique
13 # name for our 'listener' node so that multiple listeners can
14 # run simultaneously.
15 rospy.init_node('listener', anonymous=True)
16
17 rospy.Subscriber("chatter", String, callback)
18
19 # spin() simply keeps python from exiting until this node is stopped
20 rospy.spin()
21
22 if __name__ == '__main__':
23 listener()
Don't forget to make the node executable:
$ chmod +x listener.py
The Code Explained
The code for listener.py is similar to talker.py, except we've introduced a new callback-based mechanism for subscribing to messages.
This declares that your node subscribes to the chatter topic which is of type std_msgs.msgs.String. When new messages are received, callback is invoked with the message as the first argument.
We also changed up the call to rospy.init_node() somewhat. We've added the anonymous=True keyword argument. ROS requires that each node have a unique name. If a node with the same name comes up, it bumps the previous one. This is so that malfunctioning nodes can easily be kicked off the network. The anonymous=True flag tells rospy to generate a unique name for the node so that you can have multiple listener.py nodes run easily.
The final addition, rospy.spin() simply keeps your node from exiting until the node has been shutdown. Unlike roscpp, rospy.spin() does not affect the subscriber callback functions, as those have their own threads.
Building your nodes
We use CMake as our build system and, yes, you have to use it even for Python nodes. This is to make sure that the autogenerated Python code for messages and services is created.
Go to your catkin workspace and run catkin_make:
$ cd ~/catkin_ws $ catkin_make
Now that you have written a simple publisher and subscriber, let's examine the simple publisher and subscriber.
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