Note: This tutorial assumes that you have completed the previous tutorials: understanding ROS topics.
(!) Please ask about problems and questions regarding this tutorial on answers.ros.org. Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags.

编写简单的消息发布器和订阅器

Description: 本教程将介绍如何编写C++的发布器节点和订阅器节点

Tutorial Level: BEGINNER

Next Tutorial: 编写一个简单的Service和Client

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:

roscd beginner_tutorials

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。

  27 #include "ros/ros.h"
  28 

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。

  28 #include "std_msgs/String.h"
  29 

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。

  47   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——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。

  54   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。

  73   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。

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。

  75   ros::Rate loop_rate(10);

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。

  81   int count = 0;
  82   while (ros::ok())
  83   {

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。

  87     std_msgs::String msg;
  88 
  89     std::stringstream ss;
  90     ss << "hello world " << count;
  91     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。

 101     chatter_pub.publish(msg);

这里,我们向所有订阅 chatter 话题的节点发送消息。

  93     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。具体可以参考 rosconsole documentation,以获得更多信息。

 103     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。

 105     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。

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

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。

  34 void chatterCallback(const std_msgs::String::ConstPtr& msg)
  35 {
  36   ROS_INFO("I heard: [%s]", msg->data.c_str());
  37 }

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。

  75   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。roscpp overview 提供了更为详尽的信息。

  82   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。

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

roscreate-pkg 会自动为你创建 Makefile 和 CMakeLists.txt 文件

$ rosed beginner_tutorials CMakeLists.txt 

看起来应该像这样:

  • cmake_minimum_required(VERSION 2.4.6)
    include($ENV{ROS_ROOT}/core/rosbuild/rosbuild.cmake)
    
    # Set the build type.  Options are:
    #  Coverage       : w/ debug symbols, w/o optimization, w/ code-coverage
    #  Debug          : w/ debug symbols, w/o optimization
    #  Release        : w/o debug symbols, w/ optimization
    #  RelWithDebInfo : w/ debug symbols, w/ optimization
    #  MinSizeRel     : w/o debug symbols, w/ optimization, stripped binaries
    #set(ROS_BUILD_TYPE RelWithDebInfo)
    
    rosbuild_init()
    
    #set the default path for built executables to the "bin" directory
    set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin)
    #set the default path for built libraries to the "lib" directory
    set(LIBRARY_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/lib)
    
    #uncomment if you have defined messages
    #rosbuild_genmsg()
    #uncomment if you have defined services
    #rosbuild_gensrv()
    
    #common commands for building c++ executables and libraries
    #rosbuild_add_library(${PROJECT_NAME} src/example.cpp)
    #target_link_libraries(${PROJECT_NAME} another_library)
    #rosbuild_add_boost_directories()
    #rosbuild_link_boost(${PROJECT_NAME} thread)
    #rosbuild_add_executable(example examples/example.cpp)
    #target_link_libraries(example ${PROJECT_NAME})

在最后增加如下语句:

rosbuild_add_executable(talker src/talker.cpp)
rosbuild_add_executable(listener src/listener.cpp)

这会生成两个可执行文件, talkerlistener,编译结果默认存放在 "bin" 目录下.

想要了解更多关于 ROS 下 CMake 的信息,可以参考 CMakeLists 现在运行make:

$ make

编译节点

之前教程中使用 catkin_create_pkg 创建了 package.xmlCMakeLists.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):

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)
   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()

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:

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)
   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, talkerlistener, 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

运行节点

运行节点需要你先启动 ROS Core。打开终端,输入:

roscore

如果启动正常,你可以看到类似下边的输出:

... logging to /u/takayama/.ros/logs/83871c9c-934b-11de-a451-001d927076eb/roslaunch-ads-31831.log
... loading XML file [/wg/stor1a/rosbuild/shared_installation/ros/tools/roslaunch/roscore.xml]
Added core node of type [rosout/rosout] in namespace [/]
started roslaunch server http://ads:54367/

SUMMARY
========

NODES

changing ROS_MASTER_URI to [http://ads:11311/] for starting master locally
starting new master (master configured for auto start)
process[master]: started with pid [31874]
ROS_MASTER_URI=http://ads:11311/
setting /run_id to 83871c9c-934b-11de-a451-001d927076eb
+PARAM [/run_id] by /roslaunch
+PARAM [/roslaunch/uris/ads:54367] by /roslaunch
process[rosout-1]: started with pid [31889]
started core service [/rosout]
+SUB [/time] /rosout http://ads:33744/
+SERVICE [/rosout/get_loggers] /rosout http://ads:33744/
+SERVICE [/rosout/set_logger_level] /rosout http://ads:33744/
+PUB [/rosout_agg] /rosout http://ads:33744/
+SUB [/rosout] /rosout http://ads:33744/

现在已经准备好运行 talker/listener 节点了。新打开终端,跳转到 talker/listener package 并输入:

./bin/talker

在原来的终端输入:

./bin/listener

现在已经准备好运行talker/listener节点了。新打开终端,跳转到 catkin 工作空间并输入:

source ./devel/setup.bash
rosrun beginner_tutorials talker

在原来的终端输入:

rosrun beginner_tutorials listener

talker 将会输出如下类似文字:

[ INFO] I published [Hello there! This is message [0]]
[ INFO] I published [Hello there! This is message [1]]
[ INFO] I published [Hello there! This is message [2]]
...

同时 listener 也会开始输出类似的文本信息:

[ INFO] Received [Hello there! This is message [20]]
[ INFO] Received [Hello there! This is message [21]]
[ INFO] Received [Hello there! This is message [22]]
...

真棒!你已经成功运行了你的第一个 ROS 节点。需要更多的实例代码,请参考 roscpp_tutorials package.

Next Tutorial: 编写简单的Service和Client


Wiki: cn/roscpp_tutorials/Tutorials/WritingPublisherSubscriber (last edited 2017-03-15 07:09:31 by QiangQiu)