Note: This tutorial assumes that you have completed the previous tutorials: 理解ROS服务和参数.
(!) 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 (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:

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

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

Wiki: cn/ROS/Tutorials/WritingPublisherSubscriber(c++) (last edited 2017-03-15 07:12:13 by QiangQiu)