Ros学习——C++发布器publisher和订阅器subscriber
1.编写发布器
- 初始化 ROS 系统
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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
在 beginner_tutorials package 里创建 src/talker.cpp 文件：
#include "ros/ros.h" //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" //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> /** * 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"); //初始化 ROS.可指定节点的名称。节点的名称必须唯一(名称内不能包含 / 等符号) /** * 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; //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 /** * 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);//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。 //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。
//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 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()) //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());//ROS_INFO 和其他类似的函数可以用来代替 printf/cout 等函数。 /** * 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);//向所有订阅 chatter 话题的节点发送消息。 ros::spinOnce(); //ros::spinOnce()这一语句，否则你的回调函数就永远也不会被调用 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。 ++count; } return 0; }
2.编写订阅器
- 初始化ROS系统
-
订阅 chatter 话题
- 进入自循环，等待消息的到达
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当消息到达，调用 chatterCallback() 函数
在 beginner_tutorials package 目录下创建 src/listener.cpp 文件：
#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)//是一个回调函数，当接收到 chatter 话题的时候就会被调用。 { 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);
//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。
/** * 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();//ros::spin() 进入自循环，可以尽可能快的调用消息回调函数。
return 0;
}
3.编译
- 1.确认CMakeLists.txt文件
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(DIRECTORY msg FILES Num.msg) add_service_files(DIRECTORY srv FILES AddTwoInts.srv) ## Generate added messages and services generate_messages(DEPENDENCIES std_msgs) ## Declare a catkin package catkin_package(
- 2.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_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)
- 1-2会生成两个可执行文件, talker 和 listener, 默认存储到 devel space 目录下,具体在~/catkin_ws/devel/lib/<package name> 中.
- 3.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})
- 4.运行 catkin_make
4.测试结果：
5.问题：
1.当你在package.xml中，添加完run_depend后，编译出错，显示The manifest must not cotain the following tags:run_depend。
原因：软件包格式分两种，第一种的四种依赖关系分别是：
<buildtool_depend> <build_depend> <run_depend> <test_depend>
第二种的依赖关系变成：
<buildtool_depend> <build_depend> <build_export_depend> <exec_depend> <test_depend> <doc_depend>
解决：当你run_depend出错时，有可能是你的软件包格式是第二种，所以要把run_depend改成exec_depend
2.invoking "make cmake_check_build_system" failed
仔细检查：
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(DIRECTORY msg FILES Num.msg) add_service_files(DIRECTORY srv FILES AddTwoInts.srv) ## Generate added messages and services generate_messages(DEPENDENCIES std_msgs) ## Declare a catkin package catkin_package() #尤其要注意，里面为空否则报上面的错
