ROS学习之八：编写、测试发布者和订阅者
编写发布者（Publisher）
节点是连接到ROS网络的可执行文件，编写发布者节点（talker节点），首先需要进入软件包的src目录下：
roscd package_1
cd src
并在其中创建如下的源代码文件：
#include "ros/ros.h" // 包括了使用ROS系统中最常见的公共部分所需的全部头文件 #include "std_msgs/String.h" // 引用了位于std_msgs包里的std_msgs/String消息 #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，指定节点名称（使得ROS可以通过命令行进行名称重映射） /** * 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句柄（创建的第一个NodeHandle将执行节点的初始化，而最后一个被销毁的NodeHandle将清除节点所使用的任何资源） /** * 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); // 告诉主节点我们将要在chatter话题上发布一个类型为std_msgs/String的消息。这会让主节点告诉任何正在监听chatter的节点，我们将在这一话题上发布数据。第二个参数是发布队列的大小。在本例中，如果我们发布得太快，它将最多缓存1000条消息，不然就会丢弃旧消息 ros::Rate loop_rate(10); // 指定循环频率，单位Hz（录从上次调用Rate::sleep()到现在已经有多长时间，并休眠正确的时间） /** * 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()) // 当收到Ctrl+C信号、被另一个同名节点踢出网络、ros::shutdown()被程序的另一部分调用或所有的ros::NodeHandles都已被销毁时，ros::ok()返回false { /** * 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()); // 类似于printf或cout /** * 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); // 将信息广播给任何已连接的节点 ros::spinOnce(); // 如果在程序中添加订阅，但此处没有ros::spinOnce()的话，回调函数将永远不会被调用 loop_rate.sleep(); // 使用ros::Rate在剩余的时间内睡眠，使发布速率为10Hz ++count; } return 0; }
总之，在发布者的源代码文件中需要：
- 初始化ROS系统
- 通知主节点将要在chatter话题上发布std_msgs/String类型的消息
- 以每秒10次的速率向chatter循环发布消息
编写订阅者（Subscriber）
同样，在软件包的src目录下编写订阅者节点（listener节点）：
#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) // 回调函数，当有新消息到达chatter话题时被调用 { 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); // 通过主节点订阅chatter话题，每当有新消息到达时，ROS将调用chatterCallback()函数，第二个参数是队列大小，如果队列达到1000条，再有新消息到达时旧消息会被丢弃 /** * 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(); // 启动自循环，尽可能快的调用消息回调函数，一旦ros::ok()返回false，ros::spin()就会退出 return 0; }
总之，在订阅者的源代码文件中需要：
- 初始化ROS系统
- 订阅chatter话题
- 开始spin自循环，等待消息的到达
- 当消息到达后，调用chatterCallback()函数
构建节点
将如下内容添加在软件包中CMakeLists.txt文件的末尾：
add_executable(talker src/talker.cpp) target_link_libraries(talker ${catkin_LIBRARIES}) add_dependencies(talker package_1_generate_messages_cpp) add_executable(listener src/listener.cpp) target_link_libraries(listener ${catkin_LIBRARIES}) add_dependencies(listener package_1_generate_messages_cpp)
- add_executable将在工作空间的/devel/lib目录下创建两个可执行文件
- add_dependencies为可执行目标添加依赖项到消息生成目标，这确保了在使用此包之前生成了该包的消息头。如果使用了工作空间内其他包中的消息，则还需要将依赖项添加到各自的生成目标中，因为所有项目被并行构建
- target_link_libraries用来依赖所有必须的目标
完成上述步骤后，需要在工作空间路径下执行catkin_make命令
测试发布者和订阅者
- 运行roscore
roscore
- 执行source命令
cd ~/catkin_ws
source devel/setup.bash
- 运行发布者talker
rosrun beginner_tutorials talker
- 运行订阅者listener
rosrun beginner_tutorials listener
按Ctrl+C可以停止listener和talker
