ROS 官网教程02-发布者订阅者-服务和客户端
ROS 官网教程02-发布者订阅者-服务和客户端
一、编写简单的发布者和订阅者（C++）
1.1 编写发布者节点
$ roscd beginner_tutorials
$ mkdir src
$ cd src
在 src 下，创建 talker.cpp，粘贴如下代码
#include "ros/ros.h" // ros/ros.h是一个很便利的include，它包括了使用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"//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.
*/
// 指定节点名称为“talker”，不能包含“/”
ros::init(argc, argv, "talker");
/**
* 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.
*/
// 为进程创建句柄，第一句柄将执行节点初始化
ros::NodeHandle n;
/**
* 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.
*/
// 告诉主节点会在chatter话题上发布一个类型为std_msgs/String的消息，主节点会通知正在监听 chatter 的节点。第二个参数是缓存队列大小
// ros::Publisher 包含 publish 方法，在话题上发布消息
ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);
// 指定循环的频率，记录从上次调用Rate::sleep()到现在已经有多长时间，并休眠正确的时间
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;
/*安装一个SIGINT处理程序
ros::ok()在以下情况会返回false：
* 收到SIGINT信号（Ctrl+C）
* 被另一个同名的节点踢出了网络
* ros::shutdown()被程序的另一部分调用
* 所有的ros::NodeHandles都已被销毁
*/
while (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和它的朋友们可用来取代printf/cout
ROS_INFO("%s", msg.data.c_str());
/**
* 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.
*/
// 信息广播给了任何已连接的节点
chatter_pub.publish(msg);
// 处理回调函数
ros::spinOnce();
//使用ros::Rate在剩下的时间内睡眠，以让我们达到10Hz的发布速率
loop_rate.sleep();
++count;
}
return 0;
}
对上边的内容进行一下总结：
- 初始化ROS系统
- 向主节点宣告我们将要在
chatter话题上发布std_msgs/String类型的消息 - 以每秒10次的速率向
chatter循环发布消息
1.2 编写订阅者节点
在 src 下创建 chatter.cpp，并粘贴如下代码
#include "ros/ros.h"
#include "std_msgs/String.h"
/**
* This tutorial demonstrates simple receipt of messages over the ROS system.
*/
//一个回调函数，当有新消息到达chatter话题时它就会被调用
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
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.
*/
// 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。
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
/**
* 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()启动了一个自循环，它会尽可能快地调用消息回调函数。
ros::spin();
return 0;
}
同样地，我们来总结一下：
- 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
1.3 构建节点
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(FILES Num.msg)
add_service_files(FILES AddTwoInts.srv)
## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)
## Declare a catkin package
catkin_package()
## Build talker and listener
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)
1.4 测试发布者订阅者
$ roscore
# 在catkin工作空间中
$ cd ~/catkin_ws
$ source ./devel/setup.bash
# 运行 talker 发布者
$ rosrun beginner_tutorials talker # (C++)
$ rosrun beginner_tutorials talker.py # (Python)
# 运行 listener 订阅者
$ rosrun beginner_tutorials listener # (C++)
$ rosrun beginner_tutorials listener.py # (Python)
二、编写简单的服务和客户端（C++）
2.1 编写服务节点
在beginner_tutorials包中创建src/add_two_ints_server.cpp文件并粘贴以下内容进去：
#include "ros/ros.h"
// beginner_tutorials/AddTwoInts.h是从我们之前创建的srv文件中生成的头文件。
#include "beginner_tutorials/AddTwoInts.h"
// 提供了AddTwoInts服务，它接受srv文件中定义的请求（request）和响应（response）类型，并返回一个布尔值。
bool add(beginner_tutorials::AddTwoInts::Request &req,
beginner_tutorials::AddTwoInts::Response &res)
{
res.sum = req.a + req.b;
ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
ROS_INFO("sending back response: [%ld]", (long int)res.sum);
return true;
}
int main(int argc, char **argv)
{
ros::init(argc, argv, "add_two_ints_server");
ros::NodeHandle n;
// 创建服务
ros::ServiceServer service = n.advertiseService("add_two_ints", add);
ROS_INFO("Ready to add two ints.");
ros::spin();
return 0;
}
2.2 编写客户端节点
在beginner_tutorials包中创建src/add_two_ints_client.cpp文件并粘贴以下内容进去
#include "ros/ros.h"
#include "beginner_tutorials/AddTwoInts.h"
#include <cstdlib>
int main(int argc, char **argv)
{
ros::init(argc, argv, "add_two_ints_client");
if (argc != 3)
{
ROS_INFO("usage: add_two_ints_client X Y");
return 1;
}
ros::NodeHandle n;
// 为add_two_ints服务创建一个客户端。
ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");
// 实例化一个自动生成的服务类，并为它的request成员赋值。一个服务类包括2个成员变量：request和response，以及2个类定义：Request和Response。
beginner_tutorials::AddTwoInts srv;
srv.request.a = atoll(argv[1]);
srv.request.b = atoll(argv[2]);
// 实际上调用了服务。由于服务调用被阻塞，它将在调用完成后返回。
if (client.call(srv))
{
ROS_INFO("Sum: %ld", (long int)srv.response.sum);
}
else
{
ROS_ERROR("Failed to call service add_two_ints");
return 1;
}
return 0;
}
2.3 构建节点
再来编辑一下beginner_tutorials里面的CMakeLists.txt文件，文件位于~/catkin_ws/src/beginner_tutorials/CMakeLists.txt，并将下面的代码添加在文件末尾：
add_executable(add_two_ints_server src/add_two_ints_server.cpp)
target_link_libraries(add_two_ints_server ${catkin_LIBRARIES})
add_dependencies(add_two_ints_server beginner_tutorials_gencpp)
add_executable(add_two_ints_client src/add_two_ints_client.cpp)
target_link_libraries(add_two_ints_client ${catkin_LIBRARIES})
add_dependencies(add_two_ints_client beginner_tutorials_gencpp)
现在可以运行catkin_make：
# 在你的catkin工作空间下
cd ~/catkin_ws
catkin_make
2.4 测试服务和客户端
# 运行服务
$ rosrun beginner_tutorials add_two_ints_server # (C++)
$ rosrun beginner_tutorials add_two_ints_server.py # (Python)
#运行客户端
$ rosrun beginner_tutorials add_two_ints_client 1 3 # (C++)
$ rosrun beginner_tutorials add_two_ints_client.py 1 3 # (Python)
❯❯ catkin_ws 19:17 rosrun beginner_tutorials add_two_ints_server
[ INFO] [1659093578.679162016]: Ready to add two ints.
[ INFO] [1659093599.569586189]: request: x=3, y=1
[ INFO] [1659093599.569654088]: sending back response: [4]
[ INFO] [1659093609.078840971]: request: x=1, y=33
[ INFO] [1659093609.078888220]: sending back response: [34]
❯❯ catkin_ws 19:19 rosrun beginner_tutorials add_two_ints_client 1 33
[ INFO] [1659093609.079048726]: Sum: 34
