用C++编写一个简单的发布者和订阅者

Posted 2023-04-27 华为云开发者社区

摘要：节点（Node）是通过 ROS 图进行通信的可执行进程。

本文分享自华为云社区《编写一个简单的发布者和订阅者》，作者： MAVER1CK 。

@[toc]

参考官方文档：Writing a simple publisher and subscriber (C++)

背景

节点（Node）是通过 ROS 图进行通信的可执行进程。 在本教程中，节点将通过话题（Topic）以字符串消息的形式相互传递信息。 这里使用的例子是一个简单的“talker”和“listener”系统； 一个节点发布数据，另一个节点订阅该话题，以便它可以接收该数据。 可以在此处找到这些示例中使用的代码。

1.创建一个包

打开一个新的终端然后source你的ROS 2安装，以便ros2命令可以正常使用：

source /opt/ros/humble/setup.bash

回顾一下，包应该在src目录下创建，而不是在工作区的根目录下。因此，接下来，cd到ros2_ws/src，并运行包创建命令。

ros2 pkg create --build-type ament_cmake cpp_pubsub

你的终端将返回一条信息，验证你的cpp_pubsub包及其所有必要的文件和文件夹的创建。

cd到ros2_ws/src/cpp_pubsub/src。回顾一下，这是任何CMake包中包含可执行文件的源文件所在的目录。

2.编写发布者节点

下载示例代码：

wget -O publisher_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_publisher/member_function.cpp

打开之后内容如下

#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using namespace std::chrono_literals;
/* This example creates a subclass of Node and uses std::bind() to register a
* member function as a callback from the timer. */
class MinimalPublisher : public rclcpp::Node

 public:
 MinimalPublisher()
 : Node("minimal_publisher"), count_(0)
 
      publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
      timer_ = this->create_wall_timer(
 500ms, std::bind(&MinimalPublisher::timer_callback, this));
 
 private:
 void timer_callback()
 
      auto message = std_msgs::msg::String();
 message.data = "Hello, world! " + std::to_string(count_++);
 RCLCPP_INFO(this->get_logger(), "Publishing: \'%s\'", message.data.c_str());
      publisher_->publish(message);
 
 rclcpp::TimerBase::SharedPtr timer_;
 rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
 size_t count_;
;
int main(int argc, char * argv[])

 rclcpp::init(argc, argv);
 rclcpp::spin(std::make_shared<MinimalPublisher>());
 rclcpp::shutdown();
 return 0;

2.1 查看代码

代码的顶部包括你将要使用的标准C++头文件。在标准C++头文件之后是rclcpp/rclcpp.hpp，它允许你使用ROS 2系统中最常见的部分。最后是std_msgs/msg/string.hpp，它包括你将用于发布数据的内置消息类型。

这些行代表节点的依赖关系。 回想一下，必须将依赖项添加到 package.xml 和 CMakeLists.txt，您将在下一节中执行此操作。

#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using namespace std::chrono_literals;

下一行通过继承rclcpp::Node创建节点类MinimalPublisher。代码中的每个this都是指的节点。

class MinimalPublisher : public rclcpp::Node

公共构造函数将节点命名为 minimal_publisher 并将 count_ 初始化为 0。在构造函数内部，发布者使用 String 消息类型、话题名称为 topic 以及所需的队列大小，以便在发生备份时限制消息，进行初始化。 接下来，timer_ 被初始化，这导致 timer_callback 函数每秒执行两次。

public:
 MinimalPublisher()
 : Node("minimal_publisher"), count_(0)
 
    publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
    timer_ = this->create_wall_timer(
 500ms, std::bind(&MinimalPublisher::timer_callback, this));
 

timer_callback函数是设置消息数据和实际发布消息的地方。RCLCPP_INFO宏确保每个发布的消息都被打印到控制台。

private:
 void timer_callback()
 
    auto message = std_msgs::msg::String();
 message.data = "Hello, world! " + std::to_string(count_++);
 RCLCPP_INFO(this->get_logger(), "Publishing: \'%s\'", message.data.c_str());
    publisher_->publish(message);
 

最后是定时器、发布者和计数器字段的声明。

rclcpp::TimerBase::SharedPtr timer_;
rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
size_t count_;

在 MinimalPublisher 类之后是 main，节点实际执行的地方。 rclcpp::init 初始化 ROS 2，rclcpp::spin 开始处理来自节点的数据，包括来自定时器的回调。

int main(int argc, char * argv[])

 rclcpp::init(argc, argv);
 rclcpp::spin(std::make_shared<MinimalPublisher>());
 rclcpp::shutdown();
 return 0;

2.2 添加依赖

回到ros2_ws/src/cpp_pubsub目录，那里已经为你创建了CMakeLists.txt和package.xml文件。

打开 package.xml文件。正如上一个教程中提到的，确保填写<description>、<maintainer>和<license>标签。

<description>Examples of minimal publisher/subscriber using rclcpp</description>
<maintainer email="[e-mail]">Your Name</maintainer>
<license>Apache License 2.0</license>

在ament_cmake构建工具的依赖关系后增加一行，并粘贴以下与你的节点的include语句相对应的依赖关系。

<depend>rclcpp</depend>
<depend>std_msgs</depend>

这声明包在执行其代码时需要 rclcpp 和 std_msgs。

修改好后记得保存文件。

2.3 CMakeLists

现在打开CMakeLists.txt文件。在现有的依赖关系find_package(ament_cmake REQUIRED)下面，添加几行：

find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)

之后，添加可执行文件并将其命名为talker，这样你就可以用ros2 run来运行你的节点：

add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp std_msgs)

最后，添加install(TARGETS...)部分，以便ros2 run能够找到你的可执行文件：

install(TARGETS
  talker
 DESTINATION lib/$PROJECT_NAME)

你可以通过删除一些不必要的部分和注释来清理你的CMakeLists.txt，所以它看起来像这样：

cmake_minimum_required(VERSION 3.5)
project(cpp_pubsub)
Default to C++14
if(NOT CMAKE_CXX_STANDARD)
 set(CMAKE_CXX_STANDARD 14)
endif()
if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 add_compile_options(-Wall -Wextra -Wpedantic)
endif()
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp std_msgs)
install(TARGETS
  talker
 DESTINATION lib/$PROJECT_NAME)
ament_package()

你现在可以 build 你的包，source local_setup.bash，然后运行它，但让我们先创建订阅者节点，这样你就可以看到一个完整工作的系统。

3.编写订阅者节点

返回到ros2_ws/src/cpp_pubsub/src来创建下一个节点。在你的终端输入以下代码：

wget -O subscriber_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_subscriber/member_function.cpp

在终端中输入ls，现在将返回：

publisher_member_function.cpp  subscriber_member_function.cpp

打开subscriber_member_function.cpp文件：

#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using std::placeholders::_1;
class MinimalSubscriber : public rclcpp::Node

 public:
 MinimalSubscriber()
 : Node("minimal_subscriber")
 
      subscription_ = this->create_subscription<std_msgs::msg::String>(
 "topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
 
 private:
 void topic_callback(const std_msgs::msg::String & msg) const
 
 RCLCPP_INFO(this->get_logger(), "I heard: \'%s\'", msg.data.c_str());
 
 rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
;
int main(int argc, char * argv[])

 rclcpp::init(argc, argv);
 rclcpp::spin(std::make_shared<MinimalSubscriber>());
 rclcpp::shutdown();
 return 0;

3.1 查看代码

订阅者节点的代码几乎与发布者的相同。现在节点被命名为minimal_subscriber，构造函数使用节点的create_subscription类来执行回调。

没有计时器，因为无论任何时候只要数据发送到topic话题，订阅者都会作出响应：

public:
 MinimalSubscriber()
 : Node("minimal_subscriber")
 
    subscription_ = this->create_subscription<std_msgs::msg::String>(
 "topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
 

在话题教程中已经知道，发布者和订阅者使用的话题名称和消息类型必须匹配，这样他们才能进行通信。

topic_callback函数接收通过话题发布的字符串消息数据，然后使用RCLCPP_INFO宏将内容输出到终端。

这个类中唯一的字段声明是订阅：

private:
 void topic_callback(const std_msgs::msg::String & msg) const
 
 RCLCPP_INFO(this->get_logger(), "I heard: \'%s\'", msg.data.c_str());
 
 rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;

main函数是完全一样的，只是现在它spin了MinimalSubscriber节点。对于发布者节点来说，spin意味着启动定时器，但对于订阅者来说，它只是意味着准备在消息到来时接收它们。

由于这个节点与发布者节点有相同的依赖关系，所以没有什么新的东西需要添加到package.xml中。

4.构建并运行

你可能已经安装了rclcpp和std_msgs软件包作为你的ROS 2系统的一部分。在你的工作空间（ros2_ws）的根目录下运行rosdep是一个很好的做法，可以在构建之前检查是否有遗漏的依赖：

rosdep install -i --from-path src --rosdistro humble -y

然后构建软件包：

colcon build --packages-select cpp_pubsub

打开一个新的终端，cd到ros2_ws，然后source设置文件：

. install/setup.bash

现在运行 talker 节点：

ros2 run cpp_pubsub talker

终端应该开始每0.5秒发布一次信息，像这样：

[INFO] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [minimal_publisher]: Publishing: "Hello World: 2"
[INFO] [minimal_publisher]: Publishing: "Hello World: 3"
[INFO] [minimal_publisher]: Publishing: "Hello World: 4"

再打开一个新的终端，cd到ros2_ws，然后source设置文件：

. install/setup.bash

现在运行 listener 节点：

ros2 run cpp_pubsub listener

listener 将开始在终端打印消息，从发布者当时的消息计数开始，如下所示：

[INFO] [minimal_subscriber]: I heard: "Hello World: 10"
[INFO] [minimal_subscriber]: I heard: "Hello World: 11"
[INFO] [minimal_subscriber]: I heard: "Hello World: 12"
[INFO] [minimal_subscriber]: I heard: "Hello World: 13"
[INFO] [minimal_subscriber]: I heard: "Hello World: 14"

在每个终端中按Ctrl+C来停止运行节点。

 

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编写简单的发布者和订阅者(C++)---ROS学习第9篇

文章目录

• 1.编写发布者节点
• 2. 编写订阅者节点
• 3.构建节点
• 4.运行节点

1.编写发布者节点

在之前创建的study/src目录下创建talker.cpp文件并粘贴以下内容进去

#include "ros/ros.h"
#include "std_msgs/String.h"

#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");

  /**
   * 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 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);

  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())
  
    /**
     * 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());

    /**
     * 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);

    ros::spinOnce();

    loop_rate.sleep();
    ++count;
  


  return 0;

这部分代码的解读，可参考：添加链接描述

2. 编写订阅者节点

在src目录下创建listener.cpp文件并粘贴以下内容进去：

#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)

  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);

  /**
   * 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();

  return 0;

3.构建节点

前面的介绍使用了catkin_create_pkg，它创建了一个和一个package.xml和CMakeLists.txt文件。只需将这几行添加到CMakeLists.txt文件的底部：

add_executable(talker src/talker.cpp)
target_link_libraries(talker $catkin_LIBRARIES)
add_dependencies(talker study_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener $catkin_LIBRARIES)
add_dependencies(listener study_generate_messages_cpp)

这将创建两个可执行文件talker和listener，默认情况下，它们将被放到软件包目录下的devel空间中，即~/catkin_ws/devel/lib/。

现在可以运行catkin_make：

# 在你的catkin工作空间下
$ cd ~/catkin_ws
$ catkin_make

4.运行节点

打开一个终端，运行roscore；
再打开一个新的终端，运行接收节点：rosrun study listener
再打开一个新的终端，运行发布节点：rosrun study talker
再打开一个新的终端，运行rqt_graph，可以看到：