Beginner：Client libraries-4-写一个简单的发布者和订阅者(c++)

目标：创建并运行一个使用C++编写的发布者和订阅者节点

背景

 节点是通过ROS图进行通信的可执行进程。在本教程中，节点通过字符串消息的形式与其他节点进行通信。例子使用简单的"talker"和"listener"；一个节点发布数据，其他订阅该话题来接收数据。

任务

1、创建一个包

ros2 pkg create --build-type ament_cmake cpp_pubsub

2、实现一个发布者节点(wget -O publisher_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_publisher/member_function.cpp)

#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

头部为包含的标准的C++代码头文件，rclcpp/rclcpp.hpp包含ROS2系统的大部分内容，std_msgs/msg/string.hpp包含内建的消息类型，用于发布数据

#include <chrono>
#include <functional>
#include <memory>
#include <string>

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"

using namespace std::chrono_literals;

本节点的类MinimalPublisher继承了rclcpp::Node，

class MinimalPublisher : public rclcpp::Node

下面初始化相关信息

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));
  }

RCLCPP_INFO用于输出到终端

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timer的声明以及counter

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

rclcpp::init初始化ros2，使用rclcpp::spin来开始处理来自于节点的数据，包括timer的回调函数。

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

打开package.xml

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

2.3 CMakeLists.txt

添加如下行在find_package(ament_cmake REQUIRED)

find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)

之后添加如下：

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

最后，添加install ，以便ros2 run可以找到可执行文件

install(TARGETS
  talker
  DESTINATION lib/${PROJECT_NAME})

完整的如下

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()

3、编写订阅者

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

#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 分析代码

代码与publisher基本一致。节点被命名为minimal_subscriber,使用节点的create_subscription类来执行回调函数

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

接收到消息，在控制台上打印

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_;

在publisher里面spin意味着启动定时器，在subscriber里面spin意味着随时准备接收消息。

由于发布者和订阅者的依赖一样，所以在package.xml里面不需要添加其他了。

3.2 CMakeLists.txt

编辑如下内容

add_executable(listener src/subscriber_member_function.cpp)
ament_target_dependencies(listener rclcpp std_msgs)

install(TARGETS
  talker
  listener
  DESTINATION lib/${PROJECT_NAME})

4、编译和运行

colcon build --packages-select cpp_pubsub

ros2 run cpp_pubsub talker

ros2 run cpp_pubsub listener

总结

您创建了两个节点来发布和订阅主题上的数据。在编译和运行它们之前，您将它们的依赖项和可执行文件添加到包配置文件中