編程方法

了解了話題的基本原理，接下來我們就要開始編寫代碼啦。

創建工作空間

請大家先按照這個流程創建工作空間、下載課程的例程代碼，并進行編譯。

$ mkdir –p dev_ws/src
$ cd /userdata/dev_ws/src
$ git clone https://gitee.com/guyuehome/togetherros_tutorials.git
$ cd /userdata/dev_ws/
$ colcon build

運行示例程序

編譯成功后，我們嘗試運行話題通信的Hello World例程，在這個例程中，我們會先創建一個發布者，發布話題“chatter”，周期發送“Hello World”這個字符串，消息類型是ROS中標準定義的String，再創建一個訂閱者，訂閱“chatter”這個話題，從而接收到“Hello World”這個字符串。

$ source /opt/tros/local_setup.bash
$ source install/local_setup.bash
$ ros2 run learning_topic_cpp talker
$ ros2 run learning_topic_cpp listener

這就是TogetherROS系統中話題通信的方法，依然沿用了ROS2中話題通信的完整流程。

代碼解析

發布者的實現方法

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;
}

訂閱者的實現方法

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::SharedPtr 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;
}