ROS2从入门到精通1-2：详解ROS2服务通信机制与自定义服务

0 专栏介绍

本专栏旨在通过对ROS2的系统学习，掌握ROS2底层基本分布式原理，并具有机器人建模和应用ROS2进行实际项目的开发和调试的工程能力。

🚀详情：《ROS2从入门到精通》

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1 服务通信模型

服务是 ROS 图中节点之间的另一种通信方法。服务基于服务器-客户端模型，不同于话题的发布者-订阅者模型。话题允许节点订阅数据流并获取持续更新，而服务只在客户端特别调用时才提供数据。二者更详细的对比请参考第5节

2 服务模型实现(C++)

• 服务器

  本实验中无需编程,为turtlesim::Spawn定义的/spwan服务

• 客户端

  void OnResultCallBack(rclcpp::Client<turtlesim::srv::Spawn>::SharedFuture result) {
      auto response = result.get();
      RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Request service successfully! [turtle id: %s]", response->name.c_str());
  }
  
  void request() {
      auto spawn = std::make_shared<turtlesim::srv::Spawn::Request>();          
      spawn->name = "winter_turtle";
      spawn->x = 1.0;
      spawn->y = 1.0;
      spawn->theta = 1.57;
  
      while (!client_->wait_for_service(std::chrono::seconds(1))) {                                                   
          if (!rclcpp::ok()) {
              RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Interrupted while waiting for the service. Exiting.");
              return;
          }
          RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "service not available, waiting again...");
      }
  
      auto result = client_->async_send_request(spawn, std::bind(&ClientNode::OnResultCallBack, this, std::placeholders::_1));
  }
  

服务通信的效果如下所示：

3 服务模型实现(Python)

• 服务器

  本实验中无需编程,为turtlesim::Spawn定义的/spwan服务

• 客户端

  class ClientNode(Node):
      def __init__(self, name):
          super().__init__(name)
          self.client = self.create_client(Spawn, '/spawn') 
  
          while not self.client.wait_for_service(timeout_sec=1.0):
              self.get_logger().info('service not available, waiting again...') 
          self.request = Spawn.Request()
                      
      def sendRequest(self):
          self.request.name = "winter_turtle"
          self.request.x = 1.0
          self.request.y = 1.0
          self.request.theta = 1.57
          self.future = self.client.call_async(self.request)
  

服务通信的效果如下所示：

4 自定义服务

自定义服务的通用流程如下：

下面给出一个实例

添加如下自定义服务实现一个加法服务，并按上面步骤配置依赖

# client
int32 a
int32 b
---
# server
int32 sum

定义一个服务器、一个客户端，限于篇幅只贴出部分代码，完整代码见文末。

• 服务器

  class ServerNode : public rclcpp::Node
  {
      public:
          ServerNode() : Node("lab_srv_server_own") {
              server_ = create_service<own_srv_lab::srv::Add>(
                  "/add_service",
                  std::bind(&ServerNode::OnAddSrvCallBack, this, std::placeholders::_1, std::placeholders::_2)
              ); 
          }
  
      private:
          rclcpp::Service<own_srv_lab::srv::Add>::SharedPtr server_;
  
          void OnAddSrvCallBack(
              const std::shared_ptr<own_srv_lab::srv::Add::Request> request, 
              std::shared_ptr<own_srv_lab::srv::Add::Response> response
          ) {
              response->sum = request->a + request->b;
              RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Incoming request\na: %d" " b: %d", request->a, request->b);
              RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "sending back response: [%d]", response->sum);
          }
  };
  

• 客户端

  ClientNode() : Node("lab_srv_client_own") {
      client_ = create_client<own_srv_lab::srv::Add>("/add_service"); 
  }
  
  void request(int a, int b) {
      auto add_srv = std::make_shared<own_srv_lab::srv::Add::Request>();
      add_srv->a = a;          
      add_srv->b = b;
  
      while (!client_->wait_for_service(std::chrono::seconds(1))) {                                                   
          if (!rclcpp::ok()) {
              RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Interrupted while waiting for the service. Exiting.");
              return;
          }
          RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "service not available, waiting again...");
      }
  
      auto result = client_->async_send_request(add_srv, std::bind(&ClientNode::OnResultCallBack, this, std::placeholders::_1));
  }
  

服务通信效果如下所示：

5 话题、服务通信的异同

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