大纲

• Action概念
• • 组成
  • 特点
  • 使用场景
• 代码
• • 客户端
  • • 构建rclcpp_action::Client
    • 等待和服务端的连接
    • 构建目标任务
    • 构建SendGoalOptions
    • • GoalResponseCallback
      • FeedbackCallback
      • ResultCallback
    • 发送请求
• 完整代码

在ROS 2（Robot Operating System 2）中，Action是一种强大的通信机制，特别适用于需要长时间运行且需要定期反馈的任务。Action结合了话题（Topic）和服务（Service）的特性，提供了一个更加灵活和强大的方式来控制和管理机器人的复杂行为。

Action概念

组成

Action由三个主要部分组成：

1. 目标（Goal）：这是由Action客户端（Client）发送给Action服务器（Server）的请求，指示服务器需要执行的具体任务。例如，在控制一个机器人旋转的任务中，目标可能是指定机器人需要旋转的角度。
2. 反馈（Feedback）：在执行目标的过程中，Action服务器会定期向客户端发送反馈，以报告当前的执行状态和进度。这种反馈机制使得客户端能够实时了解任务的执行情况，并在必要时进行干预或调整。
3. 结果（Result）：当目标执行完成后，Action服务器会向客户端发送一个结果，表明任务的最终执行状态。这个结果可以是成功完成、失败或者部分完成等状态信息。

特点

1. 可抢占性：与服务（Service）不同，Action是可抢占的。这意味着在执行过程中，客户端可以随时取消当前的目标，或者发送一个新的目标来覆盖旧的目标。这种特性使得Action在控制复杂机器人行为时更加灵活和可靠。
2. 稳定的反馈：Action提供了持续的反馈机制，这使得客户端能够实时跟踪任务的执行情况。这种稳定的反馈不仅提高了系统的透明度和可控性，还使得客户端能够更好地适应任务执行过程中的变化。
3. 长时间的运行：由于Action适用于长时间运行的任务，因此它非常适合用于控制需要较长时间才能完成的复杂机器人行为。例如，在导航、机械臂操作等场景中，Action都能够发挥重要作用。

使用场景

Action在机器人系统中有着广泛的应用场景，包括但不限于：

1. 导航：机器人可以使用Action来发送导航指令，并实时接收路径规划的反馈。
2. 物流自动驾驶：无人驾驶汽车或物流车可以使用Action来接收路径指令并执行。
3. 人机交互：机器人可以通过Action来接收用户的指令和反馈，从而实现更加自然和流畅的交互体验。
4. 复杂运动规划：机器人可以使用Action来发送运动指令，并在执行过程中接收运动的反馈，以实现精确和可靠的运动控制。

总之，ROS 2 Action是一种强大而灵活的通信机制，它为机器人系统的控制和管理提供了更加高效和可靠的方式。通过结合话题和服务的特性，Action能够实现对长时间运行任务的精确控制和实时反馈，从而推动机器人技术的进一步发展。

代码

我们通过action_tutorials/action_tutorials_cpp/src/fibonacci_action_server.cpp和action_tutorials/action_tutorials_cpp/src/fibonacci_action_client.cpp两个例子讲解Action服务端和客户端的编码实现。

客户端

客户端代码相对简单，我们可以从它先入手，讲解Action的组成。

构建rclcpp_action::Client

class FibonacciActionClient : public rclcpp::Node { public:   using Fibonacci = action_tutorials_interfaces::action::Fibonacci;   using GoalHandleFibonacci = rclcpp_action::ClientGoalHandle;    ACTION_TUTORIALS_CPP_PUBLIC   explicit FibonacciActionClient(const rclcpp::NodeOptions & node_options = rclcpp::NodeOptions())   : Node("fibonacci_action_client", node_options)   {     this->client_ptr_ = rclcpp_action::create_client(       this->get_node_base_interface(),       this->get_node_graph_interface(),       this->get_node_logging_interface(),       this->get_node_waitables_interface(),       "fibonacci");      this->timer_ = this->create_wall_timer(       std::chrono::milliseconds(500),       [this]() {return this->send_goal();});   } ……  private:   rclcpp_action::Client::SharedPtr client_ptr_;   rclcpp::TimerBase::SharedPtr timer_; };  // class FibonacciActionClient 

首先我们需要一个rclcpp_action::Client对象，它的模板类是Fibonacci。

Fibonacci类是通过IDL文件（action_tutorials/install/action_tutorials_interfaces/share/action_tutorials_interfaces/action/Fibonacci.idl）生成的。通过下面这个结构，可以看到我们在“概念”一节中介绍的Goal、Feedback和Result三部分。

// generated from rosidl_adapter/resource/action.idl.em // with input from action_tutorials_interfaces/action/Fibonacci.action // generated code does not contain a copyright notice   module action_tutorials_interfaces {   module action {     struct Fibonacci_Goal {       int32 order;     };     struct Fibonacci_Result {       sequence sequence;     };     struct Fibonacci_Feedback {       sequence partial_sequence;     };   }; }; 

而这个文件又是由Fibonacci.action文件（action_tutorials/install/action_tutorials_interfaces/share/action_tutorials_interfaces/action/Fibonacci.action）生成

int32 order --- int32[] sequence --- int32[] partial_sequence 

准备好基本框架后，我们就要看主要逻辑——定时器具体执行了什么？

进入定时器后，我们先终止该定时器后续再次执行。这是因为后续设置逻辑只要执行一次即可。

  ACTION_TUTORIALS_CPP_PUBLIC   void send_goal()   {     using namespace std::placeholders;      this->timer_->cancel(); 

等待和服务端的连接

    if (!this->client_ptr_->wait_for_action_server(std::chrono::seconds(10))) {       RCLCPP_ERROR(this->get_logger(), "Action server not available after waiting");       rclcpp::shutdown();       return;     } 

构建目标任务

    auto goal_msg = Fibonacci::Goal();     goal_msg.order = 10; 

Fibonacci::Goal也是由上述idl文件构建。

构建SendGoalOptions

    auto send_goal_options = rclcpp_action::Client::SendGoalOptions(); 

SendGoalOptions是由三个回调函数构成

  struct SendGoalOptions   {     SendGoalOptions()     : goal_response_callback(nullptr),       feedback_callback(nullptr),       result_callback(nullptr)     {     }      /// Function called when the goal is accepted or rejected.     /**      * Takes a single argument that is a goal handle shared pointer.      * If the goal is accepted, then the pointer points to a valid goal handle.      * If the goal is rejected, then pointer has the value `nullptr`.      */     GoalResponseCallback goal_response_callback;      /// Function called whenever feedback is received for the goal.     FeedbackCallback feedback_callback;      /// Function called when the result for the goal is received.     ResultCallback result_callback;   }; 

我们看下这三个回调的实现

GoalResponseCallback

当Action客户端向服务端发送请求后，服务端会返回“接收”或者“拒绝”该请求，这个时候客户端的这个函数就会被调用。

如果服务端接收了该请求，则goal_handle非空；否则就是nullptr。

    send_goal_options.goal_response_callback = [this](       const GoalHandleFibonacci::SharedPtr & goal_handle)       {         if (!goal_handle) {           RCLCPP_ERROR(this->get_logger(), "Goal was rejected by server");         } else {           RCLCPP_INFO(this->get_logger(), "Goal accepted by server, waiting for result");         }       }; 

FeedbackCallback

在任务处理的过程中，服务端不断向客户端发送Feedback，用于告知任务处理的当前状态。客户端就会通过该回调来接收服务端的响应。

FeedbackCallback有两个参数：

1. GoalHandleFibonacci::SharedPtr和GoalResponseCallback回调中的参数一样。
2. Fibonacci::Feedback是之前介绍的IDL文件生成的。它用来承载任务中间状态。

本例中它返回了当前服务端计算出来的斐波那契数列的值。

    send_goal_options.feedback_callback = [this](       GoalHandleFibonacci::SharedPtr,       const std::shared_ptr feedback)       {         std::stringstream ss;         ss << "Next number in sequence received: ";         for (auto number : feedback->partial_sequence) {           ss << number << " ";         }         RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());       }; 

ResultCallback

当服务端的任务处理完毕，会向客户端发送结果。客户端就是通过这个回调来接收服务端的响应。

    send_goal_options.result_callback = [this](       const GoalHandleFibonacci::WrappedResult & result)       {         switch (result.code) {           case rclcpp_action::ResultCode::SUCCEEDED:             break;           case rclcpp_action::ResultCode::ABORTED:             RCLCPP_ERROR(this->get_logger(), "Goal was aborted");             return;           case rclcpp_action::ResultCode::CANCELED:             RCLCPP_ERROR(this->get_logger(), "Goal was canceled");             return;           default:             RCLCPP_ERROR(this->get_logger(), "Unknown result code");             return;         }         std::stringstream ss;         ss << "Result received: ";         for (auto number : result.result->sequence) {           ss << number << " ";         }         RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());         rclcpp::shutdown();       }; 

ResultCallback的参数WrappedResult 结构如下。它包含了Fibonacci::Result的智能指针，还有代表最终处理结果的ResultCode。

template class ClientGoalHandle { public:   RCLCPP_SMART_PTR_DEFINITIONS_NOT_COPYABLE(ClientGoalHandle)    // A wrapper that defines the result of an action   struct WrappedResult   {     /// The unique identifier of the goal     GoalUUID goal_id;     /// A status to indicate if the goal was canceled, aborted, or suceeded     ResultCode code;     /// User defined fields sent back with an action     typename ActionT::Result::SharedPtr result;   }; 

发送请求

    this->client_ptr_->async_send_goal(goal_msg, send_goal_options); 

至此，客户端的整个流程走完了。

后续的逻辑主要执行于上述的几个回调函数中。

完整代码

// Copyright 2019 Open Source Robotics Foundation, Inc. // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // //     http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License.  #include  #include  #include   #include "action_tutorials_interfaces/action/fibonacci.hpp" #include "rclcpp/rclcpp.hpp" #include "rclcpp_action/rclcpp_action.hpp" #include "rclcpp_components/register_node_macro.hpp"  #include "action_tutorials_cpp/visibility_control.h"  namespace action_tutorials_cpp { class FibonacciActionClient : public rclcpp::Node { public:   using Fibonacci = action_tutorials_interfaces::action::Fibonacci;   using GoalHandleFibonacci = rclcpp_action::ClientGoalHandle;    ACTION_TUTORIALS_CPP_PUBLIC   explicit FibonacciActionClient(const rclcpp::NodeOptions & node_options = rclcpp::NodeOptions())   : Node("fibonacci_action_client", node_options)   {     this->client_ptr_ = rclcpp_action::create_client(       this->get_node_base_interface(),       this->get_node_graph_interface(),       this->get_node_logging_interface(),       this->get_node_waitables_interface(),       "fibonacci");      this->timer_ = this->create_wall_timer(       std::chrono::milliseconds(500),       [this]() {return this->send_goal();});   }    ACTION_TUTORIALS_CPP_PUBLIC   void send_goal()   {     using namespace std::placeholders;      this->timer_->cancel();      if (!this->client_ptr_->wait_for_action_server(std::chrono::seconds(10))) {       RCLCPP_ERROR(this->get_logger(), "Action server not available after waiting");       rclcpp::shutdown();       return;     }      auto goal_msg = Fibonacci::Goal();     goal_msg.order = 10;      RCLCPP_INFO(this->get_logger(), "Sending goal");      auto send_goal_options = rclcpp_action::Client::SendGoalOptions();     send_goal_options.goal_response_callback = [this](       const GoalHandleFibonacci::SharedPtr & goal_handle)       {         if (!goal_handle) {           RCLCPP_ERROR(this->get_logger(), "Goal was rejected by server");         } else {           RCLCPP_INFO(this->get_logger(), "Goal accepted by server, waiting for result");         }       };      send_goal_options.feedback_callback = [this](       GoalHandleFibonacci::SharedPtr,       const std::shared_ptr feedback)       {         std::stringstream ss;         ss << "Next number in sequence received: ";         for (auto number : feedback->partial_sequence) {           ss << number << " ";         }         RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());       };      send_goal_options.result_callback = [this](       const GoalHandleFibonacci::WrappedResult & result)       {         switch (result.code) {           case rclcpp_action::ResultCode::SUCCEEDED:             break;           case rclcpp_action::ResultCode::ABORTED:             RCLCPP_ERROR(this->get_logger(), "Goal was aborted");             return;           case rclcpp_action::ResultCode::CANCELED:             RCLCPP_ERROR(this->get_logger(), "Goal was canceled");             return;           default:             RCLCPP_ERROR(this->get_logger(), "Unknown result code");             return;         }         std::stringstream ss;         ss << "Result received: ";         for (auto number : result.result->sequence) {           ss << number << " ";         }         RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());         rclcpp::shutdown();       };      this->client_ptr_->async_send_goal(goal_msg, send_goal_options);   }  private:   rclcpp_action::Client::SharedPtr client_ptr_;   rclcpp::TimerBase::SharedPtr timer_; };  // class FibonacciActionClient  }  // namespace action_tutorials_cpp  RCLCPP_COMPONENTS_REGISTER_NODE(action_tutorials_cpp::FibonacciActionClient)