1 前言和资料

在ROS2高效学习第二章 – ros2常用命令和相关概念学习，熟练玩起来小乌龟样例这篇博客里，我们讲解了 ros2 service 的基本概念和调试命令，本文我们讲解 ros2 service 编程。内容编排上，我们将复用ROS2高效学习第四章 – ros2 topic 编程之自定义 msg的结构和内容，也使用两种方式实现自定义服务：一是自定义服务独立成包（复用 diy_interface 包）；二是自定义服务与终端包放在一起，并实现 cpp 和 python 混合编程。
本文参考资料如下：
（1）ROS2高效学习第二章 – ros2常用命令和相关概念学习，熟练玩起来小乌龟样例 第2.5节
（2）ROS2高效学习第四章 – ros2 topic 编程之自定义 msg
（3）Writing-A-Simple-Cpp-Service-And-Client
（4）Writing-A-Simple-Py-Service-And-Client
（5）Custom-ROS2-Interfaces

2 正文

2.1 两种自定义服务的讨论

在ROS2高效学习第四章 – ros2 topic 编程之自定义 msg的2.1节，我们对自定义 msg 有详细的讨论，连带讲解了 ros2 cpp 和 python混合编程的实现方式。而自定义服务与自定义 msg 具有非常强的雷同性，同样的讨论对自定义服务完全适用，这里不再赘述，请读者移步另一篇博客阅读。

2.2 自定义服务独立成包

2.2.1 自定义服务（diy_interface）

（1）在 diy_interface 中，创建自定义服务文件

cd ~/colcon_ws/src/diy_interface
mkdir srv
touch srv/QuestionAndAnswer.srv

（2）编写QuestionAndAnswer.srv，内容就是一问一答。

string question
---
string answer

（3）添加 CMakeLists.txt

rosidl_generate_interfaces(${PROJECT_NAME}"msg/Student.msg""msg/Sphere.msg""srv/QuestionAndAnswer.srv"DEPENDENCIES geometry_msgs # Add packages that above messages depend on, in this case geometry_msgs for Sphere.msg
)

（4）编译并测试

~/colcon_ws
colcon build --packages-select diy_interface
source install/local_setup.bash
ros2 interface show diy_interface/srv/QuestionAndAnswer

2.2.2 service_cpp使用自定义服务

（1）创建 service_cpp 包和相关文件

cd ~/colcon_ws/src
ros2 pkg create --build-type ament_cmake --license Apache-2.0 service_cpp  --dependencies rclcpp
cd service_cpp
mkdir launch
touch launch/service_diy_launch.py
touch src/client_diy.cpp src/server_diy.cpp

（2）编写client_diy.cpp

#include <memory>
#include <chrono>#include "rclcpp/rclcpp.hpp"
#include "diy_interface/srv/question_and_answer.hpp"class ServiceClient : public rclcpp::Node 
{
public:ServiceClient() : Node("service_client") {client_ = this->create_client<diy_interface::srv::QuestionAndAnswer>("question_and_answer");}int init() {while (!client_->wait_for_service(std::chrono::seconds(1))) {if (!rclcpp::ok()) {RCLCPP_ERROR(this->get_logger(), "Interrupted while waiting for the service. Exiting.");return 0;}RCLCPP_INFO(this->get_logger(), "service not available, waiting again...");}return 0;}void run(const std::string &str) {auto request = std::make_shared<diy_interface::srv::QuestionAndAnswer::Request>();request->question = str;auto result = client_->async_send_request(request);if (rclcpp::spin_until_future_complete(this->get_node_base_interface(), result) == rclcpp::FutureReturnCode::SUCCESS) {RCLCPP_INFO(this->get_logger(), "[cpp client] send: %s, receive: %s", request->question.c_str(), result.get()->answer.c_str()); } else {RCLCPP_ERROR(this->get_logger(), "Failed to call service add_two_ints");} }private:rclcpp::Client<diy_interface::srv::QuestionAndAnswer>::SharedPtr client_;
};int main(int argc, char * argv[]) {rclcpp::init(argc, argv);auto node = std::make_shared<ServiceClient>();node->init();while (rclcpp::ok()) {std::string str = "how are you?";node->run(str);std::this_thread::sleep_for(std::chrono::seconds(1));}rclcpp::shutdown();return 0;
}

（3）编写server_diy.cpp

#include <memory>#include "rclcpp/rclcpp.hpp"
#include "diy_interface/srv/question_and_answer.hpp"class ServiceServer : public rclcpp::Node 
{
public:ServiceServer() : Node("service_server") {server_ = this->create_service<diy_interface::srv::QuestionAndAnswer>("question_and_answer", std::bind(&ServiceServer::answer_callback, this, std::placeholders::_1, std::placeholders::_2));}
private:void answer_callback(const std::shared_ptr<diy_interface::srv::QuestionAndAnswer::Request> request,std::shared_ptr<diy_interface::srv::QuestionAndAnswer::Response> response) {response->answer = "I'm fine, thank you.";RCLCPP_INFO(this->get_logger(), "[cpp server] receive: %s, return: %s", request->question.c_str(), response->answer.c_str());}private:rclcpp::Service<diy_interface::srv::QuestionAndAnswer>::SharedPtr server_;
};int main(int argc, char * argv[]) {rclcpp::init(argc, argv);rclcpp::spin(std::make_shared<ServiceServer>());rclcpp::shutdown();return 0;
}

（4）编写CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(service_cpp)if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")add_compile_options(-Wall -Wextra -Wpedantic)
endif()# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(diy_interface REQUIRED)add_executable(server_diy src/server_diy.cpp)
ament_target_dependencies(server_diy rclcpp diy_interface)add_executable(client_diy src/client_diy.cpp)
ament_target_dependencies(client_diy rclcpp diy_interface)install(TARGETSserver_diyclient_diyDESTINATION lib/${PROJECT_NAME})install(DIRECTORYlaunchDESTINATION share/${PROJECT_NAME}
)
ament_package()

（5）添加 package.xml

  <depend>rclcpp</depend><depend>diy_interface</depend>  

（6）编写 service_diy_launch.py

from launch import LaunchDescription
from launch_ros.actions import Nodedef generate_launch_description():return LaunchDescription([Node(package='service_cpp',namespace='cpp',executable='server_diy',name='server_diy'),Node(package='service_cpp',namespace='cpp',executable='client_diy',name='client_diy')])

（7）编译并运行

~/colcon_ws
colcon build --packages-select diy_interface service_cpp
source install/local_setup.bash
ros2 launch service_cpp service_diy_launch.py

2.2.3 service_py使用自定义服务

（1）service_py 是 service_cpp 的 python 版实现，功能完全一样，这里不再详述创建步骤，具体请参考源码：service_py，源码说明一切！
（2）编译并运行

~/colcon_ws
colcon build --packages-select diy_interface service_py
source install/local_setup.bash
ros2 launch service_py service_diy_launch.py

2.3 自定义服务放在模块包里（service_mix ）

（1）service_mix 使用 cpp 和 python 混合编程， server 端使用 cpp 实现，client 端使用 python 实现。在 service_mix 包里，添加了自定义服务：给圆半径，求圆面积。关于创建步骤，service_mix 与 ROS2高效学习第四章 – ros2 topic 编程之自定义 msg 第2.3节的 pubsub_mix 基本相同，这里不再赘述，具体请参考源码：service_mix，源码说明一切！
（2）编译并运行

~/colcon_ws
colcon build --packages-select service_mix
source install/local_setup.bash
ros2 launch service_mix service_mix_launch.py

3 总结

本文的代码托管在本人的github上：diy_interface，service_cpp，service_py，service_mix