[ros2学习]-Client Libraries - 写一个简单的服务端和客户端(C++)

① 背景

当节点使用服务进行通信时，发送数据请求的节点称为客户机节点，响应请求的节点称为服务节点。请求和响应的结构由.srv文件决定。

这里使用的示例是一个简单的整数加法系统；一个节点请求两个整数的和，另一个节点用结果响应。

其实简单得功能函数就可以解决了，这个解决了数据交互的问题，比如服务端返回客户端需要的，但是不用在客户端管理的数据或者状态，还是模块化

② 前提

学会创建工作空间和包

③ 任务

Ⅰ 创建一个包

打开终端，准备好环境，切换到dev_ws/src 路径下来创建包

ros2 pkg create --build-type ament_cmake cpp_srvcli --dependencies rclcpp example_interfaces

然后出一个文件夹

utry@utry:~/dev_ws/src$ tree cpp_srvcli/
cpp_srvcli/
├── CMakeLists.txt
├── include
│   └── cpp_srvcli
├── package.xml
└── src

加了--dependencies rclcpp example_interfaces之后，创建包的时候，会自动把package.xml 和CMakeLists.txt依赖加上，省事，example_interfaces包含例子srv文件

utry@utry:~/dev_ws/src/cpp_srvcli$ ros2 interface show example_interfaces/
example_interfaces/action/Fibonacci  example_interfaces/srv/AddTwoInts
utry@utry:~/dev_ws/src/cpp_srvcli$ ros2 interface show example_interfaces/srv/AddTwoInts 
int64 a
int64 b
---
int64 sum

修改 package.xml

这三个部分还是要改一下的，依赖已经在创建的时候解决了，所以，不用关心依赖了

<description>C++ client server tutorial</description>
<maintainer email="[email protected]">Your Name</maintainer>
<license>Apache License 2.0</license>

Ⅱ 写服务节点

到 dev_ws/src/cpp_srvcli/src 下新建文件add_two_ints_server.cpp

demo 代码：

#include "rclcpp/rclcpp.hpp"
#include "example_interfaces/srv/add_two_ints.hpp"

#include <memory>

void add(const std::shared_ptr<example_interfaces::srv::AddTwoInts::Request> request,
          std::shared_ptr<example_interfaces::srv::AddTwoInts::Response>      response)
{
  response->sum = request->a + request->b;
  RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Incoming request\na: %ld" " b: %ld",
                request->a, request->b);
  RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "sending back response: [%ld]", (long int)response->sum);
}

int main(int argc, char **argv)
{
  rclcpp::init(argc, argv);

  std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_two_ints_server");

  rclcpp::Service<example_interfaces::srv::AddTwoInts>::SharedPtr service =
    node->create_service<example_interfaces::srv::AddTwoInts>("add_two_ints", &add);

  RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Ready to add two ints.");

  rclcpp::spin(node);
  rclcpp::shutdown();
}

代码解析

这个就是add函数，把request的两个数加起来，通过response返回客户端，顺便打了两行日志

void add(const std::shared_ptr<example_interfaces::srv::AddTwoInts::Request> request,
         std::shared_ptr<example_interfaces::srv::AddTwoInts::Response>      response)
{
    response->sum = request->a + request->b;
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Incoming request\na: %ld" " b: %ld",
        request->a, request->b);
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "sending back response: [%ld]", (long int)response->sum);
}

初始化

rclcpp::init(argc, argv);

创建node

std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_two_ints_server");

创建一个服务，名字叫add_two_ints，并通过add函数发不出去

rclcpp::Service<example_interfaces::srv::AddTwoInts>::SharedPtr service =
node->create_service<example_interfaces::srv::AddTwoInts>("add_two_ints", &add);

例行日志

RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Ready to add two ints.");

死循环，不能让进程结束了

rclcpp::spin(node);

添加可执行文件

就是修改CMakeLists.txt
编译的选项

add_executable(server src/add_two_ints_server.cpp)
ament_target_dependencies(server
rclcpp example_interfaces)

安装的选项，在ament_package() 这行前面

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

Ⅲ 写客户端节点

到 dev_ws/src/cpp_srvcli/src 下新建文件add_two_ints_client.cpp
代码这样子：

#include "rclcpp/rclcpp.hpp"
#include "example_interfaces/srv/add_two_ints.hpp"

#include <chrono>
#include <cstdlib>
#include <memory>

using namespace std::chrono_literals;

int main(int argc, char **argv)
{
  rclcpp::init(argc, argv);

  if (argc != 3) {
      RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "usage: add_two_ints_client X Y");
      return 1;
  }

  std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_two_ints_client");
  rclcpp::Client<example_interfaces::srv::AddTwoInts>::SharedPtr client =
    node->create_client<example_interfaces::srv::AddTwoInts>("add_two_ints");

  auto request = std::make_shared<example_interfaces::srv::AddTwoInts::Request>();
  request->a = atoll(argv[1]);
  request->b = atoll(argv[2]);

  while (!client->wait_for_service(1s)) {
    if (!rclcpp::ok()) {
      RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Interrupted while waiting for the service. Exiting.");
      return 0;
    }
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "service not available, waiting again...");
  }

  auto result = client->async_send_request(request);
  // Wait for the result.
  if (rclcpp::spin_until_future_complete(node, result) ==
    rclcpp::executor::FutureReturnCode::SUCCESS)
  {
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Sum: %ld", result.get()->sum);
  } else {
    RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Failed to call service add_two_ints");
  }

  rclcpp::shutdown();
  return 0;
}

代码解析

创建节点和client

std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_two_ints_client");
rclcpp::Client<example_interfaces::srv::AddTwoInts>::SharedPtr client =
  node->create_client<example_interfaces::srv::AddTwoInts>("add_two_ints");

创建request，这个结构体是那个依赖里面的，

auto request = std::make_shared<example_interfaces::srv::AddTwoInts::Request>();
request->a = atoll(argv[1]);
request->b = atoll(argv[2]);

等待 服务起来

while (!client->wait_for_service(1s)) {
    if (!rclcpp::ok()) {
      RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Interrupted while waiting for the service. Exiting.");
      return 0;
    }
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "service not available, waiting again...");
  }

发送请求

 auto result = client->async_send_request(request);

等待服务端返回，并打印

 if (rclcpp::spin_until_future_complete(node, result) ==
    rclcpp::executor::FutureReturnCode::SUCCESS)
  {
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Sum: %ld", result.get()->sum);
  } else {
    RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Failed to call service add_two_ints");
  }

添加可执行文件

CMakeLists.txt改成这样：

cmake_minimum_required(VERSION 3.5)
project(cpp_srvcli)

# Default to C99
if(NOT CMAKE_C_STANDARD)
  set(CMAKE_C_STANDARD 99)
endif()

# 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 dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(example_interfaces REQUIRED)


add_executable(server src/add_two_ints_server.cpp)
ament_target_dependencies(server
rclcpp example_interfaces)

add_executable(client src/add_two_ints_client.cpp)
ament_target_dependencies(client
  rclcpp example_interfaces)

if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  # the following line skips the linter which checks for copyrights
  # uncomment the line when a copyright and license is not present in all source files
  #set(ament_cmake_copyright_FOUND TRUE)
  # the following line skips cpplint (only works in a git repo)
  # uncomment the line when this package is not in a git repo
  #set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()
install(TARGETS
  server
  client
  DESTINATION lib/${PROJECT_NAME})

ament_package()

Ⅳ 编译和运行

• 养成好习惯，编译前先解决依赖，切换到dev_ws下

udo rosdep install -i --from-path src --rosdistro eloquent -y

• 只编译新增的包

colcon build --packages-select cpp_srvcli

• 安装环境

. install/setup.bash

• 运行两个节点

ros2 run cpp_srvcli server
ros2 run cpp_srvcli  client 2 3

④ 总结

您创建了两个节点来通过服务请求和响应数据。您将它们的依赖项和可执行文件添加到程序包配置文件中，可以构建和运行它们，并看到的服务/客户端系统运行起来。