ROS2 中的服务就类似于 RPC 调用，每个节点提供服务，其它节点可以调用服务并获取返回值

服务查看

  1. 查看节点提供的服务

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    ros2 node info /turtlesim
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    Service Servers:
   /clear: std_srvs/srv/Empty
   /kill: turtlesim/srv/Kill
   /reset: std_srvs/srv/Empty
   /spawn: turtlesim/srv/Spawn
   /turtle1/set_pen: turtlesim/srv/SetPen
   /turtle1/teleport_absolute: turtlesim/srv/TeleportAbsolute
   /turtle1/teleport_relative: turtlesim/srv/TeleportRelative
   /turtlesim/describe_parameters: rcl_interfaces/srv/DescribeParameters
   /turtlesim/get_parameter_types: rcl_interfaces/srv/GetParameterTypes
   /turtlesim/get_parameters: rcl_interfaces/srv/GetParameters
   /turtlesim/get_type_description: type_description_interfaces/srv/GetTypeDescription
   /turtlesim/list_parameters: rcl_interfaces/srv/ListParameters
   /turtlesim/set_parameters: rcl_interfaces/srv/SetParameters
   /turtlesim/set_parameters_atomically: rcl_interfaces/srv/SetParametersAtomically

  2. 服务和话题一样，包括服务名和服务参数定义，可以查看服务的详细定义

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    ros2 service info /spawn
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    Type: turtlesim/srv/Spawn
Clients count: 0
Services count: 1

  3. 查看服务类型定义

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    ros2 interface show turtlesim/srv/Spawn
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    float32 x
float32 y
float32 theta
string name # Optional.  A unique name will be created and returned if this is empty
---
string name

这个定义表示输入参数为 x, y, theta, name ，其中 name 是可选的，返回值为 name

  1. ros2 命令行调用服务
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    ros2 service call /spawn turtlesim/srv/Spawn "{x: 1.0, y: 1.0, theta: 0}"
    调用后的结果
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    waiting for service to become available...
requester: making request: turtlesim.srv.Spawn_Request(x=1.0, y=1.0, theta=0.0, name='')

response:
turtlesim.srv.Spawn_Response(name='turtle2')

定义服务描述文件

  1. 在 demo_interfaces 下的 srv 目录自定义一个服务描述文件，名称叫 AddInt.srv

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    int64 a
int64 b
---
int64 sum

  2. CMakeLists.txt 中添加编译

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    rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Demo.msg"
  "srv/AddInt.srv"
  DEPENDENCIES std_msgs
)

  3. 使用 colcon build 编译

C++ 节点实现一个服务

  1. 创建一个功能包

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    ros2 pkg create  cpp_server_demo --destination-directory src --build-type ament_cmake --dependencies rclcpp demo_interfaces --license Apache-2.0

  2. 写服务文件

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    #include "rclcpp/executors.hpp"
#include "demo_interfaces/srv/add_int.hpp"

void AddInt(demo_interfaces::srv::AddInt_Request::SharedPtr request,
            demo_interfaces::srv::AddInt_Response::SharedPtr 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);

    auto node = rclcpp::Node::make_shared("add_int_server");
    auto service = node->create_service<demo_interfaces::srv::AddInt>("add_int", AddInt);

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

    rclcpp::spin(node);

    rclcpp::shutdown();
    return 0;
}

  3. 编译完成后运行就可以看到对应服务了

C++ 实现一个节点调用服务

  1. 创建一个功能包

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    ros2 pkg create  cpp_client_demo --destination-directory src --build-type ament_cmake --dependencies rclcpp demo
_interfaces --license Apache-2.0

  2. 客户端代码

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    #include "rclcpp/executors.hpp"
#include "demo_interfaces/srv/add_int.hpp"

using namespace std::chrono_literals;

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

    auto node = rclcpp::Node::make_shared("add_client");
    auto client = node->create_client<demo_interfaces::srv::AddInt>("add_int");
    auto request = std::make_shared<demo_interfaces::srv::AddInt_Request>();
    request->a = 100;
    request->b = 200;

    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);
    if (rclcpp::spin_until_future_complete(node, result) == rclcpp::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;
}