ROS2与tf2入门教程-编写tf2侦听器 (C++)

说明：

• 介绍如何在C++编写 tf2 侦听器

步骤：

• 下载示例侦听器代码

cd ~/tf2_ws/src/learning_tf2_cpp/src
wget https://raw.githubusercontent.com/ros/geometry_tutorials/ros2/turtle_tf2_cpp/src/turtle_tf2_listener.cpp

• 打开文件

#include <geometry_msgs/msg/transform_stamped.hpp>
#include <geometry_msgs/msg/twist.hpp>

#include <rclcpp/rclcpp.hpp>
#include <tf2/exceptions.h>
#include <tf2_ros/transform_listener.h>
#include <tf2_ros/buffer.h>
#include <turtlesim/srv/spawn.hpp>

#include <chrono>
#include <memory>
#include <string>

using std::placeholders::_1;
using namespace std::chrono_literals;

class FrameListener : public rclcpp::Node
{
public:
  FrameListener()
  : Node("turtle_tf2_frame_listener"),
    turtle_spawning_service_ready_(false),
    turtle_spawned_(false)
  {
    // Declare and acquire `target_frame` parameter
    this->declare_parameter<std::string>("target_frame", "turtle1");
    this->get_parameter("target_frame", target_frame_);

    tf_buffer_ =
      std::make_unique<tf2_ros::Buffer>(this->get_clock());
    transform_listener_ =
      std::make_shared<tf2_ros::TransformListener>(*tf_buffer_);

    // Create a client to spawn a turtle
    spawner_ =
      this->create_client<turtlesim::srv::Spawn>("spawn");

    // Create turtle2 velocity publisher
    publisher_ =
      this->create_publisher<geometry_msgs::msg::Twist>("turtle2/cmd_vel", 1);

    // Call on_timer function every second
    timer_ = this->create_wall_timer(
      1s, std::bind(&FrameListener::on_timer, this));
  }

private:
  void on_timer()
  {
    // Store frame names in variables that will be used to
    // compute transformations
    std::string fromFrameRel = target_frame_.c_str();
    std::string toFrameRel = "turtle2";

    if (turtle_spawning_service_ready_) {
      if (turtle_spawned_) {
        geometry_msgs::msg::TransformStamped transformStamped;

        // Look up for the transformation between target_frame and turtle2 frames
        // and send velocity commands for turtle2 to reach target_frame
        try {
          transformStamped = tf_buffer_->lookupTransform(
            toFrameRel, fromFrameRel,
            tf2::TimePointZero);
        } catch (tf2::TransformException & ex) {
          RCLCPP_INFO(
            this->get_logger(), "Could not transform %s to %s: %s",
            toFrameRel.c_str(), fromFrameRel.c_str(), ex.what());
          return;
        }

        geometry_msgs::msg::Twist msg;

        static const double scaleRotationRate = 1.0;
        msg.angular.z = scaleRotationRate * atan2(
          transformStamped.transform.translation.y,
          transformStamped.transform.translation.x);

        static const double scaleForwardSpeed = 0.5;
        msg.linear.x = scaleForwardSpeed * sqrt(
          pow(transformStamped.transform.translation.x, 2) +
          pow(transformStamped.transform.translation.y, 2));

        publisher_->publish(msg);
      } else {
        RCLCPP_INFO(this->get_logger(), "Successfully spawned");
        turtle_spawned_ = true;
      }
    } else {
      // Check if the service is ready
      if (spawner_->service_is_ready()) {
        // Initialize request with turtle name and coordinates
        // Note that x, y and theta are defined as floats in turtlesim/srv/Spawn
        auto request = std::make_shared<turtlesim::srv::Spawn::Request>();
        request->x = 4.0;
        request->y = 2.0;
        request->theta = 0.0;
        request->name = "turtle2";

        // Call request
        using ServiceResponseFuture =
          rclcpp::Client<turtlesim::srv::Spawn>::SharedFuture;
        auto response_received_callback = [this](ServiceResponseFuture future) {
            auto result = future.get();
            if (strcmp(result->name.c_str(), "turtle2") == 0) {
              turtle_spawning_service_ready_ = true;
            } else {
              RCLCPP_ERROR(this->get_logger(), "Service callback result mismatch");
            }
          };
        auto result = spawner_->async_send_request(request, response_received_callback);
      } else {
        RCLCPP_INFO(this->get_logger(), "Service is not ready");
      }
    }
  }
  // Boolean values to store the information
  // if the service for spawning turtle is available
  bool turtle_spawning_service_ready_;
  // if the turtle was successfully spawned
  bool turtle_spawned_;
  rclcpp::Client<turtlesim::srv::Spawn>::SharedPtr spawner_{nullptr};
  rclcpp::TimerBase::SharedPtr timer_{nullptr};
  rclcpp::Publisher<geometry_msgs::msg::Twist>::SharedPtr publisher_{nullptr};
  std::shared_ptr<tf2_ros::TransformListener> transform_listener_{nullptr};
  std::unique_ptr<tf2_ros::Buffer> tf_buffer_;
  std::string target_frame_;
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<FrameListener>());
  rclcpp::shutdown();
  return 0;
}

• 修改CMakeLists.txt文件，再原来的内容基础上添加turtle_tf2_listener的内容：　

add_executable(turtle_tf2_listener src/turtle_tf2_listener.cpp)
ament_target_dependencies(
  turtle_tf2_listener
  geometry_msgs
  rclcpp
  tf2
  tf2_ros
  turtlesim
)
install(TARGETS
  turtle_tf2_listener
  DESTINATION lib/${PROJECT_NAME})

• 修改turtle_tf2_demo.launch.py文件
• 修改后内容如下：　

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node


def generate_launch_description():
  return LaunchDescription([
    Node(
      package='turtlesim',
      executable='turtlesim_node',
      name='sim'
    ),
    
    Node(
      package='learning_tf2_cpp',
      executable='turtle_tf2_broadcaster',
      name='broadcaster1',
      parameters=[
          {'turtlename': 'turtle1'}
      ]
    ),

    DeclareLaunchArgument(
        'target_frame', default_value='turtle1',
        description='Target frame name.'
    ),
    Node(
        package='learning_tf2_cpp',
        executable='turtle_tf2_broadcaster',
        name='broadcaster2',
        parameters=[
            {'turtlename': 'turtle2'}
        ]
    ),
    Node(
        package='learning_tf2_cpp',
        executable='turtle_tf2_listener',
        name='listener',
        parameters=[
            {'target_frame': LaunchConfiguration('target_frame')}
        ]
    ),
  ])

• 构建

colcon build --symlink-install --packages-select learning_tf2_cpp

• 加载工作空间

. ~/tf2_ws/install/local_setup.bash

测试：　

• 运行代码

. ~/tf2_ws/install/local_setup.bash
ros2 launch learning_tf2_cpp turtle_tf2_demo.launch.py

• 应该看到带有两只海龟的海龟模拟

• 键盘控制

ros2 run turtlesim turtle_teleop_key

• 效果和python的实现类似，控制第一个海龟移动，第二个海龟会跟随

参考:

　- 资料来源