ROS2 中节点通过发布和订阅话题来传递信息，一个节点可以作为发布者发布信息，也可以同时作为订阅者订阅其它节点发布的话题。

话题由主题和消息类型组成：

  • 主题是一个字符串
  • 消息类型是传输数据的格式

话题查看

  1. 启动小海龟节点

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    ros2 run turtlesim turtlesim_node

  2. 查看小海龟节点信息

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    ros2 node info /turtlesim

    可以看到 node 的相关信息

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    /turtlesim
  Subscribers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /turtle1/cmd_vel: geometry_msgs/msg/Twist
  Publishers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /rosout: rcl_interfaces/msg/Log
    /turtle1/color_sensor: turtlesim/msg/Color
    /turtle1/pose: turtlesim/msg/Pose

    Subscribers 列出节点订阅了哪些话题，前面为主题，后面为话题消息类型，Publishers 列出发布哪些话题

  3. 查看一个话题信息

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    ros2 topic info /turtle1/cmd_vel
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    Type: geometry_msgs/msg/Twist
Publisher count: 0
Subscription count: 1

    显示出数据类型，发布者数量，被订阅的数量

  4. 查看消息类型的详细定义

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    ros2 interface show geometry_msgs/msg/Twist
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    # This expresses velocity in free space broken into its linear and angular parts.

Vector3  linear
        float64 x
        float64 y
        float64 z
Vector3  angular
        float64 x
        float64 y
        float64 z

    可以看到消息类型为两个数组，第一个为线速度，第二个为角速度

  5. 使用 ros2 命令发布话题消息

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    ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 1.0, y: 0.0}, angular: {z: 1.0}}"

C++ 发布话题

用 C++ 实现每秒发布一个字符串

  1. 创建功能包

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    ros2 pkg create  cpp_topic_publish_demo --destination-directory src --build-type ament_cmake --dependencies rclcpp std_msgs --license Apache-2.0

  2. 新建节点，每秒发布字符串

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    #include "rclcpp/node.hpp"
#include "rclcpp/executors.hpp"
#include "std_msgs/msg/string.hpp"

using namespace std::chrono_literals;

class TopicPublishDemo : public rclcpp::Node {
public:
    TopicPublishDemo(): Node("topic_publish_demo_node"), count_(0) {
        publisher_ = create_publisher<std_msgs::msg::String>("topic_string", 10);
        auto timer_callback = [this]() {
            auto msg = std_msgs::msg::String();
            msg.data = "Hello, Now is " + std::to_string(count_++);
            RCLCPP_INFO(get_logger(), "Publishing: %s", msg.data.c_str());
            publisher_->publish(msg);
        };
        timer_ = create_wall_timer(1s, timer_callback);

    }


private:
    rclcpp::TimerBase::SharedPtr timer_;
    rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
    size_t count_;
};


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

    rclcpp::spin(std::make_shared<TopicPublishDemo>());

    rclcpp::shutdown();
    return 0;
}

C++ 订阅话题

  1. 创建功能包

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    ros2 pkg create  cpp_topic_subscribe_demo --destination-directory src --build-type ament_cmake --dependencies rc
lcpp std_msgs --license Apache-2.0

  2. 节点订阅代码

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    #include "rclcpp/node.hpp"
#include "rclcpp/executors.hpp"
#include "std_msgs/msg/string.hpp"

using namespace std::chrono_literals;

class TopicSubscribeDemo : public rclcpp::Node {
public:
    TopicSubscribeDemo(): Node("topic_subscribe_demo_node") {
        auto topic_callback = [this](std_msgs::msg::String::UniquePtr msg) {
            RCLCPP_INFO(get_logger(), "I received: %s", msg->data.c_str());
        };
        subscriber_ = create_subscription<std_msgs::msg::String>("topic_string", 10, topic_callback);
    }

private:
    rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscriber_;
};


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

    rclcpp::spin(std::make_shared<TopicSubscribeDemo>());

    rclcpp::shutdown();
    return 0;
}

自定义消息类型

  1. 创建一个功能包，用来定义消息类型

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    ros2 pkg create --destination-directory=src --build-type=ament_cmake --license Apache-2.0 --dependencies std_msgs demo_interfaces

  2. 在包根目录下新建一个 msg 目录，里面创建一个 Demo.msg 文件

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    std_msgs/String name

  3. 在 CMakeLists.txt 中添加以下代码

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    find_package(rosidl_default_generators REQUIRED)

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

  4. 在 package.xml 中添加以下代码

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    <buildtool_depend>rosidl_default_generators</buildtool_depend>
<exec_depend>rosidl_default_runtime</exec_depend>
<member_of_group>rosidl_interface_packages</member_of_group>

  5. 使用 colcon build 编译

使用自定义消息

将前面的 publish 和 subscribes 改成使用自定义消息

  1. 在 package.xml 中添加依赖

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    <depend>demo_interfaces</depend>

  2. 在 CMakeLists.txt 中添加依赖

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    find_package(demo_interfaces)
ament_target_dependencies(${PROJECT_NAME} rclcpp std_msgs demo_interfaces)
  1. 将原来的 std_msgs::msg::String 改为 demo_interface::msg::Demo
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    #include "rclcpp/node.hpp"
#include "rclcpp/executors.hpp"
#include "demo_interfaces/msg/demo.hpp"

using namespace std::chrono_literals;

class TopicSubscribeDemo : public rclcpp::Node {
public:
    TopicSubscribeDemo(): Node("topic_subscribe_demo_node") {
        auto topic_callback = [this](demo_interfaces::msg::Demo::UniquePtr msg) {
            RCLCPP_INFO(get_logger(), "I received: %s", msg->name.data.c_str());
        };
        subscriber_ = create_subscription<demo_interfaces::msg::Demo>("topic_string", 10, topic_callback);
    }

private:
    rclcpp::Subscription<demo_interfaces::msg::Demo>::SharedPtr subscriber_;
};


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

    rclcpp::spin(std::make_shared<TopicSubscribeDemo>());

    rclcpp::shutdown();
    return 0;
}