ROS 2 Humble Hawksbill将于2022年5月发布

ROS 2 Humble Hawksbill应该是第一款走向成熟的ROS2长期支持版本（LTS）。

ROS 2目前各模块功能还在不断完善和修订，ROS 2 Humble Hawksbill是ROS2的第八个发行版。

rclcpp

标准格式转换案例：

std_msgs::msg::String 与 std::string 转换。

template<>
struct rclcpp::TypeAdapter<std::string,std_msgs::msg::String
>
{using is_specialized = std::true_type;using custom_type = std::string;using ros_message_type = std_msgs::msg::String;staticvoidconvert_to_ros_message(const custom_type & source,ros_message_type & destination){destination.data = source;}staticvoidconvert_to_custom(const ros_message_type & source,custom_type & destination){destination = source.data;}
};

使用案例：

using MyAdaptedType = TypeAdapter<std::string, std_msgs::msg::String>;// Publish a std::string
auto pub = node->create_publisher<MyAdaptedType>(...);
std::string custom_msg = "My std::string"
pub->publish(custom_msg);// Pass a std::string to a subscription's callback
auto sub = node->create_subscription<MyAdaptedType>("topic",10,[](const std::string & msg) {...});

在目前较新版本的ROS2（b83b18598b）仅pub或sub的实现方式cpp代码就有5种。以上面改进为例，pub代码如下（使用 `std::string` 代替 `std_msgs::msg::String`）：

#include <chrono>
#include <functional>
#include <memory>
#include <string>#include "rclcpp/type_adapter.hpp"
#include "rclcpp/rclcpp.hpp"#include "std_msgs/msg/string.hpp"using namespace std::chrono_literals;template<>
struct rclcpp::TypeAdapter<std::string, std_msgs::msg::String>
{using is_specialized = std::true_type;using custom_type = std::string;using ros_message_type = std_msgs::msg::String;staticvoidconvert_to_ros_message(const custom_type & source,ros_message_type & destination){destination.data = source;}staticvoidconvert_to_custom(const ros_message_type & source,custom_type & destination){destination = source.data;}
};class MinimalPublisher : public rclcpp::Node
{using MyAdaptedType = rclcpp::TypeAdapter<std::string, std_msgs::msg::String>;public:MinimalPublisher(): Node("minimal_publisher"), count_(0){publisher_ = this->create_publisher<MyAdaptedType>("topic", 10);timer_ = this->create_wall_timer(500ms, std::bind(&MinimalPublisher::timer_callback, this));}private:void timer_callback(){std::string message = "Hello, world! " + std::to_string(count_++);RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.c_str());publisher_->publish(message);}rclcpp::TimerBase::SharedPtr timer_;rclcpp::Publisher<MyAdaptedType>::SharedPtr publisher_;size_t count_;
};int main(int argc, char * argv[])
{rclcpp::init(argc, argv);rclcpp::spin(std::make_shared<MinimalPublisher>());rclcpp::shutdown();return 0;
}

sub代码如下：

#include <functional>
#include <memory>
#include <string>#include "rclcpp/type_adapter.hpp"
#include "rclcpp/rclcpp.hpp"#include "std_msgs/msg/string.hpp"using std::placeholders::_1;template<>
struct rclcpp::TypeAdapter<std::string, std_msgs::msg::String>
{using is_specialized = std::true_type;using custom_type = std::string;using ros_message_type = std_msgs::msg::String;staticvoidconvert_to_ros_message(const custom_type & source,ros_message_type & destination){destination.data = source;}staticvoidconvert_to_custom(const ros_message_type & source,custom_type & destination){destination = source.data;}
};class MinimalSubscriber : public rclcpp::Node
{using MyAdaptedType = rclcpp::TypeAdapter<std::string, std_msgs::msg::String>;public:MinimalSubscriber(): Node("minimal_subscriber"){subscription_ = this->create_subscription<MyAdaptedType>("topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));}private:void topic_callback(const std::string & msg) const{RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg.c_str());}rclcpp::Subscription<MyAdaptedType>::SharedPtr subscription_;
};int main(int argc, char * argv[])
{rclcpp::init(argc, argv);rclcpp::spin(std::make_shared<MinimalSubscriber>());rclcpp::shutdown();return 0;
}

这样的细节变化非常多，在dashing到foxy中，更多的是功能完善，而非bug补丁，这从侧面体现了ROS2的版本离稳定成熟还需一段时间。

ros2cli

这是命令行接口，pub和sub其实都非常常用，但是

使用标志时，将在开始发布之前等待找到一个匹配的订阅。 这避免了 ros2cli 节点在发现匹配订阅之前开始发布的问题，这会导致一些第一条消息丢失。

ros2 topic pub -1 -w 1 /chatter std_msgs/msg/String "{data: 'foo'}"

更多细节查阅官方文档。