目标:创建并运行一个使用C++编写的发布者和订阅者节点
背景
节点是通过ROS图进行通信的可执行进程。在本教程中,节点通过字符串消息的形式与其他节点进行通信。例子使用简单的"talker"和"listener";一个节点发布数据,其他订阅该话题来接收数据。
任务
1、创建一个包
ros2 pkg create --build-type ament_cmake cpp_pubsub
2、实现一个发布者节点(wget -O publisher_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_publisher/member_function.cpp)
#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using namespace std::chrono_literals;
/* This example creates a subclass of Node and uses std::bind() to register a
* member function as a callback from the timer. */
class MinimalPublisher : public rclcpp::Node
{
public:
MinimalPublisher()
: Node("minimal_publisher"), count_(0)
{
publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
timer_ = this->create_wall_timer(
500ms, std::bind(&MinimalPublisher::timer_callback, this));
}
private:
void timer_callback()
{
auto message = std_msgs::msg::String();
message.data = "Hello, world! " + std::to_string(count_++);
RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
publisher_->publish(message);
}
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<MinimalPublisher>());
rclcpp::shutdown();
return 0;
}
2.1 查看代码
头部为包含的标准的C++代码头文件,rclcpp/rclcpp.hpp包含ROS2系统的大部分内容,std_msgs/msg/string.hpp包含内建的消息类型,用于发布数据
#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using namespace std::chrono_literals;
本节点的类MinimalPublisher继承了rclcpp::Node,
class MinimalPublisher : public rclcpp::Node
下面初始化相关信息
public:
MinimalPublisher()
: Node("minimal_publisher"), count_(0)
{
publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
timer_ = this->create_wall_timer(
500ms, std::bind(&MinimalPublisher::timer_callback, this));
}
RCLCPP_INFO用于输出到终端
private:
void timer_callback()
{
auto message = std_msgs::msg::String();
message.data = "Hello, world! " + std::to_string(count_++);
RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
publisher_->publish(message);
}
最后是timer的声明以及counter
rclcpp::TimerBase::SharedPtr timer_;
rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
size_t count_;
rclcpp::init初始化ros2,使用rclcpp::spin来开始处理来自于节点的数据,包括timer的回调函数。
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalPublisher>());
rclcpp::shutdown();
return 0;
}
2.2 添加依赖
打开package.xml
<depend>rclcpp</depend>
<depend>std_msgs</depend>
2.3 CMakeLists.txt
添加如下行在find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
之后添加如下:
add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp std_msgs)
最后,添加install ,以便ros2 run可以找到可执行文件
install(TARGETS
talker
DESTINATION lib/${PROJECT_NAME})
完整的如下
cmake_minimum_required(VERSION 3.5)
project(cpp_pubsub)
# Default to C++14
if(NOT CMAKE_CXX_STANDARD)
set(CMAKE_CXX_STANDARD 14)
endif()
if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
add_compile_options(-Wall -Wextra -Wpedantic)
endif()
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp std_msgs)
install(TARGETS
talker
DESTINATION lib/${PROJECT_NAME})
ament_package()
3、编写订阅者
wget -O subscriber_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_subscriber/member_function.cpp
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using std::placeholders::_1;
class MinimalSubscriber : public rclcpp::Node
{
public:
MinimalSubscriber()
: Node("minimal_subscriber")
{
subscription_ = this->create_subscription<std_msgs::msg::String>(
"topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
}
private:
void topic_callback(const std_msgs::msg::String & msg) const
{
RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg.data.c_str());
}
rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalSubscriber>());
rclcpp::shutdown();
return 0;
}
3.1 分析代码
代码与publisher基本一致。节点被命名为minimal_subscriber,使用节点的create_subscription类来执行回调函数
public:
MinimalSubscriber()
: Node("minimal_subscriber")
{
subscription_ = this->create_subscription<std_msgs::msg::String>(
"topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
}
接收到消息,在控制台上打印
private:
void topic_callback(const std_msgs::msg::String & msg) const
{
RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg.data.c_str());
}
rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
在publisher里面spin意味着启动定时器,在subscriber里面spin意味着随时准备接收消息。
由于发布者和订阅者的依赖一样,所以在package.xml里面不需要添加其他了。
3.2 CMakeLists.txt
编辑如下内容
add_executable(listener src/subscriber_member_function.cpp)
ament_target_dependencies(listener rclcpp std_msgs)
install(TARGETS
talker
listener
DESTINATION lib/${PROJECT_NAME})
4、编译和运行
colcon build --packages-select cpp_pubsub
ros2 run cpp_pubsub talker
ros2 run cpp_pubsub listener
总结
您创建了两个节点来发布和订阅主题上的数据。在编译和运行它们之前,您将它们的依赖项和可执行文件添加到包配置文件中