节点是通过ROS图进行通信的可执行进程
引言
ROS2将复杂的系统分解为许多模块化节点。Topic 是ROS graph的重要元素,它充当节点交换消息的总线.
Node可以向任意数量的Topic发布数据,同时可以订阅任意数量的Topic。
1、创建package
打开一个新的终端进入到ros2_workspace/src,具体创建工作空间请参见
在src目录中运行并创建包,输入以下命令:
ros2 pkg create --build-type ament_cmake cpp_pubsub
创建完成目录结构如下图所示:
进入目录: ros2_workspace/src/cpp_pubsub/src
cd ros2_workspace/src/cpp_pubsub/src
2. 写publisher Node
通过输入以下命令下载示例talker代码:
wget -O publisher_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_publisher/member_function.cpp
现在将有一个名为publisher_member_function.cpp的新文件。编辑 publisher_member_function.cpp
#include <chrono>
#include <functional>
#include <memory>
#include <string> //标准C++头文件
#include "rclcpp/rclcpp.hpp" //Ros2系统中最常见的组组成部分
#include "std_msgs/msg/string.hpp" //用于发布数据的内置消息类型
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 Examine the code
#include <chrono>
#include <functional>
#include <memory>
#include <string> //标准C++头文件
#include "rclcpp/rclcpp.hpp" //Ros2系统中最常见的组组成部分
#include "std_msgs/msg/string.hpp" //用于发布数据的内置消息类型
using namespace std::chrono_literals;
通过继承rclcpp::node创建节点类MinimalPublisher。代码中的每个this都是指节点。
class MinimalPublisher : public rclcpp::Node
构造函数初始化Node count_ 回调函数timer_callback用于设置数据和实际发布消息。 RCLCPP_INFO宏确保每个发布的消息都打印到控制台
2.2 添加依赖
进入目录: ros2_workspace/src/cpp_pubsub/ 可以看到如下目录
编辑 package.xml,填写、和标签:
<description>Examples of minimal publisher/subscriber using rclcpp</description>
<maintainer email="whdxzwy@outlook.com">yang</maintainer>
<license>Apache License 2.0</license>
在 ament_cmake 下添加以下两句依赖项:
<depend>rclcpp</depend>
<depend>std_msgs</depend>
最终添加完结果如下:
2.3 CMakeLists.txt
-
打开CMakeLists.txt文件。在现有的依赖项find_package(ament_cmake REQUIRED)下面,添加以下行:
find_package(rclcpp REQUIRED) find_package(std_msgs REQUIRED)
-
之后,添加可执行文件并将其命名为talker,这样就可以使用ros2 run运行节点:
add_executable(talker src/publisher_member_function.cpp) ament_target_dependencies(talker rclcpp std_msgs)
-
最后,添加install(TARGETS…)部分,这样ros2 run就可以找到您的可执行文件:
install(TARGETS talker DESTINATION lib/${PROJECT_NAME})
3. 写Subscriber Node
返回到ros2_workspace/src/cpp_pubsub/src以创建下一个节点。在终端中输入以下代码:
wget -O subscriber_member_function.cpp https://raw.githubusercontent.com/ros2/examples/humble/rclcpp/topics/minimal_subscriber/member_function.cpp
有如下两个文件:
publisher_member_function.cpp subscriber_member_function.cpp
如图所示:
vim打开 subscriber_member_function.cpp 文件
#include <functional>
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using std::placeholders::_1;
class MinimalSubscriber : public rclcpp::Node
{
public:
MinimalSubscriber()
: Node("minimal_subscriber")
{
subscription_ = this->create_subscription<std_msgs::msg::String>(
"topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
}
private:
void topic_callback(const std_msgs::msg::String & msg) const
{
RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg.data.c_str());
}
rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalSubscriber>());
rclcpp::shutdown();
return 0;
}
3.1 CMakeLists.txt 中添加订阅者节点的可执行文件和目标。
add_executable(listener src/subscriber_member_function.cpp)
ament_target_dependencies(listener rclcpp std_msgs)
install(TARGETS
talker
listener
DESTINATION lib/${PROJECT_NAME})
4. Build and run
在构建之前,最好在工作区的根目录(ros2_workspace)中运行rosdep,以检查是否缺少依赖项:
rosdep install -i --from-path src --rosdistro humble -y
仍然在工作区的根目录ros2_workspace中构建新包:
colcon build --packages-select cpp_pubsub
打开一个新终端,进入到目录ros2_workspace下,并获取安装文件:
. install/setup.bash
现在来运行talker node:
ros2 run cpp_pubsub talker
终端应该每0.5秒开始发布一次信息消息,如下所示:
打开另一个终端,再次从ros2_workspace内部获取安装文件,然后启动listener node
ros2 run cpp_pubsub listener