ROS 2 Component

在ROS 可以將多個node 集合成一個群集nodelet，好處似乎是可以減少node 之間的通訊成本

但在ROS 2 改以component 的方式支援，目前我沒用過nodelet，所以還不確定具體差異

以下是簡單件紹component 的用法

https://github.com/ros2/ros2/wiki/Composition

https://github.com/ros2/demos/tree/master/composition

component 與一般的node 的差異是，組成component 的元件node 不具有main function，而是會編譯成shared object 的方式(library )，由一隻稱為container 的程式載入

具ROS 官方說法是，用component 運作的node 之間的溝通效率較高

啟動component 有分三種方法

• run-time command line：啟用ROS 2官方提供的container service，再透過指令載入node shared object
• compile-time coding：編寫單一執行檔，將要用的node shared object link 進去
• run-time dll open：啟用ROS 2 官方提供的執行檔，使用引數的方式代入要用的node

建立node shared object

主要是把node 的邏輯寫成一個class，初始化就放在constructor 內，而不是一個main function style 

如果是service 或者subscriber 就把要做的事情一樣放在callback 內，然後初始化就把callback function 代進去

底下程式碼是我嘗試能不能把service 與publisher 寫成同一個node 搭配component

#include "rclcpp/rclcpp.hpp" #include "std_msgs/msg/string.hpp" #include "std_srvs/srv/empty.hpp" #include "visibility_control.h" #include <chrono> using namespace std::chrono_literals; namespace component_demo { class Talker:public rclcpp::Node {     public:     COMPOSITION_PUBLIC     Talker() : Node("talker"),count_(0)     {         auto handler = [this](const std::shared_ptr<std_srvs::srv::Empty::Request> request, const std::shared_ptr<std_srvs::srv::Empty::Response> response) -> void         {             printf("receive request\n");         };         srv_ = create_service<std_srvs::srv::Empty>("test", handler);         pub_ = create_publisher<std_msgs::msg::String>("chatter");         timer_ = create_wall_timer(1s, std::bind(&Talker::on_timer, this));     };                     protected:     void on_timer()     {         auto msg = std::make_shared<std_msgs::msg::String>();         msg->data = "Hello World: " + std::to_string(++count_);         RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", msg->data.c_str())         std::flush(std::cout);   // Put the message into a queue to be processed by the middleware.   // This call is non-blocking.         pub_->publish(msg);     };     private:     size_t count_;     rclcpp::Publisher<std_msgs::msg::String>::SharedPtr pub_;     rclcpp::Service<std_srvs::srv::Empty>::SharedPtr srv_;     rclcpp::TimerBase::SharedPtr timer_; };

修改CMake

主要差異是要建library 而不是executables

cmake_minimum_required(VERSION 3.5) project(component_demo) # 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) endif() find_package(ament_cmake REQUIRED) find_package(ament_index_cpp REQUIRED) find_package(class_loader REQUIRED) find_package(rclcpp REQUIRED) find_package(rcutils REQUIRED) find_package(rosidl_cmake REQUIRED) find_package(rosidl_default_generators REQUIRED) find_package(std_msgs REQUIRED) find_package(std_srvs REQUIRED) include_directories(include) # create ament index resource which references the libraries in the binary dir set(node_plugins "") add_library(talker SHARED   src/talker.cpp) target_compile_definitions(talker   PRIVATE "COMPOSITION_BUILDING_DLL") ament_target_dependencies(talker   "class_loader"   "rclcpp"   "std_msgs"   "std_srvs") rclcpp_register_node_plugins(talker "component_demo::Talker") set(node_plugins "${node_plugins}component_demo::Talker;$<TARGET_FILE:talker>\n") # since the package installs libraries without exporting them # it needs to make sure that the library path is being exported if(NOT WIN32)   ament_environment_hooks(     "${ament_cmake_package_templates_ENVIRONMENT_HOOK_LIBRARY_PATH}") endif() install(TARGETS   talker   ARCHIVE DESTINATION lib   LIBRARY DESTINATION lib   RUNTIME DESTINATION bin   ) ament_package()

run-time command line

先開一個terminal 啟動component cotainer service

要先source setup.bash 不然會認不到自己寫的package

source install/setup.bash ros2 run composition api_composition #[INFO] [api_composition]: Load library /home/karlkwchen/ros2_overlay_ws/install/lib/libtalker.so #[INFO] [api_composition]: Instantiate class component_demo::Talker

再開一個terminal 載入shared object 到service

component_demo 是我的package name

component_demo::Talker 則是namespace 底下的Talker 物件

ros2 run composition api_composition_cli component_demo component_demo::Talker #[INFO] [api_composition_cli]: Sending request... #[INFO] [api_composition_cli]: Waiting for response... #[INFO] [api_composition_cli]: Result of load_node: success = true

compile-time coding

參考ROS 2 官方範例，改成載入剛剛自己建立的shared object 就行了

#include <memory> #include "rclcpp/rclcpp.hpp" int main(int argc, char * argv[]) {   // Force flush of the stdout buffer.   setvbuf(stdout, NULL, _IONBF, BUFSIZ);   // Initialize any global resources needed by the middleware and the client library.   // This will also parse command line arguments one day (as of Beta 1 they are not used).   // You must call this before using any other part of the ROS system.   // This should be called once per process.   rclcpp::init(argc, argv);   // Create an executor that will be responsible for execution of callbacks for a set of nodes.   // With this version, all callbacks will be called from within this thread (the main one).   rclcpp::executors::SingleThreadedExecutor exec;   // Add some nodes to the executor which provide work for the executor during its "spin" function.   // An example of available work is executing a subscription callback, or a timer callback.   auto talker = std::make_shared<component_demo::Talker>();   exec.add_node(talker);      // spin will block until work comes in, execute work as it becomes available, and keep blocking.   // It will only be interrupted by Ctrl-C.   exec.spin();   rclcpp::shutdown();   return 0; }

run-time dll open

啟動ROS 2 官方的node dlopen_composition，後面引數代入shared object 即可

ros2 run composition dlopen_composition `ros2 pkg prefix component_demo`/lib/libtalker.so [INFO] [dlopen_composition]: Load library /home/karlkwchen/ros2_overlay_ws/install/lib/libtalker.so [INFO] [dlopen_composition]: Instantiate class component_demo::Talker