ros2

# ROS2 Development Skill

## When to Use This Skill
- Building ROS2 packages, nodes, or component containers
- Setting up colcon workspaces, ament_cmake, or ament_python packages
- Writing CMakeLists.txt, package.xml, or setup.py for ROS2
- Defining custom messages, services, or actions
- Writing Python launch files with conditional logic
- Configuring DDS middleware and QoS profiles
- Implementing lifecycle (managed) nodes
- Working with Nav2, MoveIt2, or other ROS2 frameworks
- Debugging DDS discovery, QoS mismatches, or build failures
- Deploying ROS2 to production or embedded systems (micro-ROS)
- Setting up CI/CD for ROS2 packages

## Core Architecture

### 1. Node Design Patterns

**Basic Node (rclpy)**:
```python
#!/usr/bin/env python3
import rclpy
from rclpy.node import Node
from rclpy.qos import QoSProfile, ReliabilityPolicy, HistoryPolicy
from std_msgs.msg import String

class PerceptionNode(Node):
    def __init__(self):
        super().__init__('perception_node')

        # 1. Declare parameters with types and descriptions
        self.declare_parameter('rate_hz', 30.0,
            descriptor=ParameterDescriptor(
                description='Processing rate in Hz',
                floating_point_range=[FloatingPointRange(
                    from_value=1.0, to_value=120.0, step=0.0
                )]
            ))
        self.declare_parameter('confidence_threshold', 0.7)
        self.declare_parameter('frame_id', 'camera_link')

        # 2. Read parameters
        rate_hz = self.get_parameter('rate_hz').value
        self.threshold = self.get_parameter('confidence_threshold').value
        self.frame_id = self.get_parameter('frame_id').value

        # 3. Set up QoS profiles
        sensor_qos = QoSProfile(
            reliability=ReliabilityPolicy.BEST_EFFORT,
            history=HistoryPolicy.KEEP_LAST,
            depth=1
        )
        reliable_qos = QoSProfile(
            reliability=ReliabilityPolicy.RELIABLE,
            history=HistoryPolicy.KEEP_LAST,
            depth=10
        )

        # 4. Publishers first, then subscribers
        self.det_pub = self.create_publisher(
            DetectionArray, 'detections', reliable_qos)

        self.image_sub = self.create_subscription(
            Image, 'camera/image_raw', self.image_callback, sensor_qos)

        # 5. Timers for periodic work
        self.timer = self.create_timer(1.0 / rate_hz, self.timer_callback)

        # 6. Parameter change callback
        self.add_on_set_parameters_callback(self.param_callback)

        self.get_logger().info(
            f'Perception node started at {rate_hz}Hz, '
            f'threshold={self.threshold}')

    def param_callback(self, params):
        """Handle runtime parameter changes (replaces dynamic_reconfigure)"""
        for param in params:
            if param.name == 'confidence_threshold':
                self.threshold = param.value
                self.get_logger().info(f'Threshold updated to {param.value}')
        return SetParametersResult(successful=True)

    def image_callback(self, msg):
        # Process incoming images
        pass

    def timer_callback(self):
        # Periodic work
        pass

def main(args=None):
    rclpy.init(args=args)
    node = PerceptionNode()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass
    finally:
        node.destroy_node()
        rclpy.shutdown()

if __name__ == '__main__':
    main()
```

**Basic Node (rclcpp)**:
```cpp
#include <rclcpp/rclcpp.hpp>
#include <sensor_msgs/msg/image.hpp>
#include <vision_msgs/msg/detection2_d.hpp>
#include <memory>

class PerceptionNode : public rclcpp::Node {
public:
    PerceptionNode() : Node("perception_node") {
        // Declare and get parameters
        this->declare_parameter("rate_hz", 30.0);
        this->declare_parameter("confidence_threshold", 0.7);
        double rate_hz = this->get_parameter("rate_hz").as_double();

        // QoS
        auto sensor_qos = rclcpp::SensorDataQoS();
        auto reliable_qos = rclcpp::QoS(10).reliable();

        // Publishers and subscribers
        det_pub_ = this->create_publisher<vision_msgs::msg::Detection2D>("detections", reliable_qos);
        image_sub_ = this->create_subscription<sensor_msgs::msg::Image>(
            "camera/image_raw", sensor_qos, [this](const std::shared_ptr<const sensor_msgs::msg::Image>& msg){
                this->image_callback(msg);
            });

        timer_ = this->create_wall_timer(
            std::chrono::milliseconds(static_cast<int>(1000.0 / rate_hz)),
            [this](){ this->timer_callback(); });

        RCLCPP_INFO(this->get_logger(), "Perception node started at %.1fHz", rate_hz);
    }

private:
    void image_callback(const std::shared_ptr<const sensor_msgs::msg::Image>& msg) {
        // Use shared_ptr for zero-copy potential
    }
    void timer_callback() {}

    rclcpp::Publisher<vision_msgs::msg::Detection2D>::SharedPtr det_pub_;
    rclcpp::Subscription<sensor_msgs::msg::Image>::SharedPtr image_sub_;
    rclcpp::TimerBase::SharedPtr timer_;
};

int main(int argc, char** argv) {
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<PerceptionNode>());
    rclcpp::shutdown();
    return 0;
}
```

### 2. Lifecycle (Managed) Nodes

Use lifecycle nodes for production systems where you need deterministic startup, shutdown, and error recovery. This is one of ROS2's most important features over ROS1.

**State Machine**: `Unconfigured → Inactive → Active → Finalized`

```python
from rclpy.lifecycle import Node as LifecycleNode, TransitionCallbackReturn

class ManagedPerception(LifecycleNode):
    def __init__(self):
        super().__init__('managed_perception')
        self.get_logger().info('Node created (unconfigured)')

    def on_configure(self, state) -> TransitionCallbackReturn:
        """Load params, allocate memory, set up pubs/subs (but don't activate)"""
        self.declare_parameter('model_path', '')
        model_path = self.g