# ROS 2

## Prerequisites

:::info
docker 並非必要，但為了部屬方便以及讓大家盡快熟悉之後常用的開發工具，因此使用 docker 進行 ROS2 教學
:::
- ubuntu 22.04 (recommended)
- [docker](https://docs.docker.com/get-docker/)

## Installation

Please refer to: https://github.com/NCTU-AUV/orca_docker

## ROS (Robot Operating System)

### Node

在 ROS 中，Node 是最基本的運行單元(一個 process 就是一個 Node)


### Topic

Topic 是 Node 間進行通訊的方式，內容可以為字串、數字、或各種自訂的資料結構。


### Service

Service 跟 Topic 最大的差別在於 client、server 的溝通模式，且只會有一個 Service server。


## Turtlesim

- 開啟 turtlesim node:

    ros2 run turtlesim turtlesim_node


- 用鍵盤控制烏龜:

    ros2 run turtlesim turtle_teleop_key

- 用 rqt_graph 看 nodes 和 topics:

    rqt_graph

- 其他常用指令:
- node:

        ros2 node list
        ros2 node info <node name>

- topic:

        ros2 topic list
        ros2 topic info <topic name>
        ros2 topic echo <topic name>
        ros2 topic pub <topic name> <message type> <message>

## Publisher and Subscriber in Python

### Create a workspace

mkdir -p /demo_ws/src

所有的 package 都要放在一個 workspace 底下：

demo_ws/
└── src
    ├── pkg_1
    ├── pkg_2
    ├── ...
    ├── ...
    └── pkg_n

### Create a ROS2 package

cd /demo_ws/src
ros2 pkg create --build-type ament_python py_pubsub

File structure of a ROS2 package:


### Publisher Node

`py_pubsub/publisher.py`:

import rclpy
from rclpy.node import Node

from std_msgs.msg import String


class MinimalPublisher(Node):

    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello World: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1


def main(args=None):
    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_publisher.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

### Subscriber Node

`py_pubsub/subscriber.py`:

import rclpy
from rclpy.node import Node

from std_msgs.msg import String


class MinimalSubscriber(Node):

    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            String,
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning

    def listener_callback(self, msg):
        self.get_logger().info('I heard: "%s"' % msg.data)


def main(args=None):
    rclpy.init(args=args)

    minimal_subscriber = MinimalSubscriber()

    rclpy.spin(minimal_subscriber)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_subscriber.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

### Build

- In `package.xml`:

    <exec_depend>rclpy</exec_depend>
    <exec_depend>std_msgs</exec_depend>

- In `setup.py`:

    entry_points={
            'console_scripts': [
                    'talker = py_pubsub.publisher:main',
                    'listener = py_pubsub.subscriber:main',
            ],
    },

以 'talker = py_pubsub.publisher:main' 為例：
- talker: executable name
- py_pubsub: package name
- publisher: python file name (`publisher.py`)
- main: entry function (`main()` in `publisher.py`)

- Command line:

    cd /demo_ws
    rosdep install -i --from-path src --rosdistro humble -y

    colcon build --symlink-install --packages-select py_pubsub 

    source install/setup.bash

### Run

ros2 run py_pubsub talker

On the other terminal:

ros2 run py_pubsub listener


rqt_graph


## Referecnes

- https://docs.ros.org/en/humble/index.html
- https://docs.ros.org/en/humble/Tutorials/Beginner-CLI-Tools/Introducing-Turtlesim/Introducing-Turtlesim.html
- https://www.youtube.com/playlist?list=PLLSegLrePWgJudpPUof4-nVFHGkB62Izy