ROS2 for beginners

NOTE: Build files are not uploaded to this repository. To run anything, go to ros_ws and type colcon build.

1. Environment setup
2. Create workspace
3. Create custom package
4. Nodes
5. Topics

1. Environment setup

To automatically source ROS in a new terminal

 echo "source /opt/ros/humble/setup.bash" >> ~/.bashrc

To install colcon build tools and add it to source automatically

 sudo apt install python3-colcon-common-extensions
 echo "source /usr/share/colcon_argcomplete/hook/colcon-argcomplete.bash" >> ~/.bashrc

2. Create workspace

    mkdir -p ros2_ws/src/
    cd ros2_ws/
    colcon build

    echo "source /home/rishabh/Documents/ros2_ws/install/setup.bash" >> ~/.bashrc

3. Create custom package

rcl (ROS2 Client Library) is the base ROS2 library. It is used by python, c++ to interact with ROS2 and use its features. Specialized versions are used by each programming language (rclpy, rclcpp, etc.)

3.1. Python

    cd ros2_ws/src/
    ros2 pkg create [PACKAGE_NAME] --build-type ament_python --dependencies rclpy

rclpy is what enables python to use ROS2 features.

3.2. C++

    cd ros2_ws/src/
    ros2 pkg create [PACKAGE_NAME] --build-type ament_cmake --dependencies rclcpp

rclcpp is what enables C++ to use ROS2 features.

3.3. Build custom package

    cd ros2_ws/
    colcon build    // to build all packages
    colcon build --packages-select [PACKAGE_NAME]   // specific packages

4. Nodes

Nodes are sub-programs of the application, responsible for only one thing.
They communicate with each other using topics, services and parameters.
Useful for organization
Nodes written in C++ and Python can communicate with each other.

4.1. Python

Create the node file inside src/[PACKAGE_NAME]/[PACKAGE_NAME] (Sample python node).

    chmod +x [YOUR_NODE_PATH]
    ./[YOUR_NODE_PATH]

But this is not scalable for larger projects. We can make this node an executable in our project and call it from ros run. Modify the following in the src/[PACKAGE_NAME]/setup.py file.

    ...
    ...
    entry_points={
        'console_scripts': [
            "py_node = [PACKAGE_NAME].[NODE_FILE_NAME]:main"
        ]
    }

Then after building the changes,

    colcon build

This will create the py_node file in install/my_py_pkg/lib/my_py_pkg/, which can be run from there.

    cd [PATH_TO_WORKSPACE]/install/my_py_pkg/lib/my_py_pkg/
    ./py_node

Another way (much simpler and intuitive) to run the node is

    ros2 run my_py_pkg py_node

NOTE: The name of the node file (python file - my_first_node), the name of the node (inside the python file - py_test) and the name of the executable - py_node can be 3 different names for the same thing.

4.2. C++

Create the node file in the src/[PACKAGE_NAME]/src directory. You migh also need to add the ros cpp include path in your editor settings. For VSCode, add this in the c_cpp_properties.json file under includePath

    "/opt/ros/humble/include/**"

Then make the following additions to the CMakeLists.txt file,

    add_executable(cpp_node src/my_first_node.cpp)
    ament_target_dependencies(cpp_node rclcpp)

    install(TARGETS cpp_node DESTINATION lib/${PROJECT_NAME})

Steps to execute the cpp_node are similar as in the section above.

4.3. Misc

To run the same node again (as abc instead of py_test), use

 ros2 run my_py_pkg py_node --ros-args --remap __node:=[NEW_NODE_NAME]

since two nodes with same name cannot (should not) be run together.

To create a symlink to the node (so that it can be run directly without building again; only works for python)
```
 colcon build --packages-select [PACKAGE_NAME] --symlink-install
```

5. Topics

Topic is a named bus over which nodes exchange messages.

Topic is the medium over which data is transferred. A topic is associated to a msg type.
Data is unidirectional; publishers (nodes) send data to the topic, while subscribers (also nodes) receive data from the topic.
We can have multiple subscribers and publishers to a topic.
C++ publishers/subscribers can work with Python C++ subscribers.
Subscribers and publishers in python need some functions defined as callbacks, that execute some logic during publishing/subscribing.

To remap the node name and topic names, use

    ros2 run my_py_pkg py_node --ros-args -r __node:=new_node_name -r robot_news:=new_topic_name

ribhattacharya/ros2-level-1