We'll be using python3-vcstool to retrieve the ROS2 source code. On a Debian/Ubuntu system we can just install it and the rest of the dependencies with the following:

$ sudo apt install wget git python3-vcstool qemu-user-static python3-parted python3-requests

Now that we have the required dependencies for fetching the ROS2 source code, we can now create a ROS2 workspace:

$ mkdir -p ~/ros2_rpi/ros2_ws/src
$ cd ~/ros2_rpi/ros2_ws
$ wget https://raw.githubusercontent.com/ros2/ros2/master/ros2.repos
$ vcs import src < ros2.repos

For crosscompiling ROS2, we're going to use Polly. Polly is a collection of CMake scripts for compiling software for a huge variety of targets, including the Raspberry Pi, iOS and many others.

$ cd ~/ros2_rpi
$ git clone https://github.com/ruslo/polly.git

We're going to use a Docker image as the host compiler, so let's clone this repository and build the Docker image for the cross compiler:

$ cd ~/ros2_rpi
$ git clone https://github.com/esteve/ros2_raspbian_tools.git
$ cd ~/ros2_rpi/ros2_raspbian_tools
$ cat Dockerfile.bootstrap | docker build -t ros2-raspbian:crosscompiler -

The convert_raspbian_docker.py script will fetch the latest Raspbian image and convert it into a Docker image so it can be run on the host as a Docker container.

$ cd ~/ros2_rpi/ros2_raspbian_tools
$ ./convert_raspbian_docker.py ros2-raspbian

Alternatively, the convert_raspbian_docker.py script can also convert an already downloaded Raspbian image instead of fetching it:

$ cd ~/ros2_rpi/ros2_raspbian_tools
$ ./convert_raspbian_docker.py -i raspbian_lite_latest ros2-raspbian

The convert_raspbian_docker.py script can also upload the generated image to Dockerhub or fetch the Desktop variant of Raspbian, type convert_raspbian_docker.py -h to see all the options that the script accepts.

Now that we have a Raspbian Docker image, we can just run a container for it and execute any commands as if we were running them on an actual Raspberry. We'll use that to install the ROS2 dependencies on the container and export the filesystem of the container to the host machine. Let's use the export_raspbian_image.py script included in this repository to automate the process:

$ cd ~/ros2_rpi/ros2_raspbian_tools
$ ./export_raspbian_image.py ros2-raspbian:lite ros2_dependencies.bash ros2-raspbian-rootfs.tar

The script will generate a ros2-raspbian-rootfs.tar file with the contents of a Raspbian filesystem with all the ROS2 dependencies already included.

We can finally crosscompile ROS2 for the Raspberry Pi!

$ mkdir -p ~/ros2_rpi/rpi-root
$ cd ~/ros2_rpi/ros2_raspbian_tools
$ sudo tar -C ~/ros2_rpi/rpi-root -xvf ros2-raspbian-rootfs.tar
$ docker run -it --rm \
    -v ~/ros2_rpi/polly:/polly \
    -v ~/ros2_rpi/ros2_ws:/ros2_ws \
    -v ~/ros2_rpi/ros2_raspbian_tools/build_ros2.bash:/build_ros2.bash \
    -v ~/ros2_rpi/rpi-root:/raspbian_ros2_root \
    -w /ros2_ws \
    ros2-raspbian:crosscompiler \
    bash /build_ros2.bash

The newly built ROS2 workspace can be copied to a Raspberry with scp:

$ scp -r ~/ros2_rpi/ros2_ws/install_isolated RASPBERRY_PI:/home/pi/

And now the ROS2 examples and demos (both C++ and Python) can be run on the Raspberry Pi, just make sure to source the corresponding environment:

$ source /home/pi/install_isolated/local_setup.bash
% ros2 run demo_nodes_py talker