go-kwin6

Golang KWin6 interfacing example for enumerating and manipulting Plasma/Wayland windows around ¹

This project is an experiment in trying to interface the KWin window compositor with the goal of listing the registered Screens, Desktops and Windows and manipulating the Windows placement/geometry.

My OS is Arch Linux, running KDE Plasma 6 on Wayland graphics platform.

I have a case where on my home/work machine I have 4 screens and some 20 virtual desktops, each one for a specific activity, with relevant applications loaded and open there. Currently, Wayland does not support proper session storing/restoring, so each time I reboot the computer I need to manually start some 50-ish applications and move them to their relevant virtual desktop and screen. I wanted to automate this process by automatically launching them all at startup and then moving them to their places. This code deals with the "moving" activity.

I tried searching the internets for ready solutions and I didn't seem to find any, so I decided to make my own. After a bit of fiddling with wmctrl - I realized I can't make it work for Wayland, and it seemed like the only option is to utilize KWin scripting infrastructure. KWin is capable of loading JavaScript scriptlets from a file, registering them under a consecutive number, and then they can be executed (or deregistered) using this reference number. If the script needs to produce any output - it does so in the system journal and can be gathered using journalctl by specifyng filters on time window in which the script was running and a specific QT_ debug flags. In order for this to work, the following environment variable needs to be set:

export QT_LOGGING_RULES="kwin_*.debug=true"

The way this library works is it generates a dynamic JavaScript scriptlet for each activity I want KWin to execute. This is the workflow:

1. Generate the JavaScript scriptlet as a string
2. Store the scriptlet in a temp file, accessible by KWin
3. Load and register it in KWin, returning the registration ID (usually next available scriptlet number in KWin)
4. Call KWin to run the registered scriptlet using its registration number
5. Call KWin to deregister the scriptlet, thus releasing the scriptlet number
6. Delete the JavaScript scriptlet file
7. Gather all journalctl information during the time window in which the scriptlet was running

The scriptlets are written in a manner, which generates JSON strings as an output in the journal, for easy Go struct demarshalling. I tried to keep them in their relevant methods so that whoever wants to reuse them in different language or even manually, can just copy/paste/extract them and put them in their code.

To test the scripts themselves, I have written a bash script that loads, runs and deregisters a sample scriptlet and returns the collected journalctl information. In this case I have left the scriptlet with the code that returns the registered Screens. These can be found in script-tests folder.

In general the operations are as follows:

Load and register the scriptlet with KWin scripting infrastructure, and gather the script registration number:

scriptNo=$(dbus-send --print-reply --dest=org.kde.KWin /Scripting org.kde.kwin.Scripting.loadScript string:nameOfTheFileContainingTheScriptlet | tail -1 | cut -c 10-11)

Mark the time at the script start, we will need this later when we query the journal for the script output:

et=$(date "+%Y-%m-%d %H:%M:%S.%N")

Run the script:

dbus-send --print-reply --dest=org.kde.KWin /Scripting/Script${scriptNo} org.kde.kwin.Script.run

Stop and deregister the script

dbus-send --print-reply --dest=org.kde.KWin /Scripting/Script${scriptNo} org.kde.kwin.Script.stop

Mark the time at script end, we will need this later when we query the journal for the script output:

et=$(date "+%Y-%m-%d %H:%M:%S.%N")

Gather the KWin script execution output in journal, for the time script was running

journalctl QT_CATEGORY=js QT_CATEGORY=kwin_scripting -o cat --since "${st}" --until "${et}" --no-pager

This is my first attempt in both JavaScript and KWin interfacing, so the above may not be the most optimal solution. It may also stop working if KWin people change the KWin internals in future versions. For example, this will NOT work on KWin5 and below, because the internal object/method names are different.

TODO: Add capabilities for setting window geometry. I don't seem to need that at this point though.

Footnotes

1. should also work in X11 ↩