Enable only us keyboard layout in the KDE Plasma screenlocker and disable others

kscreenlocker-layout-switcher.sh

#!/bin/bash

# MIT License
#
# Copyright (c) 2024 Ofee Oficsu
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

# inspired by:
# https://alexsorokin.ru/2019/01/plasma-lock-screen-kb-layout
# https://superuser.com/a/898991
# https://superuser.com/a/820619
# https://unix.stackexchange.com/a/368632
# https://gist.github.com/bojidar-bg/5dd246cd3c442893a333b36955ae0753
# https://discuss.kde.org/t/re-read-configuration-after-setting-change/15008

PLASMA_VERSION_MAJOR=$(kwin_x11 --version | awk '{print $2}' | awk -F '.' '{print $1}')

kwriteconfig() {
  command "kwriteconfig$PLASMA_VERSION_MAJOR" "$@"
}

read-default-layout-list() {
  grep -oP 'LayoutList=\K.*' "$HOME/.config/kxkbrc"
}

update-layouts() {
  local list="$1"
  kwriteconfig --file kxkbrc --group Layout --key LayoutList "$list"
}

apply-layouts() {
  dbus-send --session --type=signal --reply-timeout=1000 \
            --dest=org.kde.keyboard /Layouts             \
            org.kde.keyboard.reloadConfig
}

monitor-screensaver() {
  local signal="type=signal"
  local saver="path=/ScreenSaver"

  local freedesktop_saver="interface=org.freedesktop.ScreenSaver"
  local kde_saver="interface=org.kde.screensaver"
  local layouts="interface=org.kde.KeyboardLayouts"

  # well-documented way to catch screensaver state changes
  local state="$signal,$saver,$freedesktop_saver,member=ActiveChanged"

  # is not well documented, but it is triggered before the
  # lockscreen appears, which hides flickering of the text
  local about="$signal,$saver,$kde_saver,member=AboutToLock"

  local layouts="$layouts,member=layoutListChanged"

  dbus-monitor "$state" "$about" "$layouts"
}

is-ready-to-lockswitch() {
  local prev="$1"
  local line="$2"

  grep -q "AboutToLock" <<< "$line" \
  && return 0

  # on kwin_x11 sometimes windows (including screenlocker)
  # are skipped, so do it second time anyway, just in case
  # also would be helpful if 'AboutToLock' api was changed
  grep -q "ActiveChanged" <<< "$prev"  \
  && grep -q "boolean true" <<< "$line" \
  && return 0

  return 1
}

is-ready-to-unlockswitch() {
  local prev="$1"
  local line="$2"

  grep -q "ActiveChanged" <<< "$prev"   \
  && grep -q "boolean false" <<< "$line"
}

request-layout-list() {
  dbus-send --print-reply --session --reply-timeout=500 \
            --dest=org.kde.keyboard /Layouts            \
            org.kde.KeyboardLayouts.getLayoutsList
}

has-us-layout() {
  local stdin="$1"
  echo "$stdin" | grep -q 'string "us"'
}

is-single-us-layout() {
  local stdin=$(cat)
  local layout_count="$(echo "$stdin" | grep struct -c)"
  local is_us=$(echo "$stdin" | grep -q 'string "us"')

  [ "$layout_count" == 1 ] \
  && has-us-layout "$stdin"
}

is-layout-list-changed() {
  local line="$1"
  local time="$2"
  local lasttime="$3"
  local state="$4"

  grep -q "layoutListChanged" <<< "$line" \
  && [ "$time" -gt "$(($lasttime + 1))" ]  \
  && [ "$state" == unlocked ]               \
  && (request-layout-list | is-single-us-layout)
}

process() {
  local default_layouts="$(read-default-layout-list)"

  local state=unlocked
  local time lasttime="$(date +%s)"
  local line prev

  while read -r line; do
    time="$(date +%s)"

    if is-ready-to-lockswitch "$prev" "$line"; then
      default_layouts="$(read-default-layout-list)"
      echo "locked, set layout to 'us'"

      update-layouts "us"
      apply-layouts

      # apply-layouts is non-blocking, which can result in
      # a race condition with the following update-layouts
      sleep 1

      # make sure default layouts outlive lockscreen crash
      update-layouts "$default_layouts"

      state=locked
    elif is-ready-to-unlockswitch "$prev" "$line"; then
      echo "unlocked, set layout to 'us,ru'"

      update-layouts "$default_layouts"
      apply-layouts

      state=unlocked
    elif is-layout-list-changed "$line" "$time" "$lasttime" "$state"; then
       echo "xorg bug: layout list wasn't recovered for a window"

       apply-layouts

       lasttime="$(date +%s)"
    fi
    prev="$line"
  done
}

monitor-screensaver | process