nixos-config/home-manager/roles/wallpaper.nix

{
  pkgs,
  config,
  ...
}: let
  modeFile = "${config.home.homeDirectory}/.mode";
  wallPapers = "${config.home.homeDirectory}/media/images/wallpapers";
  randomWallpaper = pkgs.writeHaskellScript
  {
    name = "random-wallpaper";
    imports = ["System.Random"];
    bins = [pkgs.coreutils pkgs.glib];
  } ''
    main = do
       mode <- cat "${modeFile}" |> captureTrim
       (lines . decodeUtf8 -> files) <- ls ([i|${wallPapers}/#{mode}|] :: String) |> captureTrim
       ((files Unsafe.!!) -> file) <- getStdRandom $ randomR (0, length files - 1)
       (decodeUtf8 -> current) <- gsettings "get" "org.gnome.desktop.background" "picture-uri" |> captureTrim
       let new = [i|file:///${wallPapers}/#{mode}/#{file}|] :: String
       when (new /= current) $ do
         gsettings "set" "org.gnome.desktop.background" "picture-uri" new
         gsettings "set" "org.gnome.desktop.screensaver" "picture-uri" new
  '';
in {
  home.packages = [randomWallpaper];
  systemd.user = {
    services.random-wallpaper = {
      Unit = {Description = "Random Wallpaper";};
      Service = {
        ExecStart = "${randomWallpaper}/bin/random-wallpaper";
        Type = "oneshot";
      };
    };
    timers.random-wallpaper = {
      Timer = {
        OnCalendar = "*:00/30:00";
        OnActiveSec = 1;
      };
      Install = {WantedBy = ["timers.target"];};
    };
  };
}