home-manager/modules/services/random-background.nix

{ config, lib, pkgs, ... }:

with lib;

let

  cfg = config.services.random-background;

in

{
  options = {
    services.random-background = {
      enable = mkEnableOption "random desktop background";

      imageDirectory = mkOption {
        type = types.str;
        description =
          ''
            The directory of images from which a background should be
            chosen. Should be formatted in a way understood by
            systemd, e.g., '%h' is the home directory.
          '';
      };

      interval = mkOption {
        default = null;
        type = types.nullOr types.str;
        description = ''
          The duration between changing background image, set to null
          to only set background when logging in.

          Should be formatted as a duration understood by systemd.
        '';
      };
    };
  };

  config = mkIf cfg.enable (
    mkMerge ([
      {
        systemd.user.services.random-background = {
          Unit = {
            Description = "Set random desktop background using feh";
            After = [ "graphical-session-pre.target" ];
            PartOf = [ "graphical-session.target" ];
          };

          Service = {
            Type = "oneshot";
            ExecStart = "${pkgs.feh}/bin/feh --randomize --bg-fill ${cfg.imageDirectory}";
            IOSchedulingClass = "idle";
          };

          Install = {
            WantedBy = [ "graphical-session.target" ];
          };
        };
      }
      (mkIf (cfg.interval != null) {
        systemd.user.timers.random-background = {
          Unit = {
            Description = "Set random desktop background using feh";
          };

          Timer = {
            OnUnitActiveSec = cfg.interval;
          };

          Install = {
            WantedBy = [ "timers.target" ];
          };
        };
      })
    ]));
}