nixos-config/system/desktop/suspend.nix

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

let

  cfg = config.system.custom.suspend;
in {

  options.system.custom.suspend.enable = mkEnableOption "use xlock in suspend";

  config = mkIf cfg.enable {

    systemd.services.screenlock = {
      before = [ "sleep.target" ];
      requiredBy = [ "sleep.target" ];
      environment = let
        display = if (config.services.xserver.display != null) then
          config.services.xserver.display
        else
          0;
      in { DISPLAY = ":${toString display}"; };
      script = ''
        ${pkgs.xlockmore}/bin/xlock -mode life1d -size 1 &
        sleep 1
      '';
      serviceConfig = {
        SyslogIdentifier = "screenlock";
        #Type = "simple";
        Type = "forking";
        User = config.users.users.mainUser.name;
      };
    };

    services.logind.lidSwitch = "suspend";

  };
}