cannot setup swww due to WAYLAND_DISPLAY not set in nixos

[fvrings]

I have a systemd config to setup swww on nixos:

{
  pkgs,
  config,
  ...
}:

let
  swww-daemon = "swww-daemon.service";
  graphical = config.wayland.systemd.target;

in

{
  # services.swww.enable = true;
  systemd.user = {
    services.swww = {

      Unit = {
        Description = "swww image setter";
        After = [ swww-daemon ];
        Requires = [ swww-daemon ];
      };

      Install = {
        WantedBy = [ swww-daemon ];
      };

      Service = {
        ExecStart = "${pkgs.swww}/bin/swww --transition-type random --transition-fps 60 --transition-duration 2 ${config.theme.wallpaper-anime}";
        # ExecStart = "${pkgs.swww}/bin/swww  ${config.theme.wallpaper-anime}";
        Restart = "on-failure";
        RestartSec = 1;
      };
    };
    services.swww-daemon = {

      Unit = {
        Description = "swww-daemon";
        After = [ graphical ];
        PartOf = [ graphical ];
      };

      Install = {
        WantedBy = [ graphical ];
      };

      Service = {
        ExecStart = "${pkgs.swww}/bin/swww-daemon";
        Restart = "always";
        RestartSec = 1;
      };
    };
  };
}

which results in:

⋈┈👾 systemctl status --user swww-daemon
● swww-daemon.service - swww-daemon
     Loaded: loaded (/home/ring/.config/systemd/user/swww-daemon.service; enabled; preset: ignored)
     Active: active (running) since Mon 2025-07-14 14:12:36 CST; 1min 52s ago
 Invocation: 81759462b050464a8baca572c22bf122
   Main PID: 2988 (swww-daemon)
      Tasks: 1 (limit: 23516)
     Memory: 296K (peak: 1.6M)
        CPU: 5ms
     CGroup: /user.slice/user-1000.slice/user@1000.service/app.slice/swww-daemon.service
             └─2988 /nix/store/kmjf15g45iqnqf84mgy5d6rr817n0718-swww-0.10.3/bin/swww-daemon

Jul 14 14:12:36 art systemd[2572]: Started swww-daemon.
Jul 14 14:12:36 art swww-daemon[2988]:          0ms [WARN]  WAYLAND_DISPLAY is not set! Defaulting to wayland-0
Jul 14 14:12:36 art swww-daemon[2988]: WARNING: WAYLAND_DISPLAY variable not set. Defaulting to wayland-0
Jul 14 14:12:36 art swww-daemon[2988]:          0ms [INFO]  selected pixel format: Bgr
Jul 14 14:12:36 art swww-daemon[2988]:          0ms [WARN]  failed to load cache: No such file or directory (os error 2)

I think WAYLAND_DISPLAY should be set here,because I found that there is a condition requirement in sww in home-manger.

EDIT:rename nixpkgs to home-manager.

[fvrings]

Looks like I have missed the img arg in swww's ExecStart. 😅
But I might be a problem if using services.swww.enable from home-manager.