nixpkgs/nixos/modules/services/system/systemd-lock-handler.md

systemd-lock-handler

The systemd-lock-handler module provides a service that bridges D-Bus events from logind to user-level systemd targets:

• lock.target started by loginctl lock-session,
• unlock.target started by loginctl unlock-session and
• sleep.target started by systemctl suspend.

You can create a user service that starts with any of these targets.

For example, to create a service for swaylock:

{
  services.systemd-lock-handler.enable = true;

  systemd.user.services.swaylock = {
    description = "Screen locker for Wayland";
    documentation = [ "man:swaylock(1)" ];

    # If swaylock exits cleanly, unlock the session:
    onSuccess = [ "unlock.target" ];

    # When lock.target is stopped, stops this too:
    partOf = [ "lock.target" ];

    # Delay lock.target until this service is ready:
    before = [ "lock.target" ];
    wantedBy = [ "lock.target" ];

    serviceConfig = {
      # systemd will consider this service started when swaylock forks...
      Type = "forking";

      # ... and swaylock will fork only after it has locked the screen.
      ExecStart = "${lib.getExe pkgs.swaylock} -f";

      # If swaylock crashes, always restart it immediately:
      Restart = "on-failure";
      RestartSec = 0;
    };
  };
}

See upstream documentation for more information.