light: support brightness control keys

nixos/modules/programs/light.nix

@@ -9,6 +9,7 @@ in
 {
   options = {
     programs.light = {
+
       enable = mkOption {
         default = false;
         type = types.bool;
@@ -17,11 +18,60 @@ in
           and udev rules granting access to members of the "video" group.
         '';
       };
+
+      brightnessKeys = {
+        enable = mkOption {
+          type = types.bool;
+          default = false;
+          description = ''
+            Whether to enable brightness control with keyboard keys.
+
+            This is mainly useful for minimalistic (desktop) environments. You
+            may want to leave this disabled if you run a feature-rich desktop
+            environment such as KDE, GNOME or Xfce as those handle the
+            brightness keys themselves. However, enabling brightness control
+            with this setting makes the control independent of X, so the keys
+            work in non-graphical ttys, so you might want to consider using this
+            instead of the default offered by the desktop environment.
+
+            Enabling this will turn on {option}`services.actkbd`.
+          '';
+        };
+
+        step = mkOption {
+          type = types.int;
+          default = 10;
+          description = ''
+            The percentage value by which to increase/decrease brightness.
+          '';
+        };
+
+      };
+
     };
   };
 
   config = mkIf cfg.enable {
     environment.systemPackages = [ pkgs.light ];
     services.udev.packages = [ pkgs.light ];
+    services.actkbd = mkIf cfg.brightnessKeys.enable {
+      enable = true;
+      bindings = let
+        light = "${pkgs.light}/bin/light";
+        step = toString cfg.brightnessKeys.step;
+      in [
+        {
+          keys = [ 224 ];
+          events = [ "key" ];
+          # Use minimum brightness 0.1 so the display won't go totally black.
+          command = "${light} -N 0.1 && ${light} -U ${step}";
+        }
+        {
+          keys = [ 225 ];
+          events = [ "key" ];
+          command = "${light} -A ${step}";
+        }
+      ];
+    };
   };
 }