way-displays: support stateful configuration file

Author: mightyiam

modules/services/way-displays.nix

@@ -14,22 +14,40 @@ let
     mkEnableOption
     mkIf
     mkOption
+    optionalAttrs
+    optionalString
     types
     ;
   mergeSets = sets: lists.fold attrsets.recursiveUpdate { } sets;
   cfg = config.services.way-displays;
+  serviceName = "way-displays" + optionalString cfg.statefulConfiguration "@";
   yaml = pkgs.formats.yaml { };
 in
 {
-  meta.maintainers = [ maintainers.jolars ];
+  meta.maintainers = [
+    maintainers.jolars
+    maintainers.mightyiam
+  ];
 
   options.services.way-displays = {
     enable = mkEnableOption "way-displays";
 
     package = lib.mkPackageOption pkgs "way-displays" { };
 
+    statefulConfiguration = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        A configuration file is not generated.
+        Allows way-displays to write its own configuration.
+
+        The service is _not_ started automatically.
+        It can be started by running `systemctl --user start "way-displays@$XDG_VTNR".service`.
+      '';
+    };
+
     settings = mkOption {
-      type = yaml.type;
+      type = types.nullOr yaml.type;
       default = { };
       example = literalExpression ''
         {
@@ -77,35 +95,49 @@ in
   config = mkIf cfg.enable {
     assertions = [
       (lib.hm.assertions.assertPlatform "services.way-displays" pkgs lib.platforms.linux)
+      {
+        assertion = cfg.statefulConfiguration -> cfg.settings == null;
+        message = "way-displays: when `statefulConfiguration = true` then `settings` must be `null`";
+      }
+      {
+        assertion = (!cfg.statefulConfiguration) -> cfg.settings != null;
+        message = "way-displays: `settings` must be non-null unless `statefulConfiguration = true`";
+      }
     ];
 
-    xdg.configFile."way-displays/cfg.yaml".source =
-      yaml.generate "way-displays-config.yaml"
-        (mergeSets [
-          {
-            CALLBACK_CMD = "${pkgs.libnotify}/bin/notify-send \"way-displays \${CALLBACK_LEVEL}\" \"\${CALLBACK_MSG}\"";
-          }
-          cfg.settings
-        ]);
+    home.packages = [ cfg.package ];
 
-    systemd.user.services.way-displays = {
-      Unit = {
-        Description = "Display configuration service";
-        Documentation = "man:way-displays(1)";
-        ConditionEnvironment = "WAYLAND_DISPLAY";
-        PartOf = cfg.systemdTarget;
-        Requires = cfg.systemdTarget;
-        After = cfg.systemdTarget;
-      };
+    xdg.configFile."way-displays/cfg.yaml" = mkIf (!cfg.statefulConfiguration) {
+      source = yaml.generate "way-displays-config.yaml" (mergeSets [
+        {
+          CALLBACK_CMD = "${pkgs.libnotify}/bin/notify-send \"way-displays \${CALLBACK_LEVEL}\" \"\${CALLBACK_MSG}\"";
+        }
+        cfg.settings
+      ]);
+    };
+
+    systemd.user.services.${serviceName} = {
+      Unit =
+        {
+          Description = "Display configuration service";
+          Documentation = "man:way-displays(1)";
+          ConditionEnvironment = "WAYLAND_DISPLAY";
+          Requisite = cfg.systemdTarget;
+          After = cfg.systemdTarget;
+        }
+        // optionalAttrs (!cfg.statefulConfiguration) {
+          PartOf = cfg.systemdTarget;
+        };
 
-      Install = {
+      Install = mkIf (!cfg.statefulConfiguration) {
         WantedBy = [ cfg.systemdTarget ];
       };
 
       Service = {
         Type = "simple";
         ExecStart = "${lib.getExe cfg.package}";
         Restart = "always";
+        Environment = mkIf (cfg.statefulConfiguration) "XDG_VTNR=%i";
       };
     };
   };