sway/waybar: Add abstraction for unit management

This also passes the unit state as a stream instead of requiring waybar
to poll for the state.

users/simon/modules/sway.nix

@@ -293,6 +293,19 @@ lib.mkIf nixosConfig.sbruder.gui.enable {
   # home-manager’s waybar module performs additional checks that are overly strict
   xdg.configFile."waybar/config".text =
     let
+      watchUserUnitState = unit: started: stopped: pkgs.writeShellScript "watch-user-unit-${unit}-state" ''
+        ${pkgs.systemd}/bin/journalctl --user -u ${unit} -t systemd -o cat -f \
+            | ${pkgs.gnugrep}/bin/grep --line-buffered -Eo '^(Started|Stopped)' \
+            | ${pkgs.jq}/bin/jq --unbuffered -Rc 'if . == "Started" then ${builtins.toJSON started} else ${builtins.toJSON stopped} end'
+      '';
+
+      toggleUserUnitState = unit: pkgs.writeShellScript "toggle-user-unit-${unit}-state" ''
+        if ${pkgs.systemd}/bin/systemctl --user show ${unit} | ${pkgs.gnugrep}/bin/grep -q ActiveState=active; then
+            ${pkgs.systemd}/bin/systemctl --user stop ${unit}
+        else
+            ${pkgs.systemd}/bin/systemctl --user start ${unit}
+        fi
+      '';
     in
     lib.generators.toJSON { } {
       layer = "top";
@@ -332,21 +345,11 @@ lib.mkIf nixosConfig.sbruder.gui.enable {
         spacing = 5;
       };
       "custom/redshift" = {
-        interval = 10;
+        exec = watchUserUnitState
-        exec = pkgs.writeShellScript "waybar-redshift-state" ''
+          "redshift"
-          if systemctl --user show redshift | grep -q ActiveState=active; then
+          { class = "active"; }
-            echo '{"tooltip": "Redshift active", "class": "active"}'
+          { class = "inactive"; };
-          else
+        on-click = toggleUserUnitState "redshift";
-            echo '{"tooltip": "Redshift inactive", "class": "inactive"}'
-          fi
-        '';
-        on-click = pkgs.writeShellScript "waybar-redshift-toggle" ''
-          if systemctl --user show redshift | grep -q ActiveState=active; then
-            systemctl --user stop redshift
-          else
-            systemctl --user start redshift
-          fi
-        '';
         return-type = "json";
         format = "";
         tooltip = false;