DEV: Use preferences-save-attributes transformer for custom_fields (#164)

discourse/discourse#36658 introduced a `preferences-save-attributes`
value transformer, changing `saveAttrNames` from a mutable property
to a getter. This broke the previous approach of pushing to the array
directly in the save action.

Update to use the new transformer API with backward compatibility
for older Discourse versions that don't have this transformer.

Author: ZogStriP

assets/javascripts/discourse/initializers/follow-initializer.js

@@ -1,5 +1,6 @@
 import { action } from "@ember/object";
 import { withPluginApi } from "discourse/lib/plugin-api";
+import { VALUE_TRANSFORMERS } from "discourse/lib/transformer/registry";
 import { userPath } from "discourse/lib/url";
 import { i18n } from "discourse-i18n";
 
@@ -53,21 +54,33 @@ export default {
         );
       }
 
-      // workaround to make core save custom fields when changing
-      // preferences
-      api.modifyClass(
-        "controller:preferences/notifications",
-        (Superclass) =>
-          class extends Superclass {
-            @action
-            save() {
-              if (!this.saveAttrNames.includes("custom_fields")) {
-                this.saveAttrNames.push("custom_fields");
-              }
-              super.save();
+      // Add custom_fields to the notifications page save attributes
+      if (VALUE_TRANSFORMERS.includes("preferences-save-attributes")) {
+        api.registerValueTransformer(
+          "preferences-save-attributes",
+          ({ value: attrs, context }) => {
+            if (context.page === "notifications") {
+              attrs.push("custom_fields");
             }
+            return attrs;
           }
-      );
+        );
+      } else {
+        // Backward compatibility for older Discourse versions
+        api.modifyClass(
+          "controller:preferences/notifications",
+          (Superclass) =>
+            class extends Superclass {
+              @action
+              save() {
+                if (!this.saveAttrNames.includes("custom_fields")) {
+                  this.saveAttrNames.push("custom_fields");
+                }
+                super.save();
+              }
+            }
+        );
+      }
     });
   },
 };