DEV: Introduce @dedupeTracked (#27084)

Same as `@tracked`, but skips notifying consumers if the value is unchanged. This introduces some performance overhead, so should only be used where excessive downstream re-evaluations are a problem.

This is loosely based on `@dedupeTracked` in the `tracked-toolbox` package, but without the added complexity of a customizable 'comparator'. Implementing ourselves also avoids the need for pulling in the entire package, which contains some tools which we don't want, or which are now implemented in Ember/Glimmer (e.g. `@cached`).

app/assets/javascripts/discourse/app/lib/tracked-tools.js

@@ -74,3 +74,49 @@ export function resettableTracked(prototype, key, descriptor) {
     },
   };
 }
+
+/**
+ * @decorator
+ *
+ * Same as `@tracked`, but skips notifying about updates if the value is unchanged. This introduces some
+ * performance overhead, so should only be used where excessive downstream re-evaluations are a problem.
+ *
+ * @example
+ *
+ * ```js
+ * class UserRenameForm {
+ *   ⁣@dedupeTracked fullName;
+ * }
+ *
+ * const form = new UserRenameForm();
+ * form.fullName = "Alice"; // Downstream consumers will be notified
+ * form.fullName = "Alice"; // Downstream consumers will not be re-notified
+ * form.fullName = "Bob"; // Downstream consumers will be notified
+ * ```
+ *
+ */
+export function dedupeTracked(target, key, desc) {
+  let { initializer } = desc;
+  let { get, set } = tracked(target, key, desc);
+
+  let values = new WeakMap();
+
+  return {
+    get() {
+      if (!values.has(this)) {
+        let value = initializer?.call(this);
+        values.set(this, value);
+        set.call(this, value);
+      }
+
+      return get.call(this);
+    },
+
+    set(value) {
+      if (!values.has(this) || values.get(this) !== value) {
+        values.set(this, value);
+        set.call(this, value);
+      }
+    },
+  };
+}

app/assets/javascripts/discourse/tests/unit/lib/tracked-tools-test.gjs

@@ -0,0 +1,48 @@
+import { cached } from "@glimmer/tracking";
+import { module, test } from "qunit";
+import { dedupeTracked } from "discourse/lib/tracked-tools";
+
+module("Unit | tracked-tools", function () {
+  test("@dedupeTracked", async function (assert) {
+    class Pet {
+      initialsEvaluatedCount = 0;
+
+      @dedupeTracked name;
+
+      @cached
+      get initials() {
+        this.initialsEvaluatedCount++;
+        return this.name
+          ?.split(" ")
+          .map((n) => n[0])
+          .join("");
+      }
+    }
+
+    const pet = new Pet();
+    pet.name = "Scooby Doo";
+
+    assert.strictEqual(pet.initials, "SD", "Initials are correct");
+    assert.strictEqual(
+      pet.initialsEvaluatedCount,
+      1,
+      "Initials getter evaluated once"
+    );
+
+    pet.name = "Scooby Doo";
+    assert.strictEqual(pet.initials, "SD", "Initials are correct");
+    assert.strictEqual(
+      pet.initialsEvaluatedCount,
+      1,
+      "Initials getter not re-evaluated"
+    );
+
+    pet.name = "Fluffy";
+    assert.strictEqual(pet.initials, "F", "Initials are correct");
+    assert.strictEqual(
+      pet.initialsEvaluatedCount,
+      2,
+      "Initials getter re-evaluated"
+    );
+  });
+});