[1.x] feat: conditional extend whenExtensionDisabled (#4107)

* feat: conditional extend whenExtensionDisabled

* Apply fixes from StyleCI

---------

Co-authored-by: StyleCI Bot <bot@styleci.io>

framework/core/src/Extend/Conditional.php

@@ -47,6 +47,20 @@ class Conditional implements ExtenderInterface
         }, $extenders);
     }
 
+    /**
+     * Apply extenders only if a specific extension is disabled/not installed.
+     *
+     * @param string $extensionId The ID of the extension.
+     * @param ExtenderInterface[]|callable|string $extenders A callable returning an array of extenders, or an invokable class string.
+     * @return self
+     */
+    public function whenExtensionDisabled(string $extensionId, $extenders): self
+    {
+        return $this->when(function (ExtensionManager $extensions) use ($extensionId) {
+            return ! $extensions->isEnabled($extensionId);
+        }, $extenders);
+    }
+
     /**
      * Apply extenders based on a condition.
      *

framework/core/tests/integration/extenders/ConditionalTest.php

@@ -160,6 +160,27 @@ class ConditionalTest extends TestCase
         $this->app();
     }
 
+    /** @test */
+    public function conditional_disabled_extension_not_enabled_applies_extender()
+    {
+        $this->extend(
+            (new Extend\Conditional())
+                ->whenExtensionDisabled('flarum-dummy-extension', TestExtender::class)
+        );
+
+        $this->app();
+
+        $response = $this->send(
+            $this->request('GET', '/api', [
+                'authenticatedAs' => 1,
+            ])
+        );
+
+        $payload = json_decode($response->getBody()->getContents(), true);
+
+        $this->assertArrayHasKey('customConditionalAttribute', $payload['data']['attributes']);
+    }
+
     /** @test */
     public function conditional_does_not_instantiate_extender_if_condition_is_false_using_callable()
     {