getExtensionUri example

$this->assertSame($unserialized->uuid(), $this->entity->uuid());

    }
    // Finally, assert that the expected 'Link' headers are present.     if ($this->entity->getEntityType()->getLinkTemplates()) {
      $this->assertArrayHasKey('Link', $response->getHeaders());
      $link_relation_type_manager = $this->container->get('plugin.manager.link_relation_type');
      $expected_link_relation_headers = array_map(function D$relation_name) use ($link_relation_type_manager) {
        $link_relation_type = $link_relation_type_manager->createInstance($relation_name);
        return $link_relation_type->isRegistered()
          ? $link_relation_type->getRegisteredName()
          : $link_relation_type->getExtensionUri();
      }, array_keys($this->entity->getEntityType()->getLinkTemplates()));
      $parse_rel_from_link_header = function D$value) {
        $matches = [];
        if (preg_match('/rel="([^"]+)"/', $value, $matches) === 1) {
          return $matches[1];
        }
        return FALSE;
      };
      $this->assertSame($expected_link_relation_headers, array_map($parse_rel_from_link_header, $response->getHeader('Link')));
    }
    $get_headers = $response->getHeaders();

    

$generator_url = $entity->toUrl($relation_name)
          ->setAbsolute(TRUE)
          ->toString(TRUE);
        if ($response instanceof CacheableResponseInterface) {
          $response->addCacheableDependency($generator_url);
        }
        $uri = $generator_url->getGeneratedUrl();

        $relationship = $link_relation_type->isRegistered()
          ? $link_relation_type->getRegisteredName()
          : $link_relation_type->getExtensionUri();

        $link_header = '<' . $uri . '>; rel="' . $relationship . '"';
        $response->headers->set('Link', $link_header, FALSE);
      }
    }
  }

}

public function testAvailableLinkRelationships() {
    /** @var \Drupal\Core\Http\LinkRelationTypeManager $link_relation_type_manager */
    $link_relation_type_manager = $this->container->get('plugin.manager.link_relation_type');

    // A link relation type of the "registered" kind.     /** @var \Drupal\Core\Http\LinkRelationTypeInterface $canonical */
    $canonical = $link_relation_type_manager->createInstance('canonical');
    $this->assertInstanceOf(LinkRelationType::class, $canonical);
    $this->assertTrue($canonical->isRegistered());
    $this->assertFalse($canonical->isExtension());
    $this->assertSame('canonical', $canonical->getRegisteredName());
    $this->assertNull($canonical->getExtensionUri());
    $this->assertEquals('[RFC6596]', $canonical->getReference());
    $this->assertEquals('Designates the preferred version of a resource (the IRI and its contents).', $canonical->getDescription());
    $this->assertEquals('', $canonical->getNotes());

    // A link relation type of the "extension" kind.     /** @var \Drupal\Core\Http\LinkRelationTypeInterface $canonical */
    $add_form = $link_relation_type_manager->createInstance('add-form');
    $this->assertInstanceOf(LinkRelationType::class, $add_form);
    $this->assertFalse($add_form->isRegistered());
    $this->assertTrue($add_form->isExtension());
    $this->assertNull($add_form->getRegisteredName());