Add method to check if the route is a jsonapi resource route

Closes #3198310

src/Unstable/Routing/ResourceRoutes.php

@@ -86,7 +86,7 @@ final class ResourceRoutes implements EventSubscriberInterface {
   public function decorateJsonapiResourceRoutes(RouteBuildEvent $event) {
     $route_collection = $event->getRouteCollection();
     foreach ($route_collection as $route_name => $route) {
-      if ($route->getDefault('_jsonapi_resource') === NULL) {
+      if (!self::isJsonApiResourceRequest($route->getDefaults())) {
         continue;
       }
 
@@ -211,4 +211,17 @@ final class ResourceRoutes implements EventSubscriberInterface {
     }
   }
 
+  /**
+   * Determines if the given request is for a JSON:API Resource route.
+   *
+   * * @param array $defaults
+   * *   The request's route defaults.
+   * *
+   * * @return bool
+   * *   Whether the request targets a generated route.
+   */
+  public static function isJsonApiResourceRequest(array $defaults) {
+    return !empty($defaults['_jsonapi_resource']);
+  }
+
 }