class EntityTestController

class EntityTestController extends ControllerBase {
    
    /**
     * Returns an empty page.
     *
     * @see \Drupal\entity_test\Routing\EntityTestRoutes::routes()
     */
    public function testAdmin() {
        return [];
    }
    
    /**
     * List entity_test entities referencing the given entity.
     *
     * @param string $entity_reference_field_name
     *   The name of the entity_reference field to use in the query.
     * @param string $referenced_entity_type
     *   The type of the entity being referenced.
     * @param int $referenced_entity_id
     *   The ID of the entity being referenced.
     *
     * @return array
     *   A renderable array.
     */
    public function listReferencingEntities($entity_reference_field_name, $referenced_entity_type, $referenced_entity_id) {
        // Early return if the referenced entity does not exist (or is deleted).
        $referenced_entity = $this->entityTypeManager()
            ->getStorage($referenced_entity_type)
            ->load($referenced_entity_id);
        if ($referenced_entity === NULL) {
            return [];
        }
        $query = $this->entityTypeManager()
            ->getStorage('entity_test')
            ->getQuery()
            ->accessCheck(TRUE)
            ->condition($entity_reference_field_name . '.target_id', $referenced_entity_id);
        $entities = $this->entityTypeManager()
            ->getStorage('entity_test')
            ->loadMultiple($query->execute());
        return $this->entityTypeManager()
            ->getViewBuilder('entity_test')
            ->viewMultiple($entities, 'full');
    }
    
    /**
     * List entities of the given entity type labels, sorted alphabetically.
     *
     * @param string $entity_type_id
     *   The type of the entity being listed.
     *
     * @return array
     *   A renderable array.
     */
    public function listEntitiesAlphabetically($entity_type_id) {
        $entity_type_definition = $this->entityTypeManager()
            ->getDefinition($entity_type_id);
        $query = $this->entityTypeManager()
            ->getStorage($entity_type_id)
            ->getQuery()
            ->accessCheck(TRUE);
        // Sort by label field, if any.
        if ($label_field = $entity_type_definition->getKey('label')) {
            $query->sort($label_field);
        }
        $entities = $this->entityTypeManager()
            ->getStorage($entity_type_id)
            ->loadMultiple($query->execute());
        $cache_tags = [];
        $labels = [];
        foreach ($entities as $entity) {
            $labels[] = $entity->label();
            $cache_tags = Cache::mergeTags($cache_tags, $entity->getCacheTags());
        }
        // Always associate the list cache tag, otherwise the cached empty result
        // wouldn't be invalidated. This would continue to show nothing matches the
        // query, even though a newly created entity might match the query.
        $cache_tags = Cache::mergeTags($cache_tags, $entity_type_definition->getListCacheTags());
        return [
            '#theme' => 'item_list',
            '#items' => $labels,
            '#title' => $entity_type_id . ' entities',
            '#cache' => [
                'contexts' => $entity_type_definition->getListCacheContexts(),
                'tags' => $cache_tags,
            ],
        ];
    }
    
    /**
     * Empty list of entities of the given entity type.
     *
     * Empty because no entities match the query. That may seem contrived, but it
     * is an excellent way for testing whether an entity's list cache tags are
     * working as expected.
     *
     * @param string $entity_type_id
     *   The type of the entity being listed.
     *
     * @return array
     *   A renderable array.
     */
    public function listEntitiesEmpty($entity_type_id) {
        $entity_type_definition = $this->entityTypeManager()
            ->getDefinition($entity_type_id);
        return [
            '#theme' => 'item_list',
            '#items' => [],
            '#cache' => [
                'contexts' => $entity_type_definition->getListCacheContexts(),
                'tags' => $entity_type_definition->getListCacheTags(),
            ],
        ];
    }

}