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(),
      ],
    ];
  }

}