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<?php
// Act on content entities when loaded from the storage.
// The results of this hook will be cached.
// It makes  possible to add necessary data to the fields wich will be available in preprocess or for render
// Or add data to entity object.

/**
 * Implements hook_entity_storage_load().
 */
function hook_entity_storage_load($entities, $entity_type) {
  $token_service = \Drupal::token();
  $replace_options = ['clear' => TRUE, 'sanitize' => FALSE];
  $config = \Drupal::config('file_entity.settings');

  // Loop over all the entities looking for entities with attached images.
  foreach ($entities as $entity) {
    /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
    // Examine every image field instance attached to this entity's bundle.
    foreach ($entity->getFieldDefinitions() as $field_definition) {
      $field_name = $field_definition->getName();
      $types[] = $field_definition->getType();

      if ($field_definition->getType() == 'image') {

        if (!empty($entity->{$field_name})) {

          /** @var \Drupal\image\Plugin\Field\FieldType\ImageItem $item */
          foreach ($entity->$field_name as $delta => $item) {
            // If alt and title text is not specified, fall back to alt and
            // title text on the file entity type.
            $values = $item->getValue();
            if (!empty($item->target_id) && (empty($item->alt) || empty($item->title))) {
              foreach (['alt', 'title'] as $key) {
                if (empty($item->$key)) {
                  $token_bubbleable_metadata = new BubbleableMetadata();
                  $values[$key] = $token_service->replace($config->get($key), ['file' => $item->entity], $replace_options, $token_bubbleable_metadata);
                  // Add the cacheability metadata of the token to the entity.
                  // This means attachments are discarded, but it does not ever
                  // make sense to have attachments for an image's "alt" and
                  // "title"attribute anyway, so this is acceptable.
                  $entity->addCacheableDependency($token_bubbleable_metadata);
                }
              }
              $item->setValue($values);
            }
          }
        }
      }
    }
  }

}