materio-d8/web/modules/custom/images_styles_gen/src/CreateImagesStyles.php

<?php

namespace Drupal\images_styles_gen;


// use Drupal\node\Entity\Node;

class CreateImagesStyles {

  public static function createStyles($item, &$context){
    if (!array_key_exists('progress', $context['sandbox'])) {
      $context['sandbox']['progress'] = 0;
    }
    $context['sandbox']['progress']++;
    $context['sandbox']['current_file'] = $item->uri;
    // $message = 'Creating Styles ...';
    $results = array();

    $results[] = $item->style->createDerivative($item->uri, $item->destination);

    $context['message'] = $item->uri . ' processed.';
    $context['results'][] = $item->destination;
  }

  public static function createStylesFinishedCallback($success, $results, $operations) {
    // The 'success' parameter means no fatal PHP errors were detected. All
    // other error management should be handled using 'results'.
    if ($success) {
      $message = \Drupal::translation()->formatPlural(
        count($results),
        'One image processed.', '@count image processed.'
      );
    }
    else {
      $message = t('Finished with an error.');
    }
    drupal_set_message($message);
  }
}