scroller_module.php

<?php

/**
 * @file
 * My Scroller Views module help and theme functions.
 */

// Store Scroller preprocess theme functions in a separate .inc file.
\Drupal::moduleHandler()->loadInclude('my_scroller', 'inc', 'my_scroller.theme');

/**
 * Implements hook_theme().
 */
function my_scroller_theme($existing, $type, $theme, $path) {
  return array(
    'scroller' => array(
      'file' => 'my_scroller.theme.inc',
    ),
  );
}