Random Teaser Image Field for Drupal 8 (valid for content type teaserbox in viewmode teaser)

TeaserboxRandomImage.php

<?php

namespace Drupal\yokai_helper\Plugin\DsField;

use Drupal\Component\Utility\UrlHelper;
use Drupal\Component\Utility\Html;
use Drupal\Core\Url;
use Drupal\ds\Plugin\DsField\DsFieldBase;

// more details here:
// https://github.com/upchuk/d8-demo-modules/tree/master/demo/src/Plugin/DsField
// http://bramg.github.io/reveal.js/#/9

/**
 * Plugin that renders a random image out of multiple (or just one) multi value image field.
 *
 * @DsField(
 *   id = "teaserbox_random_image",
 *   title = @Translation("Teaserbox random image"),
 *   entity_type = "node",
 *   provider = "yokai_helper"
 * )
 */
class TeaserboxRandomImage extends DsFieldBase {

  /**
   * {@inheritdoc}
   */
  public function build() {

    $random_teaser_image = array();

    $node = $this->entity();
    if ($node) {
      
      // get images from field_teaserbox_img
      $images = $node->get('field_teaserbox_img');

      if($images) {
        // pick a random image from all images
        $randomImageNumber = rand(0, ($images->count() - 1));
        $image = $images[$randomImageNumber]->entity;

        $variables = array(
          'responsive_image_style_id' => 'squared',
          'uri' => $image->getFileUri(),
        );

        // The image.factory service will check if our image is valid.
        $picture = \Drupal::service('image.factory')->get($image->getFileUri());
        if ($picture->isValid()) {
          $variables['width'] = $picture->getWidth();
          $variables['height'] = $picture->getHeight();
        }
        else {
          $variables['width'] = $variables['height'] = NULL;
        }
        $random_teaser_image = [
          '#theme' => 'responsive_image',
          '#width' => $variables['width'],
          '#height' => $variables['height'],
          '#responsive_image_style_id' => $variables['responsive_image_style_id'],
          '#uri' => $variables['uri'],
        ];
      }
    }

    // return responsive image array
    return array(
      $random_teaser_image
    );
  }

  /**
   * {@inheritdoc}
   */
  public function isAllowed() {
    if ($this->bundle() != 'teaserbox') {
      return FALSE;
    }
    else if ($this->viewMode() != 'teaser') {
      return FALSE;
    }

    return TRUE;
  }
}