materio-d8/web/modules/custom/materio_home/src/Plugin/Field/FieldType/ComputedShowroomsReferences.php

<?php
namespace Drupal\materio_home\Plugin\Field\FieldType;

use Drupal\Core\Entity\FieldableEntityInterface;
use Drupal\Core\Field\EntityReferenceFieldItemList;
use Drupal\Core\Field\EntityReferenceFieldItemListInterface;
use Drupal\Core\Field\FieldItemList;
use Drupal\Core\TypedData\TypedDataInterface;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\TypedData\ComputedItemListTrait;

// https://www.drupal.org/node/2112677
// https://www.cornel.co/article/entity-reference-computed-field-example-drupal
// https://www.caxy.com/blog/drupal-custom-form-and-computed-fields

class ComputedShowroomsReferences extends EntityReferenceFieldItemList
{
  use ComputedItemListTrait;
  /**
   * The entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityTypeManager;

  /**
   * {@inheritdoc}
   */
  public function __construct(BaseFieldDefinition $definition, $name, TypedDataInterface $parent) {
    parent::__construct($definition, $name, $parent);
    $this->entityTypeManager = \Drupal::entityTypeManager();
  }

  /**
   * Compute the values.
   */
  protected function computeValue() {
    $query = \Drupal::entityQuery('taxonomy_term')
        ->condition('status', 1)
        ->condition('vid', 'showroom');
    $tids = $query->execute();
    shuffle($tids);
    $terms = entity_load_multiple('taxonomy_term', $tids);
    // \Drupal::logger('materio_home')->notice(print_r($nodes, true));
    $key = 0;
    foreach ($terms as $tid => $term) {
      // \Drupal::logger('materio_home')->notice($nid);
      $this->list[$key] = $this->createItem($key, $term->id());
      $key++;
    }
  }

}