Drupal computed field example

DrupalComputedField.php

<?php

namespace Drupal\my_module\Fields;

use Drupal\Core\Entity\EntityPublishedInterface;
use Drupal\Core\Field\EntityReferenceFieldItemList;
use Drupal\Core\TypedData\ComputedItemListTrait;
use Drupal\Core\Url;

class MenuTeasers extends EntityReferenceFieldItemList {

  use ComputedItemListTrait;

  /**
   * Computes the field value.
   */
  protected function computeValue() {
    $context = $this->getEntity();

    $link = $context->get('link')->getValue();
    if (empty($link[0]['uri'])) {
      return;
    }

    $url = Url::fromUri($link[0]['uri']);
    if ($url->isRouted()) {
      $params = $url->getRouteParameters();
      $entity = NULL;
      $entity_type = key($params);
      if ($entity_type) {
        $entity = \Drupal::entityTypeManager()->getStorage($entity_type)->load($params[$entity_type]);
      }

      if (!is_null($entity)) {
        $language_manager = \Drupal::languageManager();

        $priorities = [];

        if (!empty($context)) {
          if (!empty($context->getFields()['langcode'])) {
            $context_language = $context->getFields()['langcode']->language;
            $priorities[] = $context_language;
          } elseif (!empty($context->getFields()['language'])) {
            $context_language = $context->getFields()['language']->language;
            $priorities[] = $context_language;
          }
        }

        $priorities[] = $language_manager->getDefaultLanguage();
        $priorities[] = $language_manager->getCurrentLanguage();

        $priorities+= $entity->getTranslationLanguages();

        foreach ($priorities as $priority) {
          if ($entity->hasTranslation($priority->getId())) {
            $translation = $entity->getTranslation($priority->getId());
            if ($translation instanceof EntityPublishedInterface && !$translation->isPublished()) {
              // link published check
              continue;
            }

            if ($translation->hasField('field_menu_teaser_enabled')) {
              $this->list[0] = $this->createItem(0, [
                'target_id' => $translation->id(),
              ]);
            }
            break;
          }
        }
      }
    }
  }

}

my_module.module

<?php

function my_module_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
  if ($entity_type->id() == 'menu_link_content') {
    $fields['teasers'] = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('Teasers reference'))
      ->setDescription(t('Field value is managed by the \'USU API\'-module.'))
      ->setComputed(TRUE)
      ->setClass('\\Drupal\\my_module\\Fields\\MenuTeasers')
      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
      ->setName('teasers') # Important for multi-lang sites.
      ->setTargetEntityTypeId('menu_link_content') # Important for multi-lang sites.
      ->setSettings(array(
        'target_type' => 'node',
        'default_value' => 0,
      ));
  }
}