materio-d8/web/modules/custom/materio_home/materio_home.module

<?php
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\mymodule\Plugin\Field\FieldType\MyFieldComputed;

/**
 * Implements hook_theme().
 */
// function materio_home_theme($existing, $type, $theme, $path) {
//   // @see https://www.drupal.org/docs/8/theming/twig/create-custom-twig-templates-from-custom-module
//
//   return array(
//     'materio_home' => array(
//       // 'render element' => '',
//       'file' => 'includes/materio_home.inc',
//       'variables' => array(
//         'frontpage_node' => NULL,
//       ),
//     ),
//   );
// }

/**
 * Implement hook_entity_bundle_field_info().
 *
 * @param EntityTypeInterface $entity_type
 * @param $bundle
 * @param array $base_field_definitions
 * @return array
 */
function materio_home_entity_bundle_field_info(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
// function materio_home_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
  // $fields = array();
  // if ($entity_type->id() == 'node' && $bundle === 'frontpage') {
  // \Drupal::logger('materio_home')->notice('bundle: '.$bundle);
  if ($entity_type->id() == 'node' && $bundle == 'frontpage') {
    $fields['computed_materials_reference'] = BaseFieldDefinition::create('entity_reference')
      ->setName('computed_materials_reference')
      ->setLabel(t('Computed Materials References'))
      ->setDescription(t('Computed Materials References.'))
      // // The Entity Type this field belongs to.
      ->setTargetEntityTypeId($entity_type->id())
      // // The Entity Type bundle this field belongs to.
      ->setTargetBundle($bundle)
      ->setSetting('target_type', 'node')
      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
      ->setComputed(TRUE)
      ->setRevisionable(FALSE)
      ->setTranslatable(FALSE)
      ->setDisplayConfigurable('view', TRUE)
      ->setDisplayOptions('view', [
          'label' => 'hidden',
          'weight' => -5,
        ])
      ->setClass(\Drupal\materio_home\Plugin\Field\FieldType\ComputedMaterialsReferences::class);

    $fields['computed_showrooms_reference'] = BaseFieldDefinition::create('entity_reference')
      ->setName('computed_showrooms_reference')
      ->setLabel(t('Computed Showrooms References'))
      ->setDescription(t('Computed Showrooms References.'))
      // // The Entity Type this field belongs to.
      ->setTargetEntityTypeId($entity_type->id())
      // // The Entity Type bundle this field belongs to.
      ->setTargetBundle($bundle)
      ->setSetting('target_type', 'taxonomy_term')
      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
      ->setComputed(TRUE)
      ->setRevisionable(FALSE)
      ->setTranslatable(FALSE)
      ->setDisplayConfigurable('view', TRUE)
      ->setDisplayOptions('view', [
          'label' => 'hidden',
          'weight' => -5,
        ])
      ->setClass(\Drupal\materio_home\Plugin\Field\FieldType\ComputedShowroomsReferences::class);

    $fields['computed_articles_reference'] = BaseFieldDefinition::create('entity_reference')
      ->setName('computed_articles_reference')
      ->setLabel(t('Computed Articles References'))
      ->setDescription(t('Computed Articles References.'))
      // // The Entity Type this field belongs to.
      ->setTargetEntityTypeId($entity_type->id())
      // // The Entity Type bundle this field belongs to.
      ->setTargetBundle($bundle)
      ->setSetting('target_type', 'node')
      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
      ->setComputed(TRUE)
      ->setRevisionable(FALSE)
      ->setTranslatable(FALSE)
      ->setDisplayConfigurable('view', TRUE)
      ->setDisplayOptions('view', [
          'label' => 'hidden',
          'weight' => -5,
        ])
      ->setClass(\Drupal\materio_home\Plugin\Field\FieldType\ComputedArticlesReferences::class);

      return $fields;
  }
}

// /**
//  * implements hook_entity_extra_field_info
//  *
//  */
// function materio_home_entity_extra_field_info() {
//   $extra = [];
//   // $extra['node']['frontpage']['form']['computed_materials_reference'] = [
//   //   'label' => t('Computed Materials References'),
//   //   'description' => t('Computed Materials References'),
//   //   'weight' => 10,
//   // ];
//   // $extra['node']['frontpage']['display']['computed_materials_reference'] = [
//   //   'label' => t('Computed Materials References'),
//   //   'description' => t('Computed Materials References'),
//   //   'weight' => 10,
//   // ];
//   return $extra;
// }

/**
 * Implements hook_install().
 */
function materio_home_install() {
  $entity_type = \Drupal::service('entity_type.manager')->getDefinition('node');
  \Drupal::service('entity.definition_update_manager')->updateEntityType($entity_type);
}

/**
 * Implements hook_uninstall().
 */
function materio_home_uninstall() {
  $entity_type = \Drupal::service('entity_type.manager')->getDefinition('node');
  \Drupal::service('entity.definition_update_manager')->updateEntityType($entity_type);
}