class EntityReferenceUuidItem

class EntityReferenceUuidItem extends EntityReferenceItem {
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $settings = $field_definition->getSettings();
    $target_type_info = \Drupal::entityTypeManager()->getDefinition($settings['target_type']);
    $properties = parent::propertyDefinitions($field_definition);
    $target_uuid_definition = DataReferenceTargetDefinition::create('string')->setLabel(new TranslatableMarkup('@label UUID', [
      '@label' => $target_type_info->getLabel(),
    ]));
    $target_uuid_definition->setRequired(TRUE);
    $properties['target_uuid'] = $target_uuid_definition;
    $properties['entity'] = DataReferenceDefinition::create('entity')->setLabel($target_type_info->getLabel())
      ->setDescription(new TranslatableMarkup('The referenced entity by UUID'))
      ->setComputed(TRUE)
      ->setReadOnly(FALSE)
      ->setTargetDefinition(EntityDataDefinition::create($settings['target_type']))
      ->addConstraint('EntityType', $settings['target_type']);
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function mainPropertyName() {
    return 'target_uuid';
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    $columns = [
      'target_uuid' => [
        'description' => 'The UUID of the target entity.',
        'type' => 'varchar_ascii',
        'length' => 128,
      ],
    ];
    return [
      'columns' => $columns,
      'indexes' => [
        'target_uuid' => [
          'target_uuid',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function setValue($values, $notify = TRUE) : void {
    if (isset($values) && !is_array($values)) {
      // If either a scalar or an object was passed as the value for the item,
      // assign it to the 'entity' or 'target_uuid' depending on values type.
      if (is_object($values)) {
        $this->set('entity', $values, $notify);
      }
      else {
        $this->set('target_uuid', $values, $notify);
      }
    }
    else {
      parent::setValue($values, FALSE);
      // Support setting the field item with only one property, but make sure
      // values stay in sync if only property is passed.
      // NULL is a valid value, so we use array_key_exists().
      if (is_array($values) && array_key_exists('target_uuid', $values) && !isset($values['entity'])) {
        $this->onChange('target_uuid', FALSE);
      }
      elseif (is_array($values) && !array_key_exists('target_uuid', $values) && isset($values['entity'])) {
        $this->onChange('entity', FALSE);
      }
      elseif (is_array($values) && array_key_exists('target_uuid', $values) && isset($values['entity'])) {
        // If both properties are passed, verify the passed values match. The
        // only exception we allow is when we have a new entity: in this case
        // its actual id and target_uuid will be different, due to the new
        // entity marker.
        $entity_uuid = $this->get('entity')
          ->get('uuid');
        // If the entity has been saved and we're trying to set both the
        // target_uuid and the entity values with a non-null target UUID, then
        // the value for target_uuid should match the UUID of the entity value.
        if (!$this->entity
          ->isNew() && $values['target_uuid'] !== NULL && $entity_uuid !== $values['target_uuid']) {
          throw new \InvalidArgumentException('The target UUID and entity passed to the entity reference item do not match.');
        }
      }
      // Notify the parent if necessary.
      if ($notify && $this->parent) {
        $this->parent
          ->onChange($this->getName());
      }
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function onChange($property_name, $notify = TRUE) : void {
    // Make sure that the target UUID and the target property stay in sync.
    if ($property_name === 'entity') {
      $property = $this->get('entity');
      if ($target_uuid = $property->isTargetNew() ? NULL : $property->getValue()
        ->uuid()) {
        $this->writePropertyValue('target_uuid', $target_uuid);
      }
    }
    elseif ($property_name === 'target_uuid') {
      $property = $this->get('entity');
      $entity_type = $property->getDataDefinition()
        ->getConstraint('EntityType');
      $entities = \Drupal::entityTypeManager()->getStorage($entity_type)
        ->loadByProperties([
        'uuid' => $this->get('target_uuid')
          ->getValue(),
      ]);
      if ($entity = array_shift($entities)) {
        assert($entity instanceof EntityInterface);
        $this->writePropertyValue('target_uuid', $entity->uuid());
        $this->writePropertyValue('entity', $entity);
      }
    }
    parent::onChange($property_name, $notify);
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    // Avoid loading the entity by first checking the 'target_uuid'.
    if ($this->target_uuid !== NULL) {
      return FALSE;
    }
    if ($this->entity && $this->entity instanceof EntityInterface) {
      return FALSE;
    }
    return TRUE;
  }
  
  /**
   * {@inheritdoc}
   */
  public function preSave() : void {
    if ($this->hasNewEntity()) {
      // Save the entity if it has not already been saved by some other code.
      if ($this->entity
        ->isNew()) {
        $this->entity
          ->save();
      }
      // Make sure the parent knows we are updating this property so it can
      // react properly.
      $this->target_uuid = $this->entity
        ->uuid();
    }
    if (!$this->isEmpty() && $this->target_uuid === NULL) {
      $this->target_uuid = $this->entity
        ->uuid();
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public static function generateSampleValue(FieldDefinitionInterface $field_definition) : array {
    $manager = \Drupal::service('plugin.manager.entity_reference_selection');
    // Instead of calling $manager->getSelectionHandler($field_definition)
    // replicate the behavior to be able to override the sorting settings.
    $options = [
      'target_type' => $field_definition->getFieldStorageDefinition()
        ->getSetting('target_type'),
      'handler' => $field_definition->getSetting('handler'),
      'handler_settings' => $field_definition->getSetting('handler_settings') ?: [],
      'entity' => NULL,
    ];
    $entity_type = \Drupal::entityTypeManager()->getDefinition($options['target_type']);
    $options['handler_settings']['sort'] = [
      'field' => $entity_type->getKey('uuid'),
      'direction' => 'DESC',
    ];
    $selection_handler = $manager->getInstance($options);
    // Select a random number of references between the last 50 referenceable
    // entities created.
    if ($referenceable = $selection_handler->getReferenceableEntities(NULL, 'CONTAINS', 50)) {
      $group = array_rand($referenceable);
      return [
        'target_uuid' => array_rand($referenceable[$group]),
      ];
    }
    return [];
  }
  
  /**
   * Determines whether the item holds an unsaved entity.
   *
   * This is notably used for "autocreate" widgets, and more generally to
   * support referencing freshly created entities (they will get saved
   * automatically as the hosting entity gets saved).
   *
   * @return bool
   *   TRUE if the item holds an unsaved entity.
   */
  public function hasNewEntity() {
    return !$this->isEmpty() && $this->target_uuid === NULL && $this->entity
      ->isNew();
  }

}