class EntityReferenceFieldItemList

class EntityReferenceFieldItemList extends FieldItemList implements EntityReferenceFieldItemListInterface {
    
    /**
     * {@inheritdoc}
     */
    public function getConstraints() {
        $constraints = parent::getConstraints();
        $constraint_manager = $this->getTypedDataManager()
            ->getValidationConstraintManager();
        $constraints[] = $constraint_manager->create('ValidReference', []);
        return $constraints;
    }
    
    /**
     * {@inheritdoc}
     */
    public function referencedEntities() {
        if ($this->isEmpty()) {
            return [];
        }
        // Collect the IDs of existing entities to load, and directly grab the
        // "autocreate" entities that are already populated in $item->entity.
        $target_entities = $ids = [];
        foreach ($this->list as $delta => $item) {
            if ($item->target_id !== NULL) {
                $ids[$delta] = $item->target_id;
            }
            elseif ($item->hasNewEntity()) {
                $target_entities[$delta] = $item->entity;
            }
        }
        // Load and add the existing entities.
        if ($ids) {
            $target_type = $this->getFieldDefinition()
                ->getSetting('target_type');
            $entities = \Drupal::entityTypeManager()->getStorage($target_type)
                ->loadMultiple($ids);
            foreach ($ids as $delta => $target_id) {
                if (isset($entities[$target_id])) {
                    $target_entities[$delta] = $entities[$target_id];
                }
            }
            // Ensure the returned array is ordered by deltas.
            ksort($target_entities);
        }
        return $target_entities;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function processDefaultValue($default_value, FieldableEntityInterface $entity, FieldDefinitionInterface $definition) {
        $default_value = parent::processDefaultValue($default_value, $entity, $definition);
        if ($default_value) {
            // Convert UUIDs to numeric IDs.
            $uuids = [];
            foreach ($default_value as $delta => $properties) {
                if (isset($properties['target_uuid'])) {
                    $uuids[$delta] = $properties['target_uuid'];
                }
            }
            if ($uuids) {
                $target_type = $definition->getSetting('target_type');
                $entity_ids = \Drupal::entityQuery($target_type)->accessCheck(TRUE)
                    ->condition('uuid', $uuids, 'IN')
                    ->execute();
                $entities = \Drupal::entityTypeManager()->getStorage($target_type)
                    ->loadMultiple($entity_ids);
                $entity_uuids = [];
                foreach ($entities as $id => $entity) {
                    $entity_uuids[$entity->uuid()] = $id;
                }
                foreach ($uuids as $delta => $uuid) {
                    if (isset($entity_uuids[$uuid])) {
                        $default_value[$delta]['target_id'] = $entity_uuids[$uuid];
                        unset($default_value[$delta]['target_uuid']);
                    }
                    else {
                        unset($default_value[$delta]);
                    }
                }
            }
            // Ensure we return consecutive deltas, in case we removed unknown UUIDs.
            $default_value = array_values($default_value);
        }
        return $default_value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultValuesFormSubmit(array $element, array &$form, FormStateInterface $form_state) {
        $default_value = parent::defaultValuesFormSubmit($element, $form, $form_state);
        // Convert numeric IDs to UUIDs to ensure config deployability.
        $ids = [];
        foreach ($default_value as $delta => $properties) {
            if (isset($properties['entity']) && $properties['entity']->isNew()) {
                // This may be a newly created term.
                $properties['entity']->save();
                $default_value[$delta]['target_id'] = $properties['entity']->id();
                unset($default_value[$delta]['entity']);
            }
            $ids[] = $default_value[$delta]['target_id'];
        }
        $entities = \Drupal::entityTypeManager()->getStorage($this->getSetting('target_type'))
            ->loadMultiple($ids);
        foreach ($default_value as $delta => $properties) {
            unset($default_value[$delta]['target_id']);
            $default_value[$delta]['target_uuid'] = $entities[$properties['target_id']]->uuid();
        }
        return $default_value;
    }

}