class ValidReferenceConstraintValidator

class ValidReferenceConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {
    
    /**
     * The selection plugin manager.
     *
     * @var \Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManagerInterface
     */
    protected $selectionManager;
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * Constructs a ValidReferenceConstraintValidator object.
     *
     * @param \Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManagerInterface $selection_manager
     *   The selection plugin manager.
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     */
    public function __construct(SelectionPluginManagerInterface $selection_manager, EntityTypeManagerInterface $entity_type_manager) {
        $this->selectionManager = $selection_manager;
        $this->entityTypeManager = $entity_type_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('plugin.manager.entity_reference_selection'), $container->get('entity_type.manager'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function validate($value, Constraint $constraint) : void {
        
        /** @var \Drupal\Core\Field\FieldItemListInterface $value */
        
        /** @var ValidReferenceConstraint $constraint */
        if (!isset($value)) {
            return;
        }
        // Collect new entities and IDs of existing entities across the field items.
        $new_entities = [];
        $target_ids = [];
        foreach ($value as $delta => $item) {
            $target_id = $item->target_id;
            // We don't use a regular NotNull constraint for the target_id property as
            // NULL is allowed if the entity property contains an unsaved entity.
            // @see \Drupal\Core\TypedData\DataReferenceTargetDefinition::getConstraints()
            if (!$item->isEmpty() && $target_id === NULL) {
                if (!$item->entity
                    ->isNew()) {
                    $this->context
                        ->buildViolation($constraint->nullMessage)
                        ->atPath((string) $delta)
                        ->addViolation();
                    return;
                }
                $new_entities[$delta] = $item->entity;
            }
            // '0' or NULL are considered valid empty references.
            if (!empty($target_id)) {
                $target_ids[$delta] = $target_id;
            }
        }
        // Early opt-out if nothing to validate.
        if (!$new_entities && !$target_ids) {
            return;
        }
        $entity = !empty($value->getParent()) ? $value->getEntity() : NULL;
        
        /** @var \Drupal\Core\Entity\EntityReferenceSelection\SelectionInterface $handler * */
        $handler = $this->selectionManager
            ->getSelectionHandler($value->getFieldDefinition(), $entity);
        $target_type_id = $value->getFieldDefinition()
            ->getSetting('target_type');
        // Add violations on deltas with a new entity that is not valid.
        if ($new_entities) {
            if ($handler instanceof SelectionWithAutocreateInterface) {
                $valid_new_entities = $handler->validateReferenceableNewEntities($new_entities);
                $invalid_new_entities = array_diff_key($new_entities, $valid_new_entities);
            }
            else {
                // If the selection handler does not support referencing newly created
                // entities, all of them should be invalidated.
                $invalid_new_entities = $new_entities;
            }
            foreach ($invalid_new_entities as $delta => $entity) {
                $this->context
                    ->buildViolation($constraint->invalidAutocreateMessage)
                    ->setParameter('%type', $target_type_id)
                    ->setParameter('%label', $entity->label())
                    ->atPath((string) $delta . '.entity')
                    ->setInvalidValue($entity)
                    ->addViolation();
            }
        }
        // Add violations on deltas with a target_id that is not valid.
        if ($target_ids) {
            // Get a list of pre-existing references.
            $previously_referenced_ids = [];
            if ($entity && !$entity->isNew()) {
                $existing_entity = $this->entityTypeManager
                    ->getStorage($entity->getEntityTypeId())
                    ->loadUnchanged($entity->id());
                foreach ($existing_entity->{$value->getFieldDefinition()
                    ->getName()}
                    ->getValue() as $item) {
                    $previously_referenced_ids[$item['target_id']] = $item['target_id'];
                }
            }
            $valid_target_ids = $handler->validateReferenceableEntities($target_ids);
            if ($invalid_target_ids = array_diff($target_ids, $valid_target_ids)) {
                // For accuracy of the error message, differentiate non-referenceable
                // and non-existent entities.
                $existing_entities = $this->entityTypeManager
                    ->getStorage($target_type_id)
                    ->loadMultiple($invalid_target_ids);
                foreach ($invalid_target_ids as $delta => $target_id) {
                    // Check if any of the invalid existing references are simply not
                    // accessible by the user, in which case they need to be excluded from
                    // validation
                    if (isset($previously_referenced_ids[$target_id]) && isset($existing_entities[$target_id]) && !$existing_entities[$target_id]->access('view')) {
                        continue;
                    }
                    $message = isset($existing_entities[$target_id]) ? $constraint->message : $constraint->nonExistingMessage;
                    $this->context
                        ->buildViolation($message)
                        ->setParameter('%type', $target_type_id)
                        ->setParameter('%id', $target_id)
                        ->atPath((string) $delta . '.target_id')
                        ->setInvalidValue($target_id)
                        ->addViolation();
                }
            }
        }
    }

}