InternalPropertyTestFieldItem.php

<?php

namespace Drupal\entity_test\Plugin\Field\FieldType;

use Drupal\Core\Field\Attribute\FieldType;
use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\TypedData\DataDefinition;
use Drupal\entity_test\TypedData\ComputedString;

/**
 * Defines the 'Internal Property' entity test field type.
 */
class InternalPropertyTestFieldItem extends StringItem {
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties = parent::propertyDefinitions($field_definition);
        // Add a computed property that is non-internal.
        $properties['non_internal_value'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Computed string, non-internal property'))
            ->setComputed(TRUE)
            ->setClass(ComputedString::class)
            ->setInternal(FALSE);
        // Add a computed property that is internal.
        $properties['internal_value'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Computed string, internal property'))
            ->setComputed(TRUE)
            ->setClass(ComputedString::class);
        return $properties;
    }

}