SingleInternalPropertyTestFieldItem.php

<?php

namespace Drupal\entity_test\Plugin\Field\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 'Single Internal Property' entity test field type.
 *
 * This is based off of the InternalPropertyTestFieldItem test field item type,
 * but only adds a single computed property. This tests that fields with a main
 * property name and one internal value are flattened.
 *
 * @see \Drupal\entity_test\Plugin\Field\FieldType\InternalPropertyTestFieldItem
 *
 * @FieldType(
 *   id = "single_internal_property_test",
 *   label = @Translation("Single Internal Property (test)"),
 *   description = @Translation("A field containing one string, from which one internal string is computed."),
 *   category = @Translation("Test"),
 *   default_widget = "string_textfield",
 *   default_formatter = "string"
 * )
 */
class SingleInternalPropertyTestFieldItem extends StringItem {
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties = parent::propertyDefinitions($field_definition);
        // 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;
    }

}