FieldType_OwlItem.php

<?php


namespace Drupal\owl\Plugin\Field\FieldType;

use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\TypedData\DataDefinition;
use Drupal\image\Plugin\Field\FieldType\ImageItem;


/**
 * Plugin implementation of the 'owl_item' field type.
 *
 * @FieldType(
 *   id = "owl_item",
 *   label = @Translation("Owl item"),
 *   module = "owl",
 *   description = @Translation("Image caption text, for the image's 'caption' attribute."),
 *   default_widget = "owl_image",
 *   default_formatter = "owl_field_formatter"
 * )
 */


class OwlItem extends ImageItem {

    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        $schema = parent::schema($field_definition);
        $schema['columns']['caption'] = array(
             'type' => 'varchar',
             'length' => '1024',
             'unsigned' => TRUE,
         );

        return $schema;

    }


    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties = parent::propertyDefinitions($field_definition);

        $properties['caption'] = DataDefinition::create('string')
            ->setLabel(t('Caption'))
            ->setDescription(t("Image caption text, for the image's 'caption' attribute."));

        return $properties;
    }

}