ComputedString.php

<?php

namespace Drupal\entity_test\TypedData;

use Drupal\Core\Cache\Cache;
use Drupal\Core\Cache\CacheableDependencyInterface;
use Drupal\Core\TypedData\TypedData;

/**
 * A computed property for test strings.
 */
class ComputedString extends TypedData implements CacheableDependencyInterface {
    
    /**
     * The data value.
     *
     * @var mixed
     */
    protected $value;
    
    /**
     * {@inheritdoc}
     */
    public function getValue() {
        
        /** @var \Drupal\Core\Field\FieldItemInterface $item */
        $item = $this->getParent();
        $computed_value = "Computed! " . $item->get('value')
            ->getString();
        return $computed_value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCastedValue() {
        return $this->getString();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheTags() {
        return [
            'you_are_it',
            'no_tag_backs',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheContexts() {
        return [
            'request_format',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheMaxAge() {
        return Cache::PERMANENT;
    }

}