class ComputedFileUrl

class ComputedFileUrl extends TypedData {
    
    /**
     * Computed root-relative file URL.
     *
     * @var string
     */
    protected $url = NULL;
    
    /**
     * {@inheritdoc}
     */
    public function getValue() {
        if ($this->url !== NULL) {
            return $this->url;
        }
        assert($this->getParent()
            ->getEntity() instanceof FileInterface);
        $uri = $this->getParent()
            ->getEntity()
            ->getFileUri();
        
        /** @var \Drupal\Core\File\FileUrlGeneratorInterface $file_url_generator */
        $file_url_generator = \Drupal::service('file_url_generator');
        $this->url = $file_url_generator->generateString($uri);
        return $this->url;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($value, $notify = TRUE) {
        $this->url = $value;
        // Notify the parent of any changes.
        if ($notify && isset($this->parent)) {
            $this->parent
                ->onChange($this->name);
        }
    }

}