class TextProcessed

class TextProcessed extends TypedData {

  /**
   * The text property.
   *
   * @var \Drupal\Core\TypedData\TypedDataInterface
   */
  protected $text;

  /**
   * The text format property.
   *
   * @var \Drupal\Core\TypedData\TypedDataInterface
   */
  protected $format;

  /**
   * Overrides TypedData::__construct().
   */
  public function __construct(array $definition, $name = NULL, TypedDataInterface $parent = NULL) {
    parent::__construct($definition, $name, $parent);
    if (!isset($definition['settings']['text source'])) {
      throw new InvalidArgumentException("The definition's 'source' key has to specify the name of the text property to be processed.");
    }
  }

  /**
   * Overrides TypedData::setContext().
   */
  public function setContext($name = NULL, TypedDataInterface $parent = NULL) {
    parent::setContext($name, $parent);
    if (isset($parent)) {
      $this->text = $parent
        ->get($this->definition['settings']['text source']);
      $this->format = $parent
        ->get('format');
    }
  }

  /**
   * Implements \Drupal\Core\TypedData\TypedDataInterface::getValue().
   */
  public function getValue($langcode = NULL) {
    if (!isset($this->text)) {
      throw new InvalidArgumentException('Computed properties require context for computation.');
    }
    $field = $this->parent
      ->getParent();
    $entity = $field
      ->getParent();
    $instance = field_info_instance($entity
      ->entityType(), $field
      ->getName(), $entity
      ->bundle());
    if (!empty($instance['settings']['text_processing']) && $this->format
      ->getValue()) {
      return check_markup($this->text
        ->getValue(), $this->format
        ->getValue(), $entity
        ->language()->langcode);
    }
    else {

      // If no format is available, still make sure to sanitize the text.
      return check_plain($this->text
        ->getValue());
    }
  }

  /**
   * Implements \Drupal\Core\TypedData\TypedDataInterface::setValue().
   */
  public function setValue($value, $notify = TRUE) {
    if (isset($value)) {
      throw new ReadOnlyException('Unable to set a computed property.');
    }
  }

  /**
   * Implements \Drupal\Core\TypedData\TypedDataInterface::validate().
   */
  public function validate() {

    // @todo: Implement.
  }

}