interface FormElementInterface

interface FormElementInterface extends ElementInterface {
  
  /**
   * Determines how user input is mapped to an element's #value property.
   *
   * @param array $element
   *   An associative array containing the properties of the element.
   * @param mixed $input
   *   The incoming input to populate the form element. If this is FALSE,
   *   the element's default value should be returned.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   *
   * @return mixed
   *   The value to assign to the element.
   */
  public static function valueCallback(&$element, $input, FormStateInterface $form_state);

}