interface FormValidatorInterface

interface FormValidatorInterface {
    
    /**
     * Executes custom validation handlers for a given form.
     *
     * Button-specific handlers are checked first. If none exist, the function
     * falls back to form-level handlers.
     *
     * @param $form
     *   An associative array containing the structure of the form.
     * @param $form_state
     *   The current state of the form. If the user submitted the form by clicking
     *   a button with custom handler functions defined, those handlers will be
     *   stored here.
     */
    public function executeValidateHandlers(&$form, FormStateInterface &$form_state);
    
    /**
     * Validates user-submitted form data in the $form_state.
     *
     * @param $form_id
     *   A unique string identifying the form for validation, submission,
     *   theming, and hook_form_alter functions.
     * @param $form
     *   An associative array containing the structure of the form, which is
     *   passed by reference. Form validation handlers are able to alter the form
     *   structure (like #process and #after_build callbacks during form building)
     *   in case of a validation error. If a validation handler alters the form
     *   structure, it is responsible for validating the values of changed form
     *   elements in $form_state->getValues() to prevent form submit handlers from
     *   receiving non validated values.
     * @param $form_state
     *   The current state of the form. The current user-submitted data is stored
     *   in $form_state->getValues(), though form validation functions are passed
     *   an explicit copy of the values for the sake of simplicity. Validation
     *   handlers can also use $form_state to pass information on to submit
     *   handlers. For example:
     *     $form_state->set('data_for_submission', $data);
     *   This technique is useful when validation requires file parsing,
     *   web service requests, or other expensive requests that should
     *   not be repeated in the submission step.
     */
    public function validateForm($form_id, &$form, FormStateInterface &$form_state);
    
    /**
     * Sets a form_token error on the given form state.
     *
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return $this
     */
    public function setInvalidTokenError(FormStateInterface $form_state);

}