Nette\Forms\Rules::addFilter PHP Method

    public function addFilter($filter)
    {
        Nette\Utils\Callback::check($filter);
        $this->rules[] = $rule = new Rule();
        $rule->control = $this->control;
        $rule->validator = function (IControl $control) use($filter) {
            $control->setValue(call_user_func($filter, $control->getValue()));
            return TRUE;
        };
        return $this;
    }