protected function processHiddenFields()
    {
        $raw_data = $this->getFormData();

        $serialized_field_name = self::HIDDEN_FIELD;
        if (isset($raw_data[$serialized_field_name])) {
            $fields = SwatString::signedUnserialize(
                $raw_data[$serialized_field_name],
                $this->salt
            );
        } else {
            return;
        }

        foreach ($fields as $name) {
            $serialized_field_name = self::SERIALIZED_PREFIX . $name;
            if (isset($raw_data[$serialized_field_name])) {
                $this->hidden_fields[$name] = $this->unserializeHiddenField(
                    $raw_data[$serialized_field_name]
                );
            }
        }
    }