private function getFields()
    {
        $fields = [];

        foreach ($this->items as $name => $field) {
            if (is_array($field) && isset($field['name'])) {
                $fields[] = $field['name'];
            } else if ($field instanceof FillableField){
                $fields[] = $field->name;
            } else if (is_string($name)){
                $fields[] = $name;
            } else {
                $fields[] = '';
            }
        }

        return $fields;
    }