public function addButtonStyle($child)
    {
        if ($child instanceof Group || $child instanceof Toolbar) {
            $child->eachChild(array($this, 'addButtonStyle'));
        } else {
            $class = $child->getAttribute('class');
            $class = array_filter($class, function ($item) {
                return strpos($item, 'btn-') !== false;
            });

            if (!$class && $this->bootstrap_buttonStyle) {
                $child->addClass($this->bootstrap_buttonStyle);
            }
        }
    }