protected function _getBtnClasses(array $attrs = array())
    {
        if (Arr::key('button', $attrs)) {
            $classes = array($this->_btm);
            foreach ((array) $attrs['button'] as $btn) {
                $classes[] = $this->_btm . '-' . $btn;
            }

            $attrs = $this->_mergeAttr($attrs, implode(' ', $classes));
            unset($attrs['button']);
        }

        return $attrs;
    }