function treeData(){
        $tree = [];
        $data = array_column($this->items,null,'id');
        foreach($data as $item){
            if(isset($data[$item['pid']])){
                $data[$item['pid']]['children'][] = &$data[$item['id']];
            }else{
                $tree[] = &$data[$item['id']];
            }
        }
        return $tree;
    }