项目中使用FractionallySizedBox的目的是什么呢，项目中下面两段代码运行出来的效果一样，为什么要使用FractionallySizedBox呢？另外FractionallySizedBox和Expanded有什么区别？

_item(BuildContext context, CommonModel item, bool first) {
    BorderSide borderSide = BorderSide(width: 0.8, color: Colors.white);
    return Container(
      decoration: BoxDecoration(
          border: Border(
        left: borderSide,
        bottom: first ? borderSide : BorderSide.none,
      )),
      child: _wrapGesture(
          context,
          Center(
            child: Text(
              item.title,
              textAlign: TextAlign.center,
              style: TextStyle(fontSize: 14, color: Colors.white),
            ),
          ),
          item),
    );
  }

_item(BuildContext context, CommonModel item, bool first) {
    BorderSide borderSide = BorderSide(width: 0.8, color: Colors.white);
    return FractionallySizedBox(
      //撑满父布局的宽度
      widthFactor: 1,
      child: Container(
        decoration: BoxDecoration(
            border: Border(
              left: borderSide,
              bottom: first ? borderSide : BorderSide.none,
            )),
        child: _wrapGesture(
            context,
            Center(
              child: Text(
                item.title,
                textAlign: TextAlign.center,
                style: TextStyle(fontSize: 14, color: Colors.white),
              ),
            ),
            item),
      ),
    );
  }