flutter 开发小技巧3

 Positioned(
                                    top: 391.w,
                                    left: 0.w,
                                    right: 0.w,
                                    child: Container(
                                      alignment: Alignment.center,
                                      child: Text(
                                        _groupInfo.sign != ""
                                            ? _groupInfo.sign
                                            : "这家伙很懒,还没有群介绍",
                                        style: TextStyle(
                                            color: Color_TitleGrey,
                                            fontSize: 28.w),
                                      ),
                                    )),