GridView
GridView可以构建一个二维网格列表，其默认构造函数定义如下：
GridView({ Key? key, Axis scrollDirection = Axis.vertical, bool reverse = false, ScrollController? controller, bool? primary, ScrollPhysics? physics, bool shrinkWrap = false, EdgeInsetsGeometry? padding, required this.gridDelegate, //下面解释 bool addAutomaticKeepAlives = true, bool addRepaintBoundaries = true, double? cacheExtent, List<Widget> children = const <Widget>[], ... }) GridVi
