Size size = MediaQuery.of(context).size;
用法：
Container( width: size.width * 0.8, height: 300.0, decoration: BoxDecoration( color: Color(0xff1f1f1f), borderRadius: BorderRadius.circular(5.0) ), ),
http://laomengit.com/flutter/widgets/MediaQuery.html#mediaquery
進階搜尋
6-1 開權限
