showDialog(
        context: context,
        barrierDismissible: false,
        builder: (context) {
           return new CupertinoAlertDialog(
           title: new Text('Data not saved?'),
           content: new Text(
               'Data will be lost. Are you sure you want to exit?'),
           actions: <Widget>[
                new CupertinoButton(
                   onPressed: () => Navigator.of(context).pop(false),
                   child: new Text('No'),
                 ),
                new CupertinoButton(
                    onPressed: () => Navigator.of(context).pop(true),
                    child: new Text('Yes'),
                   ),
                  ],
            );
   });