请给一个Flutter中使用showCupertinoModalPopup的示例

下面是一个使用 showCupertinoModalPopup 的 Flutter 示例：

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: RaisedButton(
            onPressed: () {
              showCupertinoModalPopup(
                context: context,
                builder: (BuildContext context) {
                  return CupertinoActionSheet(
                    actions: <Widget>[
                      CupertinoActionSheetAction(
                        child: Text('选项 1'),
                        onPressed: () {},
                      ),
                      CupertinoActionSheetAction(
                        child: Text('选项 2'),
                        onPressed: () {},
                      ),
                      CupertinoActionSheetAction(
                        child: Text('选项 3'),
                        onPressed: () {},
                      ),
                    ],
                    cancelButton: CupertinoActionSheetAction(
                      child: Text('取消'),
                      onPressed: () {
                        Navigator.pop(context);
                      },
                    ),
                  );
                },
              );
            },
            child: Text('打开弹窗'),
          ),
        ),
      ),
    );
  }
}