main.dart

import 'package:flutter/cupertino.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(context) {
    return CupertinoApp(
      home: CupertinoPageScaffold(
        child: Center(
          child: Builder(
            builder: (context) {
              return CupertinoButton(
                color: CupertinoColors.activeBlue,
                child: Text('Open popup'),
                onPressed: () {
                  showCupertinoDialog(
                    context: context,
                    builder: (context) => CupertinoAlertDialog(
                      title: Text('My message!'),
                      actions: [
                        CupertinoDialogAction(
                          child: Text('Close'),
                          onPressed: () => Navigator.pop(context)
                        ),
                      ],
                    )
                  );
                },
              );
            }
          )
        ),
      ),
    );
  }
}