flutter 如何更新dialog UI？

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Countdown Dialog'),
        ),
        body: Center(
          child: RaisedButton(
            child: Text('Show Dialog'),
            onPressed: () {
              showDialog(
                context: context,
                builder: (context) => CountdownDialog(),
              );
            },
          ),
        ),
      ),
    );
  }
}

class CountdownDialog extends StatefulWidget {
  @override
  _CountdownDialogState createState() => _CountdownDialogState();
}

class _CountdownDialogState extends State<CountdownDialog> {
  int countdown = 5;

  @override
  void initState() {
    super.initState();
    Timer.periodic(Duration(seconds: 1), (timer) {
      if (countdown == 0) {
        timer.cancel();
        Navigator.of(context).pop();
      } else {
        setState(() {
          countdown--;
        });
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: Text('Countdown Dialog'),
      content: Text('Closing in $countdown seconds...'),
    );
  }
}