如何创建在使用AnimationController创建的倒计时计时器期间发出的警报？

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: CountdownPage(),
    );
  }
}

class CountdownPage extends StatefulWidget {
  @override
  _CountdownPageState createState() => _CountdownPageState();
}

class _CountdownPageState extends State<CountdownPage>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  FlutterLocalNotificationsPlugin _notificationsPlugin;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: 10),
    );
    _controller.addListener(() {
      if (_controller.isCompleted) {
        _showNotification();
      }
    });

    _notificationsPlugin = FlutterLocalNotificationsPlugin();
    var initializationSettingsAndroid =
        AndroidInitializationSettings('app_icon');
    var initializationSettingsIOS = IOSInitializationSettings();
    var initializationSettings = InitializationSettings(
        android: initializationSettingsAndroid, iOS: initializationSettingsIOS);
    _notificationsPlugin.initialize(initializationSettings);
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  void _showNotification() async {
    var androidPlatformChannelSpecifics = AndroidNotificationDetails(
      'channel_id',
      'channel_name',
      'channel_description',
      importance: Importance.max,
      priority: Priority.high,
    );
    var iOSPlatformChannelSpecifics = IOSNotificationDetails();
    var platformChannelSpecifics = NotificationDetails(
        android: androidPlatformChannelSpecifics,
        iOS: iOSPlatformChannelSpecifics);
    await _notificationsPlugin.show(
      0,
      '倒计时结束',
      '时间到了！',
      platformChannelSpecifics,
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('倒计时计时器'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              '倒计时',
              style: TextStyle(fontSize: 24),
            ),
            SizedBox(height: 20),
            AnimatedBuilder(
              animation: _controller,
              builder: (context, child) {
                return Text(
                  '${(_controller.duration.inSeconds - _controller.value.toInt())}秒',
                  style: TextStyle(fontSize: 48),
                );
              },
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _controller.reverse(from: _controller.value == 0.0 ? 1.0 : _controller.value);
        },
        child: Icon(Icons.play_arrow),
      ),
    );
  }
}