Flutter timer by AnimationBuilder

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> with SingleTickerProviderStateMixin

{
  
  int timerValue = 10;
  int _counter = 10;
  bool _showCompleted = false;
  late Animation<double> animation;
  late AnimationController controller;
  
  @override
  void initState() {
    super.initState();
    
    controller = AnimationController(duration : Duration(seconds : timerValue.toInt()), vsync : this);
    animation = Tween<double>(begin : timerValue.toDouble(), end : 0)
      .animate(controller)
      ..addStatusListener((status) {
        if (AnimationStatus.completed == status) {
            _showCompleted = true;

        } else {
           _showCompleted = false;

        }
           setState((){
          });
      });
//       ..addListener(() {
//         setState((){
//           _counter = animation.value.toInt();
//         }) ;
//       });
  }
  
  
  @override
  Widget build(BuildContext context) {
    print("build()");
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            if (_showCompleted)
              ...[Text('Complete')],
            AnimatedBuilder(
              animation: controller,
              child : Text('$_counter', style: Theme.of(context).textTheme.headline2),
              builder : (buildContext, child) {
                return Text('${animation.value.toInt()}', style: Theme.of(context).textTheme.headline2);
              }
              
             ),
            ElevatedButton( onPressed: () {
                        print('start');

              controller.reset();
              controller.forward();
            }
               , child: Text('Start'), 
            )
          ],
        ),
      ),
    );
  }
}