Count Down Widget with Tween Animation in 24 lines of code.

countdown_widget.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: CountDown(
              onEnd: () {}, durationInSeconds: 10, onEndText: 'onEnd'),
        ),
      ),
    );
  }
}

class CountDown extends StatelessWidget {
  final int durationInSeconds;
  final Function onEnd;
  final String? onEndText;
  const CountDown(
      {Key? key,
      required this.onEnd,
      required this.durationInSeconds,
      this.onEndText})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    final _duration = Duration(seconds: durationInSeconds);
    return TweenAnimationBuilder<Duration>(
        onEnd: () => onEnd(),
        duration: _duration,
        tween: Tween(
          begin: _duration,
          end: Duration.zero,
        ),
        builder: (context, Duration value, Widget? child) {
          return Text((value == Duration.zero)
              ? (onEndText ?? '0')
              : (value.inMilliseconds / 1000).toStringAsFixed(3));
        });
  }
}