countdown_widget.dart

import 'package:flutter/material.dart';

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

class App extends StatelessWidget {
  const App({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: CountdownWidget(style: TextStyle(fontSize: 256)),
        ),
      ),
    );
  }
}

class CountdownWidget extends StatefulWidget {
  const CountdownWidget({
    super.key,
    this.initialValue = 9,
    this.style,
    this.onEnd,
  }) : assert(initialValue > 0);

  final int initialValue;
  final TextStyle? style;
  final VoidCallback? onEnd;

  @override
  State<CountdownWidget> createState() => _CountdownWidgetState();
}

class _CountdownWidgetState extends State<CountdownWidget> with SingleTickerProviderStateMixin {
  late final AnimationController _controller = AnimationController(vsync: this, duration: const Duration(seconds: 1));
  late int _currentValue = widget.initialValue;

  @override
  void initState() {
    super.initState();
    _controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        setState(() {
          if (_currentValue > 1) {
            _currentValue--;
            _controller.reset();
            _controller.forward();
          } else {
            widget.onEnd?.call();
          }
        });
      }
    });
    _controller.forward();
  }

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

  @override
  void didUpdateWidget(CountdownWidget oldWidget) {
    super.didUpdateWidget(oldWidget);
    _currentValue = widget.initialValue;
    _controller.reset();
    _controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _controller,
      builder: (context, child) {
        final value = 1 - Curves.easeInCubic.transform(_controller.value);
        return Opacity(
          opacity: value,
          child: Transform.scale(
            scale: value,
            child: Text(
              '$_currentValue',
              style: widget.style ?? Theme.of(context).textTheme.displayLarge,
            ),
          ),
        );
      },
    );
  }
}