value_key_with_animation_example.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: const Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class ScaleAnimated extends StatefulWidget {
  final Widget child;
  final Duration duration;

  const ScaleAnimated({Key? key, required this.child, required this.duration})
      : super(key: key);

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

class _ScaleAnimatedState extends State<ScaleAnimated>
    with SingleTickerProviderStateMixin {
  late final AnimationController _controller;
  late final Animation<double> _animation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
        vsync: this,
        duration: widget.duration,
        upperBound: 1.0,
        lowerBound: 0.0);
    _controller.forward();
    _animation = CurvedAnimation(parent: _controller, curve: Curves.linear);
  }

  @override
  Widget build(BuildContext context) {
    return ScaleTransition(
      scale: _animation,
      child: widget.child,
    );
  }

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

class MyWidget extends StatefulWidget {
  const MyWidget({Key? key}) : super(key: key);

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

class _MyWidgetState extends State<MyWidget> {
  int counter = 0;
  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        ScaleAnimated(
          duration: const Duration(seconds: 2),
          child: Text("Widget without key animates only once: $counter"),
        ),
         ScaleAnimated(
           key: ValueKey(counter),
          duration: const Duration(seconds: 2),
          child: Text("Widget with key animates each time the key is different: $counter"),
        ),
        ElevatedButton(child: const Text("Press me"), onPressed: _pressed),
      ],
    );
  }
  
  void _pressed() {
    setState(() {
      counter++;
    });
  }
}