flutter pan to adjust opacity

main.dart

import 'package:flutter/material.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
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  double _opacity = 0.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Container(
          width: 200,
          height: 200,
          color: Colors.yellow,
          child: GestureDetector(
            child: FittedBox(
              child: Center(
                child: Opacity(
                  opacity: _opacity,
                  child: const Text(
                    "hello",
                    style: TextStyle(
                      fontSize: 40,
                    ),
                  ),
                ),
              ),
            ),
            onPanUpdate: (details) {
              double x = details.localPosition.dx;
              double normX = normalize(x, 0, 200);
              setState(() {
                _opacity = normX;
              });
            },
          ),
        ),
      ),
    );
  }
}

double normalize(double x, double min, double max) {
  if (min == max) {
    throw UnsupportedError("Cannot divide by zero");
  }

  double normalized = (x - min) / (max - min);
  return normalized.clamp(0, 1);
}