import 'dart:ui';

import 'package:flutter/material.dart';

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

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

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const MyHomePage(title: 'Flutter Reorderable List Index Bug'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int newestIndex = 0;
  int expectedIndex = 0;
  final ScrollController c = ScrollController();

  @override
  Widget build(BuildContext context) {
    final Size size = MediaQuery.of(context).size;
    final padding = MediaQuery.of(context).viewPadding;
    final height = (size.height - padding.top - kToolbarHeight) / 2;
    final items = Iterable.generate(10, (index) => 'Draggable Tile');

    final list = SliverReorderableList(
      proxyDecorator: _proxyDecorator,
      itemBuilder: (context, index) {
        final item = items.elementAt(index);

        return Container(
          key: Key('$item-$index'),

          decoration: BoxDecoration(border: Border.all(color: Colors.blue)),
          padding: EdgeInsets.only(bottom: 20),
          margin: EdgeInsets.only(bottom: 5),
          child: ListTile(
            title: Text(item),
            leading: ReorderableDragStartListener(
              enabled: true,
              index: index,
              child: Icon(Icons.reorder),
            ),
          ),
        );
      },

      itemCount: items.length,
      onReorder: (oldIndex, newIndex) {
        setState(() {
          //BUG HERE
          //Dragging down an object shows newest index as 1 greater than expected
          newestIndex = newIndex;
          expectedIndex = oldIndex > newIndex ? newIndex: newIndex - 1 ;
        });
      },
    );

    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('Newest index:'),
            Text(
              '$newestIndex',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
            const Text('Expected index:'),
            Text(
              '$expectedIndex',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
            SizedBox(
              height: height,
              child: RawScrollbar(
                controller: c,
                child: CustomScrollView(
                  controller: c,
                  scrollDirection: Axis.vertical,
                  shrinkWrap: true,
                  slivers: [list],
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Widget _proxyDecorator(Widget child, int index, Animation<double> animation) {
  return AnimatedBuilder(
    animation: animation,
    builder: (BuildContext context, Widget? child) {
      final double animValue = Curves.easeInOut.transform(animation.value);
      final double elevation = lerpDouble(0, 6, animValue)!;
      return Material(elevation: elevation, child: child);
    },
    child: child,
  );
}