import 'package:flutter/material.dart';

//github.com/ Flutter code sample for [CustomScrollView].

void main() => runApp(const CustomScrollViewExampleApp());

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

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(home: CustomScrollViewExample());
  }
}

class CustomScrollViewExample extends StatefulWidget {
  const CustomScrollViewExample({super.key});

  @override
  State<CustomScrollViewExample> createState() =>
      _CustomScrollViewExampleState();
}

class _CustomScrollViewExampleState extends State<CustomScrollViewExample> {
  Widget _buildLabel(String label) {
    return SliverToBoxAdapter(
      child: Container(
        height: 50,
        color: Colors.green,
        alignment: Alignment.center,
        child: Text(label),
      ),
    );
  }

  Widget _buildList() {
    return SliverList.builder(
      itemBuilder:
          (context, index) => Container(
            alignment: Alignment.center,
            color: Colors.blue[200 + index % 4 * 100],
            height: 100,
            child: Text('Item: $index'),
          ),
      itemCount: 5,
    );
  }

  @override
  Widget build(BuildContext context) {
    const Key centerKey = ValueKey<String>('bottom-sliver-list');
    return Scaffold(
      appBar: AppBar(
        title: const Text('Press on the plus to add items above and below'),
      ),
      body: CustomScrollView(
        center: centerKey,
        slivers: <Widget>[
          SliverMainAxisGroup(
            slivers: [
              SliverMainAxisGroup(slivers: [_buildLabel('-E'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('-D'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('-C'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('-B'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('-A'), _buildList()]),
            ],
          ),
          SliverMainAxisGroup(
            key: centerKey,
            slivers: [
              SliverMainAxisGroup(slivers: [_buildLabel('A'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('B'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('C'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('D'), _buildList()]),
              SliverMainAxisGroup(slivers: [_buildLabel('E'), _buildList()]),
            ],
          ),
        ],
      ),
    );
  }
}