import 'dart:math' as math;
import 'package:flutter/material.dart';

extension SliverListEx on SliverList {
  static SliverList separated({
    required int itemCount,
    required NullableIndexedWidgetBuilder itemBuilder,
    required NullableIndexedWidgetBuilder separatorBuilder,
  }) {
    return SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, index) {
          final itemIndex = index ~/ 2;
          return index.isEven
              ? itemBuilder(context, itemIndex)
              : separatorBuilder(context, itemIndex);
        },
        childCount: math.max(0, itemCount * 2 - 1),
      ),
    );
  }
}

class StickyFixedHeightHeaderDelegate extends SliverPersistentHeaderDelegate {
  const StickyFixedHeightHeaderDelegate({
    required this.height,
    required this.child,
  });

  final double height;
  final Widget child;

  @override
  double get minExtent => height;

  @override
  double get maxExtent => height;

  @override
  Widget build(
    BuildContext context,
    double shrinkOffset,
    bool overlapsContent,
  ) {
    return child;
  }

  @override
  bool shouldRebuild(StickyFixedHeightHeaderDelegate oldDelegate) {
    return height != oldDelegate.height;
  }
}

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 Page(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Page'),
      ),
      body: CustomScrollView(
        slivers: [
          SliverToBoxAdapter(
              child: Container(
            width: double.infinity,
            height: 300,
            color: Colors.red,
          )),
          SliverPersistentHeader(
            pinned: true,
            delegate: StickyFixedHeightHeaderDelegate(
                height: 200,
                child: Container(
                  color: Colors.green,
                ),),
          ),
          SliverListEx.separated(
            itemCount: 100,
            itemBuilder: (_, index) => ListTile(
              key: ValueKey(index),
              title: Text('item: $index'),
            ),
            separatorBuilder: (_, __) => const Divider(
              color: Colors.blueGrey,
            ),
          )
        ],
      ),
    );
  }
}