Showcase of SliverSeparatedChildBuilderDelegate

sliver_separated_child_builder_delegate.dart

import 'dart:math' as math;

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

final elements = List<int>.generate(100, (i) => i);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: CustomScrollView(
          slivers: [
            SliverList(
              delegate: SliverSeparatedChildBuilderDelegate(
                builder: builder,
                separatorBuilder: separatorBuilder,
                childCount: elements.length,
              ),
            ),
          ],
        ),
      ),
    );
  }

  Widget builder(BuildContext context, int index) {
    final element = elements[index];
    return ListTile(
      title: Text('$element'),
    );
  }

  Widget separatorBuilder(BuildContext context, int index) => Container(
        width: double.maxFinite,
        height: 16,
        color: Colors.orange,
      );
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text(
      'Hello, World!',
      style: Theme.of(context).textTheme.headline4,
    );
  }
}

class SliverSeparatedChildBuilderDelegate extends SliverChildBuilderDelegate {
  SliverSeparatedChildBuilderDelegate({
    required NullableIndexedWidgetBuilder builder,
    required IndexedWidgetBuilder separatorBuilder,
    required int childCount,
  }) : super(
          (context, index) => _separatedBuilder(
            context,
            index,
            builder,
            separatorBuilder,
          ),
          childCount: math.max(0, childCount * 2 - 1),
          semanticIndexCallback: _separatedSemanticIndexCallback,
        );

  static int? _separatedSemanticIndexCallback(Widget _, int localIndex) {
    if (localIndex.isEven) {
      return localIndex ~/ 2;
    }
    return null;
  }

  static Widget? _separatedBuilder(
    BuildContext context,
    int index,
    NullableIndexedWidgetBuilder builder,
    IndexedWidgetBuilder separatorBuilder,
  ) {
    final int itemIndex = index ~/ 2;
    if (index.isEven) {
      return builder(context, itemIndex);
    }
    return separatorBuilder(context, itemIndex);
  }
}