This code snippets provide a delegate to build separators in a SliverList. Original code from https://github.com/BreX900/easy_widget. I rewrote it, so that it's null-safety conform.

sliver_layout_delegate.dart

import 'package:flutter/widgets.dart';

class SliverLayoutDelegate extends SliverChildBuilderDelegate {
  SliverLayoutDelegate({
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    SemanticIndexCallback? semanticIndexCallback,
    int semanticIndexOffset = 0,
    required int childCount,
    bool surround: false,
    IndexedWidgetBuilder? separatorBuilder,
    required IndexedWidgetBuilder builder,
  }) : super(
          (separatorBuilder == null
              ? builder
              : WidgetBuilderUtility.builderWithSeparator(
                  builder,
                  separatorBuilder,
                  surround: surround,
                )),
          childCount: WidgetBuilderUtility.childCount(
            childCount,
            separatorBuilder != null,
            surround: surround,
          ),
          addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
          addSemanticIndexes: addSemanticIndexes,
          semanticIndexCallback: (Widget _, int localIndex) => localIndex,
          semanticIndexOffset: semanticIndexOffset,
        );

  factory SliverLayoutDelegate.childrenBuilder({
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    SemanticIndexCallback? semanticIndexCallback,
    int semanticIndexOffset = 0,
    bool surround: false,
    required int childCount,
    required Widget separator,
    required IndexedWidgetBuilder builder,
  }) {
    return SliverLayoutDelegate(
      surround: surround,
      childCount: childCount,
      separatorBuilder: (_, __) => separator,
      builder: builder,
      addAutomaticKeepAlives: addAutomaticKeepAlives,
      addRepaintBoundaries: addRepaintBoundaries,
      addSemanticIndexes: addSemanticIndexes,
      semanticIndexCallback: (Widget _, int localIndex) => localIndex,
      semanticIndexOffset: semanticIndexOffset,
    );
  }

  factory SliverLayoutDelegate.children({
    bool surround: false,
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    SemanticIndexCallback? semanticIndexCallback,
    int semanticIndexOffset = 0,
    Widget? separator,
    required List<Widget> children,
  }) {
    return SliverLayoutDelegate(
      surround: surround,
      builder: (context, index) => children[index],
      childCount: children.length,
      separatorBuilder:
          separator == null ? (_, __) => SizedBox() : (_, __) => separator,
      addAutomaticKeepAlives: addAutomaticKeepAlives,
      addRepaintBoundaries: addRepaintBoundaries,
      addSemanticIndexes: addSemanticIndexes,
      semanticIndexCallback: (Widget _, int localIndex) => localIndex,
      semanticIndexOffset: semanticIndexOffset,
    );
  }
}

class ListUtility {
  static List<E> joiner<E>(List<E> list, E builder(int index)) {
    if (list.length == 1) return list.toList();
    final length = (list.length * 2) - 1;
    // ignore: deprecated_member_use
    final newList = <E>[]..length = length;
    for (int i = 0; i < (list.length - 1); i++) {
      final index = i * 2;
      newList
        ..[index] = list[i]
        ..[index + 1] = builder(i);
    }
    return newList..[length - 1] = list.last;
  }

  static List<E> nullGenerator<E>(E builder(int index), {int? itemCount}) {
    if (itemCount != null) return List.generate(itemCount, builder);
    E element = builder(0);
    if (element == null) return <E>[];
    List<E> newList = <E>[element];
    for (int i = 1; element != null; i++) {
      newList..add(element);
      element = builder(i);
    }
    return newList;
  }
}

class WidgetBuilderUtility {
  static IndexedWidgetBuilder builderWithSeparator(
      IndexedWidgetBuilder builder, IndexedWidgetBuilder separatorBuilder,
      {bool surround: false}) {
    if (separatorBuilder == null) return builder;

    return (context, index) {
      final itemIndex = index ~/ 2;
      if (surround ? !(index % 2 == 0) : (index % 2 == 0))
        return builder(context, itemIndex);
      else
        return separatorBuilder(context, itemIndex);
    };
  }

  static int separatorCount(int childCount, [bool surround = false]) {
    return childCount > 0
        ? ((childCount > 1 ? childCount - 1 : 0) + (surround ? 2 : 0))
        : 0;
  }

  static int childCount(int childCount, bool existSeparator,
      {bool surround: false}) {
    return childCount +
        (existSeparator ? separatorCount(childCount, surround) : 0);
  }
}