Flutter - Infinite scrolling / loading list view

infinite_list.dart

import 'package:flutter/material.dart';

extension on List {
  Object lastOrNull() {
    return this.isNotEmpty ? this.last : null;
  }
}

typedef ItemWidgetBuilder = Widget Function(Object item);
typedef FutureItemsCallback = Future<List<Object>> Function(Object lastLoadedItem);

typedef ItemCallback = void Function(Object item);

class InfiniteList extends StatefulWidget {
  final ItemWidgetBuilder widgetBuilder;
  final FutureItemsCallback loadMore;
  final ItemCallback onItemSelected;

  InfiniteList({Key key, @required this.widgetBuilder, @required this.loadMore, this.onItemSelected}) : super(key: key);

  @override
  State<StatefulWidget> createState() {
    return InfiniteListState();
  }
}

class InfiniteListState extends State<InfiniteList> {
  List<Object> items = [];
  bool shouldTryToLoadMore = true;

  @override
  void initState() {
    super.initState();
    waitOnItems();
  }

  void waitOnItems() async {
    try {
      final items = await widget.loadMore(this.items.lastOrNull());
      this.shouldTryToLoadMore = items.isNotEmpty;
      setState(() {
        this.items.addAll(items);
      });
    } catch(error) {
      print(error);
    }
  }

  @override
  Widget build(BuildContext context) {
    if (items.isEmpty) {
      return initiallyLoading();
    } else {
      //TODO: show progress bar at the bottom if loading more
      return list();
    }
  }

  Widget list() {
    return ListView.builder(
        itemCount: shouldTryToLoadMore ? null : items.length,
        itemBuilder: (context, index) {
          if (shouldTryToLoadMore && index == items.length - 1) {
            waitOnItems();
            return null;
          } else if (index >= items.length) {
            return null;
          } else if (widget.onItemSelected != null) {
            return InkWell(
              onTap: () => {
                widget.onItemSelected(items[index])
              },
              child: widget.widgetBuilder(items[index]),
            );
          } else {
            return widget.widgetBuilder(items[index]);
          }
        }
      );
  }

  Widget initiallyLoading() {
    return Center(
      child: CircularProgressIndicator(),
    );
  }
}

usage.dart

import 'package:flutter/material.dart';

class Example extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return InfiniteList(
      widgetBuilder: (item) {
        return Text(item);
      },
      loadMore: (lastLoaded) {
        if (lastLoaded == null) {
          //first load request
          return ["hello", "world"];
        } else {
          //subsequent load request(s)
          return [];
        }
      },
      onItemSelected: (item) {
        print(item);
      },
    );
  }
}