An example of ScrollController.

main.dart

import 'package:flutter/material.dart';

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

void main() {
  print(Colors.black.hashCode.toString());
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: MyWidget(),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();  
}

class _MyWidgetState extends State<MyWidget> {
  
  ScrollController _controller = ScrollController();
  
  void _onScrollEvent() {
    final extentAfter = _controller.position.extentAfter;
    print("Extent after: $extentAfter");
    if (extentAfter < 300) {
      // Load new items
    }
  }
  
  @override
  void initState() {
    _controller.addListener(_onScrollEvent);
    super.initState();
  }
  
  @override
  void dispose() {
    _controller.removeListener(_onScrollEvent);
    super.dispose();
  }
  
  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      controller: _controller,
      itemCount: 50,
      itemBuilder: (context, index) {
        return ListTile(
          title: Text('List Item ${index + 1}')
        );
      }
    );
  }
}