Flutter scroll testing

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  final ScrollController scrollEndController = ScrollController();
  final ValueNotifier<bool> scrollEndValue = ValueNotifier<bool>(false);
  
  final ScrollController addListenerController = ScrollController();
  final ValueNotifier<bool> addListenerValue = ValueNotifier<bool>(false);
  
  final ScrollController scrollMetricsController = ScrollController();
  final ValueNotifier<bool> scrollMetricsValue = ValueNotifier<bool>(false);
  
  void onAddListenerChange() {
    final position = addListenerController.position;
    if (position.extentAfter == 0) {
      addListenerValue.value = false;
    } else {
      addListenerValue.value = true;
    } 
  }
  
  @override
  Widget build(BuildContext context) {
    addListenerController.addListener(onAddListenerChange);
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Row(
          children: <Widget>[
            NotificationListener<ScrollEndNotification>(
              onNotification: (scrollEnd) {
                final metrics = scrollEnd.metrics;
                if (metrics.extentAfter == 0) {
                  scrollEndValue.value = false;
                } else {
                  scrollEndValue.value = true;
                }
                return true;
              },
              child: MyScrollable(color: Colors.red, controller: scrollEndController, notifier: scrollEndValue),
            ),
            MyScrollable(color: Colors.green, controller: addListenerController, notifier: addListenerValue),
            NotificationListener<ScrollMetricsNotification>(
              onNotification: (scrollMetrics) {
                final metrics = scrollMetrics.metrics;
                if (metrics.extentAfter == 0) {
                  scrollMetricsValue.value = false;
                } else {
                  scrollMetricsValue.value = true;
                }
                return true;
              },            
              child: MyScrollable(color: Colors.blue, controller: scrollMetricsController, notifier: scrollMetricsValue),
            ),
          ])
    );
  }
}

class MyScrollable extends StatelessWidget {
  const MyScrollable({super.key, required this.color, required this.controller, required this.notifier});
  final Color color;
  final ScrollController controller;
  final ValueNotifier<bool> notifier;
  
  @override
  Widget build(BuildContext context) {
    return Stack(
        children: [
          SingleChildScrollView(
            controller: controller,
            child: Container(width: 100, height: 400, color: color),
          ),
          Positioned(
            bottom: 0,
            right: 0,
            child: ScrollToBottomButton(controller: controller, notifier: notifier)
          )]);
  }
}

class ScrollToBottomButton extends StatelessWidget {
  const ScrollToBottomButton({super.key, required this.controller, required this.notifier});
  final ValueNotifier<bool> notifier;
  final ScrollController controller;
                
  Future<void> _navigateToBottom(BuildContext context) async {
    controller.animateTo(0, duration: const Duration(milliseconds: 300), curve: Curves.easeIn);
  }       
  
  @override
  Widget build(BuildContext context) {
    return ValueListenableBuilder<bool>(
      builder: (BuildContext context, bool value, Widget? child) {
        return (value && child != null) ? child : const SizedBox.shrink();
      },
      valueListenable: notifier,
      child: TextButton(
        child: const Text("Go", style: TextStyle(color: Colors.black)),
        onPressed: () => _navigateToBottom(context)));
  }
}