Show scrollbar in ListView

scrollbar_demo.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  final ScrollController _scrollController = ScrollController();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Scrollbar(
            isAlwaysShown: true,
            controller: _scrollController,
            child: ListView.builder(
                controller: _scrollController,
                itemCount: 100,
                itemBuilder: (context, index) {
                  return Card(
                      child: ListTile(
                    title: Text("Item: ${index + 1}"),
                  ));
                }),
          ),
        ),
      ),
    );
  }
}

what is final ScrollController _scrollController = ScrollController(); do ? why we must add that code

You are initializing new ScrollController so then you can pass it to Scrollbar and ListView. You have to pass the same object and that's why it's defined above.