Vertical scroll with alphabet scroll - Flutter

main.dart

import 'package:flutter/material.dart';
import 'package:draggable_scrollbar/draggable_scrollbar.dart';
import 'package:flutter/foundation.dart';
import 'package:faker/faker.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  ScrollController _semicircleController = ScrollController();
  List<String> usersList = <String>[];

  @override
  void initState() {
    // TODO: implement initState
    super.initState();

    //generate the list
    for (int i = 0; i < 1000; i++) {
      usersList.add(faker.person.name());
    }

    //sort the list
    usersList.sort((a, b) {
      return a.toLowerCase().compareTo(b.toLowerCase());
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: const Text('vertical scroll indicator'),
      ),
      body: SemicircleDemo(
        controller: _semicircleController,
        nameList: usersList,
      ),
    );
  }
}

class SemicircleDemo extends StatelessWidget {
  final List<String> nameList;

  final ScrollController controller;

  const SemicircleDemo({Key key, @required this.controller, this.nameList})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return DraggableScrollbar.semicircle(
      labelTextBuilder: (offset) {
        final int currentItem = offset ~/ 40;
        var letter = currentItem <= nameList.length - 1
            ? nameList[currentItem].substring(0, 1)
            : nameList.last.substring(0, 1);
        return Text("$letter");
      },
      alwaysVisibleScrollThumb: true,
      labelConstraints: BoxConstraints.tightFor(width: 80.0, height: 30.0),
      controller: controller,
      child: ListView.builder(
        controller: controller,
        itemBuilder: (BuildContext context, int index) {
          return new Card(
            child: new Container(
              decoration: new BoxDecoration(
                gradient: LinearGradient(
                  colors: [const Color(0xFF4286f4), const Color(0xFFad3e67)],
                ),
              ),
              alignment: Alignment.centerLeft,
              padding: const EdgeInsets.all(10.0),
              child: new Text(nameList[index]),
            ),
          );
        },
        itemCount: nameList.length,
      ),
    );
  }
}

pubspec.yaml

name: vertical_scroll_indicator
description: A new Flutter application to test vertical scroll indicator like contacts.

# The following defines the version and build number for your application.
# A version number is three numbers separated by dots, like 1.2.43
# followed by an optional build number separated by a +.
# Both the version and the builder number may be overridden in flutter
# build by specifying --build-name and --build-number, respectively.
# In Android, build-name is used as versionName while build-number used as versionCode.
# Read more about Android versioning at https://developer.android.com/studio/publish/versioning
# In iOS, build-name is used as CFBundleShortVersionString while build-number used as CFBundleVersion.
# Read more about iOS versioning at
# https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
version: 1.0.0+1

environment:
  sdk: ">=2.1.0 <3.0.0"

dependencies:
  draggable_scrollbar: ^0.0.4
  faker: ^1.0.1
  flutter:
    sdk: flutter

  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2

dev_dependencies:
  flutter_test:
    sdk: flutter


# For information on the generic Dart part of this file, see the
# following page: https://www.dartlang.org/tools/pub/pubspec

# The following section is specific to Flutter.
flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the material Icons class.
  uses-material-design: true

  # To add assets to your application, add an assets section, like this:
  # assets:
  #  - images/a_dot_burr.jpeg
  #  - images/a_dot_ham.jpeg

  # An image asset can refer to one or more resolution-specific "variants", see
  # https://flutter.io/assets-and-images/#resolution-aware.

  # For details regarding adding assets from package dependencies, see
  # https://flutter.io/assets-and-images/#from-packages

  # To add custom fonts to your application, add a fonts section here,
  # in this "flutter" section. Each entry in this list should have a
  # "family" key with the font family name, and a "fonts" key with a
  # list giving the asset and other descriptors for the font. For
  # example:
  # fonts:
  #   - family: Schyler
  #     fonts:
  #       - asset: fonts/Schyler-Regular.ttf
  #       - asset: fonts/Schyler-Italic.ttf
  #         style: italic
  #   - family: Trajan Pro
  #     fonts:
  #       - asset: fonts/TrajanPro.ttf
  #       - asset: fonts/TrajanPro_Bold.ttf
  #         weight: 700
  #
  # For details regarding fonts from package dependencies,
  # see https://flutter.io/custom-fonts/#from-packages

Dude thank you!