Collapsing AppBar when scrolling with SliverAppBar

main_collapsing_toolbar.dart

import 'package:flutter/material.dart';
import 'package:flutter_icons/flutter_icons.dart';

class MaterialsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialPageView();
  }
}

class MaterialPageView extends StatefulWidget {
  @override
  _MaterialPageViewState createState() => _MaterialPageViewState();
}

class _MaterialPageViewState extends State<MaterialPageView> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: DefaultTabController(
        length: 2,
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return [
              SliverAppBar(
                floating: true,
                pinned: true,
                backgroundColor: Colors.grey[300],
                iconTheme: IconThemeData(color: Colors.black),
                actionsIconTheme: IconThemeData(color: Colors.black),
                actions: [
                  IconButton(icon: Icon(FontAwesome5.question_circle), onPressed: null)
                ],
                flexibleSpace: FlexibleSpaceBar(
                  title: SafeArea(
                    child: Container(
                      alignment: Alignment.centerLeft,
                      margin: EdgeInsets.only(top: 8.333),
                      child: Text('Material', style: TextStyle(fontSize: 14, color: Colors.black)),
                    ),
                  ),
                ),
                bottom: TabBar(
                  labelColor: Colors.black,
                  tabs: [
                    Tab(
                      child: Container(
                        child: Text('Daftar Material'),
                      ),
                    ),
                    Tab(
                      child: Container(
                        child: Text('Daftar Supplier'),
                      ),
                    )
                  ],
                ),
              ),
            ];
          },
          body: Container(
            margin: EdgeInsets.all(6),
            child: TabBarView(
              children: [
                Text('Coba'),
                Text('Supplier')
              ],
            ),
          )
        )
      ),
    );
  }
}


class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SliverAppBarDelegate(this._tabBar);

  final TabBar _tabBar;

  @override
  double get minExtent => _tabBar.preferredSize.height;
  @override
  double get maxExtent => _tabBar.preferredSize.height;

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return new Container(
      color: Colors.white,
      child: _tabBar,
    );
  }

  @override
  bool shouldRebuild(_SliverAppBarDelegate oldDelegate) {
    return false;
  }
}

Preview Video : https://youtu.be/1apZ-HjrwcM