TabView preserve scroll position

main.dart

import 'dart:math';

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter/foundation.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Test(),
    );
  }
}


@immutable
class Test extends StatelessWidget {
  Test({
    Key key,
  }) : super(key: key);

  List<String> _tabs = ["1", "2"];
  PageStorageBucket _bucket = PageStorageBucket();

  @override
  Widget build(BuildContext context) {
    return Material(
      child: Scaffold(
        body: DefaultTabController(
          length: _tabs.length, // This is the number of tabs.
          child: NestedScrollView(
            headerSliverBuilder:
                (BuildContext context, bool innerBoxIsScrolled) {
              // These are the slivers that show up in the "outer" scroll view.
              return <Widget>[
                // This widget takes the overlapping behavior of the SliverAppBar,
                // and redirects it to the SliverOverlapInjector below. If it is
                // missing, then it is possible for the nested "inner" scroll view
                // below to end up under the SliverAppBar even when the inner
                // scroll view thinks it has not been scrolled.
                // This is not necessary if the "headerSliverBuilder" only builds
                // widgets that do not overlap the next sliver.

                SliverPadding(
                  //top: false,
                  padding: EdgeInsets.only(top: 24),
                  sliver: SliverAppBar(
                    backgroundColor: Colors.black,
                    title: const Text('Books'),
                    floating: true,
                    pinned: true,
                    snap: false,
                    primary: false,
                    //collapsedHeight: kToolbarHeight,
                    // The "forceElevated" property causes the SliverAppBar to show
                    // a shadow. The "innerBoxIsScrolled" parameter is true when the
                    // inner scroll view is scrolled beyond its "zero" point, i.e.
                    // when it appears to be scrolled below the SliverAppBar.
                    // Without this, there are cases where the shadow would appear
                    // or not appear inappropriately, because the SliverAppBar is
                    // not actually aware of the precise position of the inner
                    // scroll views.
                    //forceElevated: innerBoxIsScrolled,
                    bottom: TabBar(
                      // These are the widgets to put in each tab in the tab bar.
                      tabs:
                          _tabs.map((String name) => Tab(text: name)).toList(),
                    ),
                  ),
                ),
              ];
            },
            body: TabBarView(
              // These are the contents of the tab views, below the tabs.
              children: _tabs.map((String name) {
                return SafeArea(
                  top: false,
                  bottom: false,
                  child: Builder(
                    // This Builder is needed to provide a BuildContext that is "inside"
                    // the NestedScrollView, so that sliverOverlapAbsorberHandleFor() can
                    // find the NestedScrollView.
                    builder: (BuildContext context) {
                      return CustomScrollView(
                        // The "controller" and "primary" members should be left
                        // unset, so that the NestedScrollView can control this
                        // inner scroll view.
                        // If the "controller" property is set, then this scroll
                        // view will not be associated with the NestedScrollView.
                        // The PageStorageKey should be unique to this ScrollView;
                        // it allows the list to remember its scroll position when
                        // the tab view is not on the screen.
                        key: PageStorageKey<String>(name),
                        slivers: <Widget>[
                          /* SliverOverlapInjector(
                            // This is the flip side of the SliverOverlapAbsorber above.
                            handle:
                                NestedScrollView.sliverOverlapAbsorberHandleFor(
                                    context),
                          ),*/
                          SliverPadding(
                            padding: const EdgeInsets.all(8.0),
                            // In this example, the inner scroll view has
                            // fixed-height list items, hence the use of
                            // SliverFixedExtentList. However, one could use any
                            // sliver widget here, e.g. SliverList or SliverGrid.
                            sliver: SliverFixedExtentList(
                              // The items in this example are fixed to 48 pixels
                              // high. This matches the Material Design spec for
                              // ListTile widgets.
                              itemExtent: 60.0,
                              delegate: SliverChildBuilderDelegate(
                                (BuildContext context, int index) {
                                  // This builder is called for each child.
                                  // In this example, we just number each list item.
                                  return Container(
                                      color: Color(
                                              (Random().nextDouble() * 0xFFFFFF)
                                                      .toInt() <<
                                                  0)
                                          .withOpacity(1.0));
                                },
                                // The childCount of the SliverChildBuilderDelegate
                                // specifies how many children this inner list
                                // has. In this example, each tab has a list of
                                // exactly 30 items, but this is arbitrary.
                                childCount: 30,
                              ),
                            ),
                          ),
                        ],
                      );
                    },
                  ),
                );
              }).toList(),
            ),
          ),
        ),
      ),
    );
  }
}