import 'package:flutter/material.dart';

void main() => runApp(
  MaterialApp(
    home:     MyApp()
    ,
  )
);


class MyApp extends StatefulWidget{
  MyAppState createState() => MyAppState();
}

class MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {

  TabController tabController;
  Widget _tabBarView;

  @override
  void initState() {
    super.initState();
    tabController = TabController(length: 2, vsync: this,);
    _tabBarView = TabBarView(
        children: [
          DemoTab(),
          DemoTab(),
        ]);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: NestedScrollView(
          controller: ScrollController(keepScrollOffset: true),
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return <Widget>[
              SliverList(
                delegate: SliverChildListDelegate(
                    [
                      Container(height: 300, color: Colors.blue)
                    ]
                ),
              ),
            ];
          },
          body: DefaultTabController(
            length: 2,
            child: Column(
              children: <Widget>[
                Expanded(
                  child: Container(
                      child: _tabBarView
                  ),
                ),
              ],
            ),
          )
      ),
    );
  }
}

class DemoTab extends StatefulWidget{
  DemoTabState createState() => DemoTabState();
}

class DemoTabState extends State<DemoTab> with AutomaticKeepAliveClientMixin<DemoTab>{
  @override
  // TODO: implement wantKeepAlive
  bool get wantKeepAlive => true;


  @override
  Widget build(BuildContext context) {

    return ListView.builder(
      key: UniqueKey(),
      itemBuilder: (b, i) {
      return Container(
        height: 50,
        color: Colors.green,
        margin: EdgeInsets.only(bottom: 3),
        child: Text(i.toString(),),
      );
    }, itemCount: 30,) ;

  }
}