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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MainPage(),
    );
  }
}

class MainPage extends StatefulWidget {
  @override
  _MainPageState createState() => _MainPageState();
}

class _MainPageState extends State<MainPage> with TickerProviderStateMixin {
  ScrollController _scrollController;

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    print('MainPage: ${_scrollController.positions}');
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.black,
        title: Text('Demo'),
        actions: [
          CupertinoButton(
            child: Text('Page 1'),
            onPressed: () {
              Navigator.of(context).push(CupertinoPageRoute(builder: (context) {
                return Page1();
              }));
            },
          )
        ],
      ),
      body: NestedScrollView(
        controller: _scrollController,
        headerSliverBuilder: (context, _) {
          return [
            OpacityView(
              controller: _scrollController,
            )
          ];
        },
        body: Container(
          height: 1000,
          color: Colors.red,
        ),
      ),
    );
  }
}

class OpacityView extends AnimatedWidget {
  OpacityView({Key key, this.controller})
      : super(key: key, listenable: controller);

  final ScrollController controller;

  @override
  Widget build(BuildContext context) {
    double opacity = 1;
    print('OpacityView: ${controller.positions}');
    opacity = 1 - (controller.offset / 200).clamp(0, 1);

    return SliverToBoxAdapter(
      child: Opacity(
        opacity: opacity,
        child: Container(
          height: 200,
          color: Colors.blue,
        ),
      ),
    );
  }
}

class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Page 1'),
        backgroundColor: Colors.black,
      ),
    );
  }
}