import 'package:nested_scroll_view_plus/nested_scroll_view_plus.dart';

NestedScrollViewPlus(
  headerSliverBuilder: (context, innerScrolled) => <Widget>[
    // 1. Wrap your SliverAppBar with OverlapAbsorberPlus
    OverlapAbsorberPlus(
      sliver: SliverAppBar(), // Your SliverAppBar
    ),
  ],
  body: TabBarView(
    children: [
      CustomScrollView(
        // 2. [IMPORTANT] Change the physics of CustomScrollView to AlwaysScrollableScrollPhysics
        physics: const BouncingScrollPhysics(
          parent: AlwaysScrollableScrollPhysics(),
        ),
        slivers: <Widget>[
          // 3. Use OverlapInjectorPlus on top of your inner CustomScrollView
          OverlapInjectorPlus(),
          // Other children of CustomScrollView
          // ...,
        ],
      ),
    ],
  ),
);

class _ExampleState extends State<Example> {
  // 1. Create a GlobalKey
  final GlobalKey<NestedScrollViewStatePlus> myKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return NestedScrollViewPlus(
      // 2. Set the key to NestedScrollViewStatePlus
      key: myKey,
      // ...,
    );
  }

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      // 3. Access the inner or outer scroll controller using GlobalKey<NestedScrollViewStatePlus>
      myKey.currentState!.innerController.addListener(_handleInnerScroll);
      myKey.currentState!.outerController.addListener(_handleOuterScroll);
    });
  }

  void _handleInnerScroll() {
    final innerController = myKey.currentState!.innerController;
    if (innerController.positions.length == 1) {
      print('Scrolling inner nested scrollview: ${innerController.offset}');
    }
  }

  void _handleOuterScroll() {
    final outerController = myKey.currentState!.outerController;
    if (outerController.positions.length == 1) {
      print('Scrolling outer nested scrollview: ${outerController.offset}');
    }
  }
}

CustomScrollView(
  key: PageStorageKey<String>('unique-key'),
  slivers: <Widget>[
    // ...,
  ],
),