import 'package:example/home_page.dart';
import 'package:flutter/material.dart';
void main() {
  runApp(const MyApp());
}
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(title: 'Scroll to top', home: HomePage());
  }
}

import 'package:flutter/material.dart';
class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);
  @override
  State<HomePage> createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _list(),
      appBar: AppBar(title: const Text('Scroll to top')),
    );
  }
  Widget _list() {
    return ListView.builder(itemBuilder: _itemBuilder, itemCount: 100);
  }
    Widget _itemBuilder(BuildContext context, int index) {
    return Container(
      decoration: BoxDecoration(
        border: Border.all(color: Colors.blue),
      ),
      padding: const EdgeInsets.all(20),
      child: Text('$index', style: const TextStyle(fontSize: 20)),
    );
  }
}

import 'package:flutter/material.dart';
class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);
  @override
  State<HomePage> createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
  late ScrollController _scrollController;
  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
  }
  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _list(),
      appBar: AppBar(title: const Text('Scroll to top')),
    );
  }
  Widget _list() {
    return ListView.builder(
      itemBuilder: _itemBuilder,
      itemCount: 100,
      controller: _scrollController,
    );
  }
Widget _itemBuilder(BuildContext context, int index) {
return Container(
  decoration: BoxDecoration(
    border: Border.all(color: Colors.blue),
  ),
  padding: const EdgeInsets.all(20),
  child: Text('$index', style: const TextStyle(fontSize: 20)),
);
  }
}

// iOS FEATURES - status bar tap, back gesture
  // On iOS, tapping the status bar scrolls the app's primary scrollable to the
  // top. We implement this by looking up the  primary scroll controller and
  // scrolling it to the top when tapped.
  void _handleStatusBarTap() {
    final ScrollController? _primaryScrollController = PrimaryScrollController.of(context);
    if (_primaryScrollController != null && _primaryScrollController.hasClients) {
      _primaryScrollController.animateTo(
        0.0,
        duration: const Duration(milliseconds: 300),
        curve: Curves.linear, // TODO(ianh): Use a more appropriate curve.
      );
    }
  }

/// Associates a [ScrollController] with a subtree.
///
/// When a [ScrollView] has [ScrollView.primary] set to true and is not given
/// an explicit [ScrollController], the [ScrollView] uses [of] to find the
/// [ScrollController] associated with its subtree.
///
/// This mechanism can be used to provide default behavior for scroll views in a
/// subtree. For example, the [Scaffold] uses this mechanism to implement the
/// scroll-to-top gesture on iOS.
///
/// Another default behavior handled by the PrimaryScrollController is default
/// [ScrollAction]s. If a ScrollAction is not handled by an otherwise focused
/// part of the application, the ScrollAction will be evaluated using the scroll
/// view associated with a PrimaryScrollController, for example, when executing
/// [Shortcuts] key events like page up and down.
///
/// See also:
///   * [ScrollAction], an [Action] that scrolls the [Scrollable] that encloses
///     the current [primaryFocus] or is attached to the PrimaryScrollController.
///   * [Shortcuts], a widget that establishes a [ShortcutManager] to be used
///     by its descendants when invoking an [Action] via a keyboard key
///     combination that maps to an [Intent].
class PrimaryScrollController extends InheritedWidget {
  /// Creates a widget that associates a [ScrollController] with a subtree.
  const PrimaryScrollController({
    Key? key,
    required ScrollController this.controller,
    required Widget child,
  }) : assert(controller != null),
       super(key: key, child: child);

final ScrollController? scrollController =
        primary ? PrimaryScrollController.of(context) : controller

/// Also when true, the scroll view is used for default [ScrollAction]s. If a
/// ScrollAction is not handled by an otherwise focused part of the application,
/// the ScrollAction will be evaluated using this scroll view, for example,
/// when executing [Shortcuts] key events like page up and down.
///
/// On iOS, this also identifies the scroll view that will scroll to top in
/// response to a tap in the status bar.
/// {@endtemplate}
///

/// Defaults to true when [scrollDirection] is [Axis.vertical] and
/// [controller] is null.
final bool primary;

primary = primary ?? controller == null && identical(scrollDirection, Axis.vertical),

//...
@override
Widget build(BuildContext context) {
  return AnimatedBuilder(
    animation: widget.route.restorationScopeId,
    builder: (BuildContext context, Widget? child) {
      assert(child != null);
      return RestorationScope(
        restorationId: widget.route.restorationScopeId.value,
        child: child!,
      );
    },
    child: _ModalScopeStatus(
      route: widget.route,
      isCurrent: widget.route.isCurrent, // _routeSetState is called if this updates
      canPop: widget.route.canPop, // _routeSetState is called if this updates
      child: Offstage(
        offstage: widget.route.offstage, // _routeSetState is called if this updates
        child: PageStorage(
          bucket: widget.route._storageBucket, // immutable
          child: Builder(
            builder: (BuildContext context) {
              return Actions(
                actions: <Type, Action<Intent>>{
                  DismissIntent: _DismissModalAction(context),
                },
                child: PrimaryScrollController( //<- Вот это место
//...

@override
  Widget build(BuildContext context) {
    return PrimaryScrollController(
      controller: _scrollController,
      child: Scaffold(
        body: _list(),
        appBar: AppBar(title: const Text('Scroll to top')),
      ),
    );
  }

import 'package:example/content_page.dart';
import 'package:flutter/material.dart';
class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);
  @override
  State<HomePage> createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
    int _currentIndex = 0;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Scroll to top')),
      body: IndexedStack(
        index: _currentIndex,
        children: const [
          ContentPage(backgroundColor: Colors.white),
          ContentPage(backgroundColor: Colors.deepOrange),
          ContentPage(backgroundColor: Colors.green),
        ],
      ),
      bottomNavigationBar: BottomNavigationBar(
        onTap: _onTabTapped,
        currentIndex: _currentIndex,
        items: const [
          BottomNavigationBarItem(icon: Icon(Icons.home), label: 'Home'),
          BottomNavigationBarItem(icon: Icon(Icons.mail), label: 'Messages'),
          BottomNavigationBarItem(icon: Icon(Icons.person), label: 'Profile')
        ],
      ),
    );
  }
  void _onTabTapped(int index) {
    setState(() {
      _currentIndex = index;
    });
  }
}

...
  @override
  Widget build(BuildContext context) {
    return PrimaryScrollController(
      controller: _scrollController,
      child: Scaffold(
        primary: true,
        body: _list(),
        backgroundColor: widget.backgroundColor,
      ),
    );
  }
...

class _FakeScrollPositionWithSingleContext
    extends ScrollPositionWithSingleContext {
  _FakeScrollPositionWithSingleContext({
    required BuildContext context,
    required ScrollsToTopCallback callback,
  })  : _callback = callback,
        super(
          physics: const NeverScrollableScrollPhysics(),
          context: _FakeScrollContext(context),
        );
  final ScrollsToTopCallback _callback;
  @override
  Future<void> animateTo(
    double to, {
    required Duration duration,
    required Curve curve,
  }) {
    return _callback(
      ScrollsToTopEvent(to, duration: duration, curve: curve),
    );
  }
}

void _attach(BuildContext context) {
    final primaryScrollController = PrimaryScrollController.of(context);
    if (primaryScrollController == null) return;
final scrollPositionWithSingleContext =
    _FakeScrollPositionWithSingleContext(
  context: context,
  callback: widget.onScrollsToTop,
);
primaryScrollController.attach(scrollPositionWithSingleContext);
_primaryScrollController = primaryScrollController;
_scrollPositionWithSingleContext = scrollPositionWithSingleContext;
  }

/// Widget for catch scrolls-to-top event
class ScrollsToTop extends StatefulWidget {
  /// Creates new ScrollsToTop widget
  const ScrollsToTop({
    Key? key,
    required this.scaffold,
    required this.onScrollsToTop,
  }) : super(key: key);
  /// Primary scaffold of your app
  final Scaffold scaffold;
  /// Callback for handle scrolls-to-top event
  final ScrollsToTopCallback onScrollsToTop;
  @override
  State<ScrollsToTop> createState() => _ScrollsToTopState();
}