packages/flutter/lib/src/widgets/nested_scroll_view.dart

// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';
import 'dart:math' as math;

import 'package:flutter/gestures.dart';
import 'package:flutter/painting.dart';
import 'package:flutter/physics.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart';

import 'basic.dart';
import 'framework.dart';
import 'primary_scroll_controller.dart';
import 'scroll_activity.dart';
import 'scroll_context.dart';
import 'scroll_controller.dart';
import 'scroll_metrics.dart';
import 'scroll_physics.dart';
import 'scroll_position.dart';
import 'scroll_view.dart';
import 'sliver.dart';
import 'ticker_provider.dart';
import 'viewport.dart';

// Examples can assume:
// List<String> _tabs;

/// Signature used by [NestedScrollView] for building its header.
///
/// The `innerBoxIsScrolled` argument is typically used to control the
/// [SliverAppBar.forceElevated] property to ensure that the app bar shows a
/// shadow, since it would otherwise not necessarily be aware that it had
/// content ostensibly below it.
typedef List<Widget> NestedScrollViewHeaderSliversBuilder(BuildContext context, bool innerBoxIsScrolled);

/// A scrolling view inside of which can be nested other scrolling views, with
/// their scroll positions being intrinsically linked.
///
/// The most common use case for this widget is a scrollable view with a
/// flexible [SliverAppBar] containing a [TabBar] in the header (build by
/// [headerSliverBuilder], and with a [TabBarView] in the [body], such that the
/// scrollable view's contents vary based on which tab is visible.
///
/// ## Motivation
///
/// In a normal [ScrollView], there is one set of slivers (the components of the
/// scrolling view). If one of those slivers hosted a [TabBarView] which scrolls
/// in the opposite direction (e.g. allowing the user to swipe horizontally
/// between the pages represented by the tabs, while the list scrolls
/// vertically), then any list inside that [TabBarView] would not interact with
/// the outer [ScrollView]. For example, flinging the inner list to scroll to
/// the top would not cause a collapsed [SliverAppBar] in the outer [ScrollView]
/// to expand.
///
/// [NestedScrollView] solves this problem by providing custom
/// [ScrollController]s for the outer [ScrollView] and the inner [ScrollView]s
/// (those inside the [TabBarView], hooking them together so that they appear,
/// to the user, as one coherent scroll view.
///
/// ## Sample code
///
/// This example shows a [NestedScrollView] whose header is the combination of a
/// [TabBar] in a [SliverAppBar] and whose body is a [TabBarView]. It uses a
/// [SliverOverlapAbsorber]/[SliverOverlapInjector] pair to make the inner lists
/// align correctly, and it uses [SafeArea] to avoid any horizontal disturbances
/// (e.g. the "notch" on iOS when the phone is horizontal). In addition,
/// [PageStorageKey]s are used to remember the scroll position of each tab's
/// list.
///
/// In the example below, `_tabs` is a list of strings, one for each tab, giving
/// the tab labels. In a real application, it would be replaced by the actual
/// data model being represented.
///
/// ```dart
/// new DefaultTabController(
///   length: _tabs.length, // This is the number of tabs.
///   child: new NestedScrollView(
///     headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
///       // These are the slivers that show up in the "outer" scroll view.
///       return <Widget>[
///         new SliverOverlapAbsorber(
///           // 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.
///           handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
///           child: new SliverAppBar(
///             title: const Text('Books'), // This is the title in the app bar.
///             pinned: true,
///             expandedHeight: 150.0,
///             // 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: new TabBar(
///               // These are the widgets to put in each tab in the tab bar.
///               tabs: _tabs.map((String name) => new Tab(text: name)).toList(),
///             ),
///           ),
///         ),
///       ];
///     },
///     body: new TabBarView(
///       // These are the contents of the tab views, below the tabs.
///       children: _tabs.map((String name) {
///         return new SafeArea(
///           top: false,
///           bottom: false,
///           child: new 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 new 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: new PageStorageKey<String>(name),
///                 slivers: <Widget>[
///                   new SliverOverlapInjector(
///                     // This is the flip side of the SliverOverlapAbsorber above.
///                     handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
///                   ),
///                   new 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: new SliverFixedExtentList(
///                       // The items in this example are fixed to 48 pixels
///                       // high. This matches the Material Design spec for
///                       // ListTile widgets.
///                       itemExtent: 48.0,
///                       delegate: new SliverChildBuilderDelegate(
///                         (BuildContext context, int index) {
///                           // This builder is called for each child.
///                           // In this example, we just number each list item.
///                           return new ListTile(
///                             title: new Text('Item $index'),
///                           );
///                         },
///                         // 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(),
///     ),
///   ),
/// )
/// ```
class NestedScrollView extends StatefulWidget {
  /// Creates a nested scroll view.
  ///
  /// The [reverse], [headerSliverBuilder], and [body] arguments must not be
  /// null.
  const NestedScrollView({
    Key key,
    this.controller,
    this.scrollDirection: Axis.vertical,
    this.reverse: false,
    this.physics,
    @required this.headerSliverBuilder,
    @required this.body,
  }) : assert(scrollDirection != null),
       assert(reverse != null),
       assert(headerSliverBuilder != null),
       assert(body != null),
       super(key: key);

  /// An object that can be used to control the position to which the outer
  /// scroll view is scrolled.
  final ScrollController controller;

  /// The axis along which the scroll view scrolls.
  ///
  /// Defaults to [Axis.vertical].
  final Axis scrollDirection;

  /// Whether the scroll view scrolls in the reading direction.
  ///
  /// For example, if the reading direction is left-to-right and
  /// [scrollDirection] is [Axis.horizontal], then the scroll view scrolls from
  /// left to right when [reverse] is false and from right to left when
  /// [reverse] is true.
  ///
  /// Similarly, if [scrollDirection] is [Axis.vertical], then the scroll view
  /// scrolls from top to bottom when [reverse] is false and from bottom to top
  /// when [reverse] is true.
  ///
  /// Defaults to false.
  final bool reverse;

  /// How the scroll view should respond to user input.
  ///
  /// For example, determines how the scroll view continues to animate after the
  /// user stops dragging the scroll view (providing a custom implementation of
  /// [ScrollPhysics.createBallisticSimulation] allows this particular aspect of
  /// the physics to be overridden).
  ///
  /// Defaults to matching platform conventions.
  ///
  /// The [ScrollPhysics.applyBoundaryConditions] implementation of the provided
  /// object should not allow scrolling outside the scroll extent range
  /// described by the [ScrollMetrics.minScrollExtent] and
  /// [ScrollMetrics.maxScrollExtent] properties passed to that method. If that
  /// invariant is not maintained, the nested scroll view may respond to user
  /// scrolling erratically.
  final ScrollPhysics physics;

  /// A builder for any widgets that are to precede the inner scroll views (as
  /// given by [body]).
  ///
  /// Typically this is used to create a [SliverAppBar] with a [TabBar].
  final NestedScrollViewHeaderSliversBuilder headerSliverBuilder;

  /// The widget to show inside the [NestedScrollView].
  ///
  /// Typically this will be [TabBarView].
  ///
  /// The [body] is built in a context that provides a [PrimaryScrollController]
  /// that interacts with the [NestedScrollView]'s scroll controller. Any
  /// [ListView] or other [Scrollable]-based widget inside the [body] that is
  /// intended to scroll with the [NestedScrollView] should therefore not be
  /// given an explicit [ScrollController], instead allowing it to default to
  /// the [PrimaryScrollController] provided by the [NestedScrollView].
  final Widget body;

  /// Returns the [SliverOverlapAbsorberHandle] of the nearest ancestor
  /// [NestedScrollView].
  ///
  /// This is necessary to configure the [SliverOverlapAbsorber] and
  /// [SliverOverlapInjector] widgets.
  ///
  /// For sample code showing how to use this method, see the [NestedScrollView]
  /// documentation.
  static SliverOverlapAbsorberHandle sliverOverlapAbsorberHandleFor(BuildContext context) {
    final _InheritedNestedScrollView target = context.inheritFromWidgetOfExactType(_InheritedNestedScrollView);
    assert(target != null, 'NestedScrollView.sliverOverlapAbsorberHandleFor must be called with a context that contains a NestedScrollView.');
    return target.state._absorberHandle;
  }

  List<Widget> _buildSlivers(BuildContext context, ScrollController innerController, bool bodyIsScrolled) {
    final List<Widget> slivers = <Widget>[];
    slivers.addAll(headerSliverBuilder(context, bodyIsScrolled));
    slivers.add(new SliverFillRemaining(
      child: new PrimaryScrollController(
        controller: innerController,
        child: body,
      ),
    ));
    return slivers;
  }

  @override
  _NestedScrollViewState createState() => new _NestedScrollViewState();
}

class _NestedScrollViewState extends State<NestedScrollView> {
  final SliverOverlapAbsorberHandle _absorberHandle = new SliverOverlapAbsorberHandle();

  _NestedScrollCoordinator _coordinator;

  @override
  void initState() {
    super.initState();
    _coordinator = new _NestedScrollCoordinator(this, widget.controller, _handleHasScrolledBodyChanged);
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    _coordinator.setParent(widget.controller);
  }

  @override
  void didUpdateWidget(NestedScrollView oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.controller != widget.controller)
      _coordinator.setParent(widget.controller);
  }

  @override
  void dispose() {
    _coordinator.dispose();
    _coordinator = null;
    super.dispose();
  }

  bool _lastHasScrolledBody;

  void _handleHasScrolledBodyChanged() {
    if (!mounted)
      return;
    final bool newHasScrolledBody = _coordinator.hasScrolledBody;
    if (_lastHasScrolledBody != newHasScrolledBody) {
      setState(() {
        // _coordinator.hasScrolledBody changed (we use it in the build method)
        // (We record _lastHasScrolledBody in the build() method, rather than in
        // this setState call, because the build() method may be called more
        // often than just from here, and we want to only call setState when the
        // new value is different than the last built value.)
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return new _InheritedNestedScrollView(
      state: this,
      child: new Builder(
        builder: (BuildContext context) {
          _lastHasScrolledBody = _coordinator.hasScrolledBody;
          return new _NestedScrollViewCustomScrollView(
            scrollDirection: widget.scrollDirection,
            reverse: widget.reverse,
            physics: widget.physics != null
                ? widget.physics.applyTo(const ClampingScrollPhysics())
                : const ClampingScrollPhysics(),
            controller: _coordinator._outerController,
            slivers: widget._buildSlivers(
              context,
              _coordinator._innerController,
              _lastHasScrolledBody,
            ),
            handle: _absorberHandle,
          );
        },
      ),
    );
  }
}

class _NestedScrollViewCustomScrollView extends CustomScrollView {
  _NestedScrollViewCustomScrollView({
    @required Axis scrollDirection,
    @required bool reverse,
    @required ScrollPhysics physics,
    @required ScrollController controller,
    @required List<Widget> slivers,
    @required this.handle,
  }) : super(
         scrollDirection: scrollDirection,
         reverse: reverse,
         physics: physics,
         controller: controller,
         slivers: slivers,
       );

  final SliverOverlapAbsorberHandle handle;

  @override
  Widget buildViewport(
    BuildContext context,
    ViewportOffset offset,
    AxisDirection axisDirection,
    List<Widget> slivers,
  ) {
    assert(!shrinkWrap);
    return new NestedScrollViewViewport(
      axisDirection: axisDirection,
      offset: offset,
      slivers: slivers,
      handle: handle,
    );
  }
}

class _InheritedNestedScrollView extends InheritedWidget {
  const _InheritedNestedScrollView({
    Key key,
    @required this.state,
    @required Widget child,
  }) : assert(state != null),
       assert(child != null),
       super(key: key, child: child);

  final _NestedScrollViewState state;

  @override
  bool updateShouldNotify(_InheritedNestedScrollView old) => state != old.state;
}

class _NestedScrollMetrics extends FixedScrollMetrics {
  _NestedScrollMetrics({
    @required double minScrollExtent,
    @required double maxScrollExtent,
    @required double pixels,
    @required double viewportDimension,
    @required AxisDirection axisDirection,
    @required this.minRange,
    @required this.maxRange,
    @required this.correctionOffset,
  }) : super(
    minScrollExtent: minScrollExtent,
    maxScrollExtent: maxScrollExtent,
    pixels: pixels,
    viewportDimension: viewportDimension,
    axisDirection: axisDirection,
  );

  @override
  _NestedScrollMetrics copyWith({
    double minScrollExtent,
    double maxScrollExtent,
    double pixels,
    double viewportDimension,
    AxisDirection axisDirection,
    double minRange,
    double maxRange,
    double correctionOffset,
  }) {
    return new _NestedScrollMetrics(
      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
      pixels: pixels ?? this.pixels,
      viewportDimension: viewportDimension ?? this.viewportDimension,
      axisDirection: axisDirection ?? this.axisDirection,
      minRange: minRange ?? this.minRange,
      maxRange: maxRange ?? this.maxRange,
      correctionOffset: correctionOffset ?? this.correctionOffset,
    );
  }

  final double minRange;

  final double maxRange;

  final double correctionOffset;
}

typedef ScrollActivity _NestedScrollActivityGetter(_NestedScrollPosition position);

class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldController {
  _NestedScrollCoordinator(this._state, this._parent, this._onHasScrolledBodyChanged) {
    final double initialScrollOffset = _parent?.initialScrollOffset ?? 0.0;
    _outerController = new _NestedScrollController(this, initialScrollOffset: initialScrollOffset, debugLabel: 'outer');
    _innerController = new _NestedScrollController(this, initialScrollOffset: 0.0, debugLabel: 'inner');
  }

  final _NestedScrollViewState _state;
  ScrollController _parent;
  final VoidCallback _onHasScrolledBodyChanged;

  _NestedScrollController _outerController;
  _NestedScrollController _innerController;

  _NestedScrollPosition get _outerPosition {
    if (!_outerController.hasClients)
      return null;
    return _outerController.nestedPositions.single;
  }

  Iterable<_NestedScrollPosition> get _innerPositions {
    return _innerController.nestedPositions;
  }

  bool get canScrollBody {
    final _NestedScrollPosition outer = _outerPosition;
    if (outer == null)
      return true;
    return outer.haveDimensions && outer.extentAfter == 0.0;
  }

  bool get hasScrolledBody {
    for (_NestedScrollPosition position in _innerPositions) {
      if (position.pixels > position.minScrollExtent)
        return true;
    }
    return false;
  }

  void updateShadow() {
    if (_onHasScrolledBodyChanged != null)
      _onHasScrolledBodyChanged();
  }

  ScrollDirection get userScrollDirection => _userScrollDirection;
  ScrollDirection _userScrollDirection = ScrollDirection.idle;

  void updateUserScrollDirection(ScrollDirection value) {
    assert(value != null);
    if (userScrollDirection == value)
      return;
    _userScrollDirection = value;
    _outerPosition.didUpdateScrollDirection(value);
    for (_NestedScrollPosition position in _innerPositions)
      position.didUpdateScrollDirection(value);
  }

  ScrollDragController _currentDrag;

  void beginActivity(ScrollActivity newOuterActivity, _NestedScrollActivityGetter innerActivityGetter) {
    _outerPosition.beginActivity(newOuterActivity);
    bool scrolling = newOuterActivity.isScrolling;
    for (_NestedScrollPosition position in _innerPositions) {
      final ScrollActivity newInnerActivity = innerActivityGetter(position);
      position.beginActivity(newInnerActivity);
      scrolling = scrolling && newInnerActivity.isScrolling;
    }
    _currentDrag?.dispose();
    _currentDrag = null;
    if (!scrolling)
      updateUserScrollDirection(ScrollDirection.idle);
  }

  @override
  AxisDirection get axisDirection => _outerPosition.axisDirection;

  static IdleScrollActivity _createIdleScrollActivity(_NestedScrollPosition position) {
    return new IdleScrollActivity(position);
  }

  @override
  void goIdle() {
    beginActivity(_createIdleScrollActivity(_outerPosition), _createIdleScrollActivity);
  }

  @override
  void goBallistic(double velocity) {
    beginActivity(
      createOuterBallisticScrollActivity(velocity),
      (_NestedScrollPosition position) => createInnerBallisticScrollActivity(position, velocity),
    );
  }

  ScrollActivity createOuterBallisticScrollActivity(double velocity) {
    // This function creates a ballistic scroll for the outer scrollable.
    //
    // It assumes that the outer scrollable can't be overscrolled, and sets up a
    // ballistic scroll over the combined space of the innerPositions and the
    // outerPosition.

    // First we must pick a representative inner position that we will care
    // about. This is somewhat arbitrary. Ideally we'd pick the one that is "in
    // the center" but there isn't currently a good way to do that so we
    // arbitrarily pick the one that is the furthest away from the infinity we
    // are heading towards.
    _NestedScrollPosition innerPosition;
    if (velocity != 0.0) {
      for (_NestedScrollPosition position in _innerPositions) {
        if (innerPosition != null) {
          if (velocity > 0.0) {
            if (innerPosition.pixels < position.pixels)
              continue;
          } else {
            assert(velocity < 0.0);
            if (innerPosition.pixels > position.pixels)
              continue;
          }
        }
        innerPosition = position;
      }
    }

    if (innerPosition == null) {
      // It's either just us or a velocity=0 situation.
      return _outerPosition.createBallisticScrollActivity(
        _outerPosition.physics.createBallisticSimulation(_outerPosition, velocity),
        mode: _NestedBallisticScrollActivityMode.independent,
      );
    }

    final _NestedScrollMetrics metrics = _getMetrics(innerPosition, velocity);

    return _outerPosition.createBallisticScrollActivity(
      _outerPosition.physics.createBallisticSimulation(metrics, velocity),
      mode: _NestedBallisticScrollActivityMode.outer,
      metrics: metrics,
    );
  }

  @protected
  ScrollActivity createInnerBallisticScrollActivity(_NestedScrollPosition position, double velocity) {
    return position.createBallisticScrollActivity(
      position.physics.createBallisticSimulation(
        velocity == 0 ? position : _getMetrics(position, velocity),
        velocity,
      ),
      mode: _NestedBallisticScrollActivityMode.inner,
    );
  }

  _NestedScrollMetrics _getMetrics(_NestedScrollPosition innerPosition, double velocity) {
    assert(innerPosition != null);
    double pixels, minRange, maxRange, correctionOffset, extra;
    if (innerPosition.pixels == innerPosition.minScrollExtent) {
      pixels = _outerPosition.pixels.clamp(_outerPosition.minScrollExtent, _outerPosition.maxScrollExtent); // TODO(ianh): gracefully handle out-of-range outer positions
      minRange = _outerPosition.minScrollExtent;
      maxRange = _outerPosition.maxScrollExtent;
      assert(minRange <= maxRange);
      correctionOffset = 0.0;
      extra = 0.0;
    } else {
      assert(innerPosition.pixels != innerPosition.minScrollExtent);
      if (innerPosition.pixels < innerPosition.minScrollExtent) {
        pixels = innerPosition.pixels - innerPosition.minScrollExtent + _outerPosition.minScrollExtent;
      } else {
        assert(innerPosition.pixels > innerPosition.minScrollExtent);
        pixels = innerPosition.pixels - innerPosition.minScrollExtent + _outerPosition.maxScrollExtent;
      }
      if ((velocity > 0.0) && (innerPosition.pixels > innerPosition.minScrollExtent)) {
        // This handles going forward (fling up) and inner list is scrolled past
        // zero. We want to grab the extra pixels immediately to shrink.
        extra = _outerPosition.maxScrollExtent - _outerPosition.pixels;
        assert(extra >= 0.0);
        minRange = pixels;
        maxRange = pixels + extra;
        assert(minRange <= maxRange);
        correctionOffset = _outerPosition.pixels - pixels;
      } else if ((velocity < 0.0) && (innerPosition.pixels < innerPosition.minScrollExtent)) {
        // This handles going backward (fling down) and inner list is
        // underscrolled. We want to grab the extra pixels immediately to grow.
        extra = _outerPosition.pixels - _outerPosition.minScrollExtent;
        assert(extra >= 0.0);
        minRange = pixels - extra;
        maxRange = pixels;
        assert(minRange <= maxRange);
        correctionOffset = _outerPosition.pixels - pixels;
      } else {
        // This handles going forward (fling up) and inner list is
        // underscrolled, OR, going backward (fling down) and inner list is
        // scrolled past zero. We want to skip the pixels we don't need to grow
        // or shrink over.
        if (velocity > 0.0) {
          // shrinking
          extra = _outerPosition.minScrollExtent - _outerPosition.pixels;
        } else {
          assert(velocity < 0.0);
          // growing
          extra = _outerPosition.pixels - (_outerPosition.maxScrollExtent - _outerPosition.minScrollExtent);
        }
        assert(extra <= 0.0);
        minRange = _outerPosition.minScrollExtent;
        maxRange = _outerPosition.maxScrollExtent + extra;
        assert(minRange <= maxRange);
        correctionOffset = 0.0;
      }
    }
    return new _NestedScrollMetrics(
      minScrollExtent: _outerPosition.minScrollExtent,
      maxScrollExtent: _outerPosition.maxScrollExtent + innerPosition.maxScrollExtent - innerPosition.minScrollExtent + extra,
      pixels: pixels,
      viewportDimension: _outerPosition.viewportDimension,
      axisDirection: _outerPosition.axisDirection,
      minRange: minRange,
      maxRange: maxRange,
      correctionOffset: correctionOffset,
    );
  }

  double unnestOffset(double value, _NestedScrollPosition source) {
    if (source == _outerPosition)
      return value.clamp(_outerPosition.minScrollExtent, _outerPosition.maxScrollExtent);
    if (value < source.minScrollExtent)
      return value - source.minScrollExtent + _outerPosition.minScrollExtent;
    return value - source.minScrollExtent + _outerPosition.maxScrollExtent;
  }

  double nestOffset(double value, _NestedScrollPosition target) {
    if (target == _outerPosition)
      return value.clamp(_outerPosition.minScrollExtent, _outerPosition.maxScrollExtent);
    if (value < _outerPosition.minScrollExtent)
      return value - _outerPosition.minScrollExtent + target.minScrollExtent;
    if (value > _outerPosition.maxScrollExtent)
      return value - _outerPosition.maxScrollExtent + target.minScrollExtent;
    return target.minScrollExtent;
  }

  void updateCanDrag() {
    if (!_outerPosition.haveDimensions)
      return;
    double maxInnerExtent = 0.0;
    for (_NestedScrollPosition position in _innerPositions) {
      if (!position.haveDimensions)
        return;
      maxInnerExtent = math.max(maxInnerExtent, position.maxScrollExtent - position.minScrollExtent);
    }
    _outerPosition.updateCanDrag(maxInnerExtent);
  }

  Future<Null> animateTo(double to, {
    @required Duration duration,
    @required Curve curve,
  }) async {
    final DrivenScrollActivity outerActivity = _outerPosition.createDrivenScrollActivity(
      nestOffset(to, _outerPosition),
      duration,
      curve,
    );
    final List<Future<Null>> resultFutures = <Future<Null>>[outerActivity.done];
    beginActivity(
      outerActivity,
      (_NestedScrollPosition position) {
        final DrivenScrollActivity innerActivity = position.createDrivenScrollActivity(
          nestOffset(to, position),
          duration,
          curve,
        );
        resultFutures.add(innerActivity.done);
        return innerActivity;
      },
    );
    await Future.wait<Null>(resultFutures);
  }

  void jumpTo(double to) {
    goIdle();
    _outerPosition.localJumpTo(nestOffset(to, _outerPosition));
    for (_NestedScrollPosition position in _innerPositions)
      position.localJumpTo(nestOffset(to, position));
    goBallistic(0.0);
  }

  @override
  double setPixels(double newPixels) {
    assert(false);
    return 0.0;
  }

  ScrollHoldController hold(VoidCallback holdCancelCallback) {
    beginActivity(
      new HoldScrollActivity(delegate: _outerPosition, onHoldCanceled: holdCancelCallback),
      (_NestedScrollPosition position) => new HoldScrollActivity(delegate: position),
    );
    return this;
  }

  @override
  void cancel() {
    goBallistic(0.0);
  }

  Drag drag(DragStartDetails details, VoidCallback dragCancelCallback) {
    final ScrollDragController drag = new ScrollDragController(
      delegate: this,
      details: details,
      onDragCanceled: dragCancelCallback,
    );
    beginActivity(
      new DragScrollActivity(_outerPosition, drag),
      (_NestedScrollPosition position) => new DragScrollActivity(position, drag),
    );
    assert(_currentDrag == null);
    _currentDrag = drag;
    return drag;
  }

  @override
  void applyUserOffset(double delta) {
    updateUserScrollDirection(delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse);
    assert(delta != 0.0);
    if (_innerPositions.isEmpty) {
      _outerPosition.applyFullDragUpdate(delta);
    } else if (delta < 0.0) {
      // dragging "up"
      // TODO(ianh): prioritize first getting rid of overscroll, and then the
      // outer view, so that the app bar will scroll out of the way asap.
      // Right now we ignore overscroll. This works fine on Android but looks
      // weird on iOS if you fling down then up. The problem is it's not at all
      // clear what this should do when you have multiple inner positions at
      // different levels of overscroll.
      final double innerDelta = _outerPosition.applyClampedDragUpdate(delta);
      if (innerDelta != 0.0) {
        for (_NestedScrollPosition position in _innerPositions)
          position.applyFullDragUpdate(innerDelta);
      }
    } else {
      // dragging "down" - delta is positive
      // prioritize the inner views, so that the inner content will move before the app bar grows
      double outerDelta = 0.0; // it will go positive if it changes
      final List<double> overscrolls = <double>[];
      final List<_NestedScrollPosition> innerPositions = _innerPositions.toList();
      for (_NestedScrollPosition position in innerPositions) {
        final double overscroll = position.applyClampedDragUpdate(delta);
        outerDelta = math.max(outerDelta, overscroll);
        overscrolls.add(overscroll);
      }
      if (outerDelta != 0.0)
        outerDelta -= _outerPosition.applyClampedDragUpdate(outerDelta);
      // now deal with any overscroll
      for (int i = 0; i < innerPositions.length; ++i) {
        final double remainingDelta = overscrolls[i] - outerDelta;
        if (remainingDelta > 0.0)
          innerPositions[i].applyFullDragUpdate(remainingDelta);
      }
    }
  }

  void setParent(ScrollController value) {
    _parent = value;
    updateParent();
  }

  void updateParent() {
    _outerPosition?.setParent(_parent ?? PrimaryScrollController.of(_state.context));
  }

  @mustCallSuper
  void dispose() {
    _currentDrag?.dispose();
    _currentDrag = null;
    _outerController.dispose();
    _innerController.dispose();
  }

  @override
  String toString() => '$runtimeType(outer=$_outerController; inner=$_innerController)';
}

class _NestedScrollController extends ScrollController {
  _NestedScrollController(this.coordinator, {
    double initialScrollOffset: 0.0,
    String debugLabel,
  }) : super(initialScrollOffset: initialScrollOffset, debugLabel: debugLabel);

  final _NestedScrollCoordinator coordinator;

  @override
  ScrollPosition createScrollPosition(
    ScrollPhysics physics,
    ScrollContext context,
    ScrollPosition oldPosition,
  ) {
    return new _NestedScrollPosition(
      coordinator: coordinator,
      physics: physics,
      context: context,
      initialPixels: initialScrollOffset,
      oldPosition: oldPosition,
      debugLabel: debugLabel,
    );
  }

  @override
  void attach(ScrollPosition position) {
    assert(position is _NestedScrollPosition);
    super.attach(position);
    coordinator.updateParent();
    coordinator.updateCanDrag();
    position.addListener(_scheduleUpdateShadow);
    _scheduleUpdateShadow();
  }

  @override
  void detach(ScrollPosition position) {
    assert(position is _NestedScrollPosition);
    position.removeListener(_scheduleUpdateShadow);
    super.detach(position);
    _scheduleUpdateShadow();
  }

  void _scheduleUpdateShadow() {
    // We do this asynchronously for attach() so that the new position has had
    // time to be initialized, and we do it asynchronously for detach() and from
    // the position change notifications because those happen synchronously
    // during a frame, at a time where it's too late to call setState. Since the
    // result is usually animated, the lag incurred is no big deal.
    SchedulerBinding.instance.addPostFrameCallback(
      (Duration timeStamp) {
        coordinator.updateShadow();
      }
    );
  }

  Iterable<_NestedScrollPosition> get nestedPositions sync* {
    // TODO(vegorov) use instance method version of castFrom when it is available.
    yield* Iterable.castFrom<ScrollPosition, _NestedScrollPosition>(positions);
  }
}

// The _NestedScrollPosition is used by both the inner and outer viewports of a
// NestedScrollView. It tracks the offset to use for those viewports, and knows
// about the _NestedScrollCoordinator, so that when activities are triggered on
// this class, they can defer, or be influenced by, the coordinator.
class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDelegate {
  _NestedScrollPosition({
    @required ScrollPhysics physics,
    @required ScrollContext context,
    double initialPixels: 0.0,
    ScrollPosition oldPosition,
    String debugLabel,
    @required this.coordinator,
  }) : super(
    physics: physics,
    context: context,
    oldPosition: oldPosition,
    debugLabel: debugLabel,
  ) {
    if (pixels == null && initialPixels != null)
      correctPixels(initialPixels);
    if (activity == null)
      goIdle();
    assert(activity != null);
    saveScrollOffset(); // in case we didn't restore but could, so that we don't restore it later
  }

  final _NestedScrollCoordinator coordinator;

  TickerProvider get vsync => context.vsync;

  ScrollController _parent;

  void setParent(ScrollController value) {
    _parent?.detach(this);
    _parent = value;
    _parent?.attach(this);
  }

  @override
  AxisDirection get axisDirection => context.axisDirection;

  @override
  void absorb(ScrollPosition other) {
    super.absorb(other);
    activity.updateDelegate(this);
  }

  @override
  void restoreScrollOffset() {
    if (coordinator.canScrollBody)
      super.restoreScrollOffset();
  }

  // Returns the amount of delta that was not used.
  //
  // Positive delta means going down (exposing stuff above), negative delta
  // going up (exposing stuff below).
  double applyClampedDragUpdate(double delta) {
    assert(delta != 0.0);
    // If we are going towards the maxScrollExtent (negative scroll offset),
    // then the furthest we can be in the minScrollExtent direction is negative
    // infinity. For example, if we are already overscrolled, then scrolling to
    // reduce the overscroll should not disallow the overscroll.
    //
    // If we are going towards the minScrollExtent (positive scroll offset),
    // then the furthest we can be in the minScrollExtent direction is wherever
    // we are now, if we are already overscrolled (in which case pixels is less
    // than the minScrollExtent), or the minScrollExtent if we are not.
    //
    // In other words, we cannot, via applyClampedDragUpdate, _enter_ an
    // overscroll situation.
    //
    // An overscroll situation might be nonetheless entered via several means.
    // One is if the physics allow it, via applyFullDragUpdate (see below). An
    // overscroll situation can also be forced, e.g. if the scroll position is
    // artificially set using the scroll controller.
    final double min = delta < 0.0 ? -double.infinity : math.min(minScrollExtent, pixels);
    // The logic for max is equivalent but on the other side.
    final double max = delta > 0.0 ? double.infinity : math.max(maxScrollExtent, pixels);
    final double oldPixels = pixels;
    final double newPixels = (pixels - delta).clamp(min, max);
    final double clampedDelta = newPixels - pixels;
    if (clampedDelta == 0.0)
      return delta;
    final double overscroll = physics.applyBoundaryConditions(this, newPixels);
    final double actualNewPixels = newPixels - overscroll;
    final double offset = actualNewPixels - oldPixels;
    if (offset != 0.0) {
      forcePixels(actualNewPixels);
      didUpdateScrollPositionBy(offset);
    }
    return delta + offset;
  }

  // Returns the overscroll.
  double applyFullDragUpdate(double delta) {
    assert(delta != 0.0);
    final double oldPixels = pixels;
    // Apply friction:
    final double newPixels = pixels - physics.applyPhysicsToUserOffset(this, delta);
    if (oldPixels == newPixels)
      return 0.0; // delta must have been so small we dropped it during floating point addition
    // Check for overscroll:
    final double overscroll = physics.applyBoundaryConditions(this, newPixels);
    final double actualNewPixels = newPixels - overscroll;
    if (actualNewPixels != oldPixels) {
      forcePixels(actualNewPixels);
      didUpdateScrollPositionBy(actualNewPixels - oldPixels);
    }
    if (overscroll != 0.0) {
      didOverscrollBy(overscroll);
      return overscroll;
    }
    return 0.0;
  }

  @override
  ScrollDirection get userScrollDirection => coordinator.userScrollDirection;

  DrivenScrollActivity createDrivenScrollActivity(double to, Duration duration, Curve curve) {
    return new DrivenScrollActivity(
      this,
      from: pixels,
      to: to,
      duration: duration,
      curve: curve,
      vsync: vsync,
    );
  }

  @override
  double applyUserOffset(double delta) {
    assert(false);
    return 0.0;
  }

  // This is called by activities when they finish their work.
  @override
  void goIdle() {
    beginActivity(new IdleScrollActivity(this));
  }

  // This is called by activities when they finish their work and want to go ballistic.
  @override
  void goBallistic(double velocity) {
    Simulation simulation;
    if (velocity != 0.0 || outOfRange)
      simulation = physics.createBallisticSimulation(this, velocity);
    beginActivity(createBallisticScrollActivity(
      simulation,
      mode: _NestedBallisticScrollActivityMode.independent,
    ));
  }

  ScrollActivity createBallisticScrollActivity(Simulation simulation, {
    @required _NestedBallisticScrollActivityMode mode,
    _NestedScrollMetrics metrics,
  }) {
    if (simulation == null)
      return new IdleScrollActivity(this);
    assert(mode != null);
    switch (mode) {
      case _NestedBallisticScrollActivityMode.outer:
        assert(metrics != null);
        if (metrics.minRange == metrics.maxRange)
          return new IdleScrollActivity(this);
        return new _NestedOuterBallisticScrollActivity(coordinator, this, metrics, simulation, context.vsync);
      case _NestedBallisticScrollActivityMode.inner:
        return new _NestedInnerBallisticScrollActivity(coordinator, this, simulation, context.vsync);
      case _NestedBallisticScrollActivityMode.independent:
        return new BallisticScrollActivity(this, simulation, context.vsync);
    }
    return null;
  }

  @override
  Future<Null> animateTo(double to, {
    @required Duration duration,
    @required Curve curve,
  }) {
    return coordinator.animateTo(coordinator.unnestOffset(to, this), duration: duration, curve: curve);
  }

  @override
  void jumpTo(double value) {
    return coordinator.jumpTo(coordinator.unnestOffset(value, this));
  }

  @override
  void jumpToWithoutSettling(double value) {
    assert(false);
  }

  void localJumpTo(double value) {
    if (pixels != value) {
      final double oldPixels = pixels;
      forcePixels(value);
      didStartScroll();
      didUpdateScrollPositionBy(pixels - oldPixels);
      didEndScroll();
    }
  }

  @override
  void applyNewDimensions() {
    super.applyNewDimensions();
    coordinator.updateCanDrag();
  }

  void updateCanDrag(double totalExtent) {
    context.setCanDrag(totalExtent > (viewportDimension - maxScrollExtent) || minScrollExtent != maxScrollExtent);
  }

  @override
  ScrollHoldController hold(VoidCallback holdCancelCallback) {
    return coordinator.hold(holdCancelCallback);
  }

  @override
  Drag drag(DragStartDetails details, VoidCallback dragCancelCallback) {
    return coordinator.drag(details, dragCancelCallback);
  }

  @override
  void dispose() {
    _parent?.detach(this);
    super.dispose();
  }
}

enum _NestedBallisticScrollActivityMode { outer, inner, independent }

class _NestedInnerBallisticScrollActivity extends BallisticScrollActivity {
  _NestedInnerBallisticScrollActivity(
    this.coordinator,
    _NestedScrollPosition position,
    Simulation simulation,
    TickerProvider vsync,
  ) : super(position, simulation, vsync);

  final _NestedScrollCoordinator coordinator;

  @override
  _NestedScrollPosition get delegate => super.delegate;

  @override
  void resetActivity() {
    delegate.beginActivity(coordinator.createInnerBallisticScrollActivity(delegate, velocity));
  }

  @override
  void applyNewDimensions() {
    delegate.beginActivity(coordinator.createInnerBallisticScrollActivity(delegate, velocity));
  }

  @override
  bool applyMoveTo(double value) {
    return super.applyMoveTo(coordinator.nestOffset(value, delegate));
  }
}

class _NestedOuterBallisticScrollActivity extends BallisticScrollActivity {
  _NestedOuterBallisticScrollActivity(
    this.coordinator,
    _NestedScrollPosition position,
    this.metrics,
    Simulation simulation,
    TickerProvider vsync,
  ) : assert(metrics.minRange != metrics.maxRange),
      assert(metrics.maxRange > metrics.minRange),
      super(position, simulation, vsync);

  final _NestedScrollCoordinator coordinator;
  final _NestedScrollMetrics metrics;

  @override
  _NestedScrollPosition get delegate => super.delegate;

  @override
  void resetActivity() {
    delegate.beginActivity(coordinator.createOuterBallisticScrollActivity(velocity));
  }

  @override
  void applyNewDimensions() {
    delegate.beginActivity(coordinator.createOuterBallisticScrollActivity(velocity));
  }

  @override
  bool applyMoveTo(double value) {
    bool done = false;
    if (velocity > 0.0) {
      if (value < metrics.minRange)
        return true;
      if (value > metrics.maxRange) {
        value = metrics.maxRange;
        done = true;
      }
    } else if (velocity < 0.0) {
      if (value > metrics.maxRange)
        return true;
      if (value < metrics.minRange) {
        value = metrics.minRange;
        done = true;
      }
    } else {
      value = value.clamp(metrics.minRange, metrics.maxRange);
      done = true;
    }
    final bool result = super.applyMoveTo(value + metrics.correctionOffset);
    assert(result); // since we tried to pass an in-range value, it shouldn't ever overflow
    return !done;
  }

  @override
  String toString() {
    return '$runtimeType(${metrics.minRange} .. ${metrics.maxRange}; correcting by ${metrics.correctionOffset})';
  }
}

/// Handle to provide to a [SliverOverlapAbsorber], a [SliverOverlapInjector],
/// and an [NestedScrollViewViewport], to shift overlap in a [NestedScrollView].
///
/// A particular [SliverOverlapAbsorberHandle] can only be assigned to a single
/// [SliverOverlapAbsorber] at a time. It can also be (and normally is) assigned
/// to one or more [SliverOverlapInjector]s, which must be later descendants of
/// the same [NestedScrollViewViewport] as the [SliverOverlapAbsorber]. The
/// [SliverOverlapAbsorber] must be a direct descendant of the
/// [NestedScrollViewViewport], taking part in the same sliver layout. (The
/// [SliverOverlapInjector] can be a descendant that takes part in a nested
/// scroll view's sliver layout.)
///
/// Whenever the [NestedScrollViewViewport] is marked dirty for layout, it will
/// cause its assigned [SliverOverlapAbsorberHandle] to fire notifications. It
/// is the responsibility of the [SliverOverlapInjector]s (and any other
/// clients) to mark themselves dirty when this happens, in case the geometry
/// subsequently changes during layout.
///
/// See also:
///
///  * [NestedScrollView], which uses a [NestedScrollViewViewport] and a
///    [SliverOverlapAbsorber] to align its children, and which shows sample
///    usage for this class.
class SliverOverlapAbsorberHandle extends ChangeNotifier {
  // Incremented when a RenderSliverOverlapAbsorber takes ownership of this
  // object, decremented when it releases it. This allows us to find cases where
  // the same handle is being passed to two render objects.
  int _writers = 0;

  /// The current amount of overlap being absorbed by the
  /// [SliverOverlapAbsorber].
  ///
  /// This corresponds to the [SliverGeometry.layoutExtent] of the child of the
  /// [SliverOverlapAbsorber].
  ///
  /// This is updated during the layout of the [SliverOverlapAbsorber]. It
  /// should not change at any other time. No notifications are sent when it
  /// changes; clients (e.g. [SliverOverlapInjector]s) are responsible for
  /// marking themselves dirty whenever this object sends notifications, which
  /// happens any time the [SliverOverlapAbsorber] might subsequently change the
  /// value during that layout.
  double get layoutExtent => _layoutExtent;
  double _layoutExtent;

  /// The total scroll extent of the gap being absorbed by the
  /// [SliverOverlapAbsorber].
  ///
  /// This corresponds to the [SliverGeometry.scrollExtent] of the child of the
  /// [SliverOverlapAbsorber].
  ///
  /// This is updated during the layout of the [SliverOverlapAbsorber]. It
  /// should not change at any other time. No notifications are sent when it
  /// changes; clients (e.g. [SliverOverlapInjector]s) are responsible for
  /// marking themselves dirty whenever this object sends notifications, which
  /// happens any time the [SliverOverlapAbsorber] might subsequently change the
  /// value during that layout.
  double get scrollExtent => _scrollExtent;
  double _scrollExtent;

  void _setExtents(double layoutValue, double scrollValue) {
    assert(_writers == 1, 'Multiple RenderSliverOverlapAbsorbers have been provided the same SliverOverlapAbsorberHandle.');
    _layoutExtent = layoutValue;
    _scrollExtent = scrollValue;
  }

  void _markNeedsLayout() => notifyListeners();

  @override
  String toString() {
    String extra;
    switch (_writers) {
      case 0:
        extra = ', orphan';
        break;
      case 1:
        // normal case
        break;
      default:
        extra = ', $_writers WRITERS ASSIGNED';
        break;
    }
    return '$runtimeType($layoutExtent$extra)';
  }
}

/// A sliver that wraps another, forcing its layout extent to be treated as
/// overlap.
///
/// The difference between the overlap requested by the [child] sliver and the
/// overlap reported by this widget, called the _absorbed overlap_, is reported
/// to the [SliverOverlapAbsorberHandle], which is typically passed to a
/// [SliverOverlapInjector].
///
/// See also:
///
///  * [NestedScrollView], whose documentation has sample code showing how to
///    use this widget.
class SliverOverlapAbsorber extends SingleChildRenderObjectWidget {
  /// Creates a sliver that absorbs overlap and reports it to a
  /// [SliverOverlapAbsorberHandle].
  ///
  /// The [handle] must not be null.
  ///
  /// The [child] must be a sliver.
  const SliverOverlapAbsorber({
    Key key,
    @required this.handle,
    Widget child,
  }) : assert(handle != null),
       super(key: key, child: child);

  /// The object in which the absorbed overlap is recorded.
  ///
  /// A particular [SliverOverlapAbsorberHandle] can only be assigned to a
  /// single [SliverOverlapAbsorber] at a time.
  final SliverOverlapAbsorberHandle handle;

  @override
  RenderSliverOverlapAbsorber createRenderObject(BuildContext context) {
    return new RenderSliverOverlapAbsorber(
      handle: handle,
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderSliverOverlapAbsorber renderObject) {
    renderObject
      ..handle = handle;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<SliverOverlapAbsorberHandle>('handle', handle));
  }
}

/// A sliver that wraps another, forcing its layout extent to be treated as
/// overlap.
///
/// The difference between the overlap requested by the [child] sliver and the
/// overlap reported by this widget, called the _absorbed overlap_, is reported
/// to the [SliverOverlapAbsorberHandle], which is typically passed to a
/// [RenderSliverOverlapInjector].
class RenderSliverOverlapAbsorber extends RenderSliver with RenderObjectWithChildMixin<RenderSliver> {
  /// Create a sliver that absorbs overlap and reports it to a
  /// [SliverOverlapAbsorberHandle].
  ///
  /// The [handle] must not be null.
  ///
  /// The [child] must be a [RenderSliver].
  RenderSliverOverlapAbsorber({
    @required SliverOverlapAbsorberHandle handle,
    RenderSliver child,
  }) : assert(handle != null), _handle = handle {
    this.child = child;
  }

  /// The object in which the absorbed overlap is recorded.
  ///
  /// A particular [SliverOverlapAbsorberHandle] can only be assigned to a
  /// single [RenderSliverOverlapAbsorber] at a time.
  SliverOverlapAbsorberHandle get handle => _handle;
  SliverOverlapAbsorberHandle _handle;
  set handle(SliverOverlapAbsorberHandle value) {
    assert(value != null);
    if (handle == value)
      return;
    if (attached) {
      handle._writers -= 1;
      value._writers += 1;
      value._setExtents(handle.layoutExtent, handle.scrollExtent);
    }
    _handle = value;
  }

  @override
  void attach(PipelineOwner owner) {
    super.attach(owner);
    handle._writers += 1;
  }

  @override
  void detach() {
    handle._writers -= 1;
    super.detach();
  }

  @override
  void performLayout() {
    assert(handle._writers == 1, 'A SliverOverlapAbsorberHandle cannot be passed to multiple RenderSliverOverlapAbsorber objects at the same time.');
    if (child == null) {
      geometry = const SliverGeometry();
      return;
    }
    child.layout(constraints, parentUsesSize: true);
    final SliverGeometry childLayoutGeometry = child.geometry;
    geometry = new SliverGeometry(
      scrollExtent: childLayoutGeometry.scrollExtent - childLayoutGeometry.maxScrollObstructionExtent,
      paintExtent: childLayoutGeometry.paintExtent,
      paintOrigin: childLayoutGeometry.paintOrigin,
      layoutExtent: childLayoutGeometry.paintExtent - childLayoutGeometry.maxScrollObstructionExtent,
      maxPaintExtent: childLayoutGeometry.maxPaintExtent,
      maxScrollObstructionExtent: childLayoutGeometry.maxScrollObstructionExtent,
      hitTestExtent: childLayoutGeometry.hitTestExtent,
      visible: childLayoutGeometry.visible,
      hasVisualOverflow: childLayoutGeometry.hasVisualOverflow,
      scrollOffsetCorrection: childLayoutGeometry.scrollOffsetCorrection,
    );
    handle._setExtents(childLayoutGeometry.maxScrollObstructionExtent, childLayoutGeometry.maxScrollObstructionExtent);
  }

  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    // child is always at our origin
  }

  @override
  bool hitTestChildren(HitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition }) {
    if (child != null)
      return child.hitTest(result, mainAxisPosition: mainAxisPosition, crossAxisPosition: crossAxisPosition);
    return false;
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child != null)
      context.paintChild(child, offset);
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<SliverOverlapAbsorberHandle>('handle', handle));
  }
}

/// A sliver that has a sliver geometry based on the values stored in a
/// [SliverOverlapAbsorberHandle].
///
/// The [SliverOverlapAbsorber] must be an earlier descendant of a common
/// ancestor [Viewport], so that it will always be laid out before the
/// [SliverOverlapInjector] during a particular frame.
///
/// See also:
///
///  * [NestedScrollView], which uses a [SliverOverlapAbsorber] to align its
///    children, and which shows sample usage for this class.
class SliverOverlapInjector extends SingleChildRenderObjectWidget {
  /// Creates a sliver that is as tall as the value of the given [handle]'s
  /// layout extent.
  ///
  /// The [handle] must not be null.
  const SliverOverlapInjector({
    Key key,
    @required this.handle,
    Widget child,
  }) : assert(handle != null),
       super(key: key, child: child);

  /// The handle to the [SliverOverlapAbsorber] that is feeding this injector.
  ///
  /// This should be a handle owned by a [SliverOverlapAbsorber] and a
  /// [NestedScrollViewViewport].
  final SliverOverlapAbsorberHandle handle;

  @override
  RenderSliverOverlapInjector createRenderObject(BuildContext context) {
    return new RenderSliverOverlapInjector(
      handle: handle,
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderSliverOverlapInjector renderObject) {
    renderObject
      ..handle = handle;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<SliverOverlapAbsorberHandle>('handle', handle));
  }
}

/// A sliver that has a sliver geometry based on the values stored in a
/// [SliverOverlapAbsorberHandle].
///
/// The [RenderSliverOverlapAbsorber] must be an earlier descendant of a common
/// ancestor [RenderViewport] (probably a [RenderNestedScrollViewViewport]), so
/// that it will always be laid out before the [RenderSliverOverlapInjector]
/// during a particular frame.
class RenderSliverOverlapInjector extends RenderSliver {
  /// Creates a sliver that is as tall as the value of the given [handle]'s extent.
  ///
  /// The [handle] must not be null.
  RenderSliverOverlapInjector({
    @required SliverOverlapAbsorberHandle handle,
    RenderSliver child,
  }) : assert(handle != null), _handle = handle;

  double _currentLayoutExtent;
  double _currentMaxExtent;

  /// The object that specifies how wide to make the gap injected by this render
  /// object.
  ///
  /// This should be a handle owned by a [RenderSliverOverlapAbsorber] and a
  /// [RenderNestedScrollViewViewport].
  SliverOverlapAbsorberHandle get handle => _handle;
  SliverOverlapAbsorberHandle _handle;
  set handle(SliverOverlapAbsorberHandle value) {
    assert(value != null);
    if (handle == value)
      return;
    if (attached) {
      handle.removeListener(markNeedsLayout);
    }
    _handle = value;
    if (attached) {
      handle.addListener(markNeedsLayout);
      if (handle.layoutExtent != _currentLayoutExtent ||
          handle.scrollExtent != _currentMaxExtent)
        markNeedsLayout();
    }
  }

  @override
  void attach(PipelineOwner owner) {
    super.attach(owner);
    handle.addListener(markNeedsLayout);
    if (handle.layoutExtent != _currentLayoutExtent ||
        handle.scrollExtent != _currentMaxExtent)
      markNeedsLayout();
  }

  @override
  void detach() {
    handle.removeListener(markNeedsLayout);
    super.detach();
  }

  @override
  void performLayout() {
    _currentLayoutExtent = handle.layoutExtent;
    _currentMaxExtent = handle.layoutExtent;
    final double clampedLayoutExtent = math.min(_currentLayoutExtent - constraints.scrollOffset, constraints.remainingPaintExtent);
    geometry = new SliverGeometry(
      scrollExtent: _currentLayoutExtent,
      paintExtent: math.max(0.0, clampedLayoutExtent),
      maxPaintExtent: _currentMaxExtent,
    );
  }

  @override
  void debugPaint(PaintingContext context, Offset offset) {
    assert(() {
      if (debugPaintSizeEnabled) {
        final Paint paint = new Paint()
          ..color = const Color(0xFFCC9933)
          ..strokeWidth = 3.0
          ..style = PaintingStyle.stroke;
        Offset start, end, delta;
        switch (constraints.axis) {
          case Axis.vertical:
            final double x = offset.dx + constraints.crossAxisExtent / 2.0;
            start = new Offset(x, offset.dy);
            end = new Offset(x, offset.dy + geometry.paintExtent);
            delta = new Offset(constraints.crossAxisExtent / 5.0, 0.0);
            break;
          case Axis.horizontal:
            final double y = offset.dy + constraints.crossAxisExtent / 2.0;
            start = new Offset(offset.dx, y);
            end = new Offset(offset.dy + geometry.paintExtent, y);
            delta = new Offset(0.0, constraints.crossAxisExtent / 5.0);
            break;
        }
        for (int index = -2; index <= 2; index += 1) {
          paintZigZag(context.canvas, paint, start - delta * index.toDouble(), end - delta * index.toDouble(), 10, 10.0);
        }
      }
      return true;
    }());
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<SliverOverlapAbsorberHandle>('handle', handle));
  }
}

/// The [Viewport] variant used by [NestedScrollView].
///
/// This viewport takes a [SliverOverlapAbsorberHandle] and notifies it any time
/// the viewport needs to recompute its layout (e.g. when it is scrolled).
class NestedScrollViewViewport extends Viewport {
  /// Creates a variant of [Viewport] that has a [SliverOverlapAbsorberHandle].
  ///
  /// The [handle] must not be null.
  NestedScrollViewViewport({
    Key key,
    AxisDirection axisDirection: AxisDirection.down,
    AxisDirection crossAxisDirection,
    double anchor: 0.0,
    @required ViewportOffset offset,
    Key center,
    List<Widget> slivers: const <Widget>[],
    @required this.handle,
  }) : assert(handle != null),
       super(
         key: key,
         axisDirection: axisDirection,
         crossAxisDirection: crossAxisDirection,
         anchor: anchor,
         offset: offset,
         center: center,
         slivers: slivers,
       );

  /// The handle to the [SliverOverlapAbsorber] that is feeding this injector.
  final SliverOverlapAbsorberHandle handle;

  @override
  RenderNestedScrollViewViewport createRenderObject(BuildContext context) {
    return new RenderNestedScrollViewViewport(
      axisDirection: axisDirection,
      crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
      anchor: anchor,
      offset: offset,
      handle: handle,
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderNestedScrollViewViewport renderObject) {
    renderObject
      ..axisDirection = axisDirection
      ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
      ..anchor = anchor
      ..offset = offset
      ..handle = handle;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<SliverOverlapAbsorberHandle>('handle', handle));
  }
}

/// The [RenderViewport] variant used by [NestedScrollView].
///
/// This viewport takes a [SliverOverlapAbsorberHandle] and notifies it any time
/// the viewport needs to recompute its layout (e.g. when it is scrolled).
class RenderNestedScrollViewViewport extends RenderViewport {
  /// Create a variant of [RenderViewport] that has a [SliverOverlapAbsorberHandle].
  ///
  /// The [handle] must not be null.
  RenderNestedScrollViewViewport({
    AxisDirection axisDirection: AxisDirection.down,
    @required AxisDirection crossAxisDirection,
    @required ViewportOffset offset,
    double anchor: 0.0,
    List<RenderSliver> children,
    RenderSliver center,
    @required SliverOverlapAbsorberHandle handle,
  }) : assert(handle != null),
       _handle = handle,
       super(
         axisDirection: axisDirection,
         crossAxisDirection: crossAxisDirection,
         offset: offset,
         anchor: anchor,
         children: children,
         center: center,
       );

  /// The object to notify when [markNeedsLayout] is called.
  SliverOverlapAbsorberHandle get handle => _handle;
  SliverOverlapAbsorberHandle _handle;
  /// Setting this will trigger notifications on the new object.
  set handle(SliverOverlapAbsorberHandle value) {
    assert(value != null);
    if (handle == value)
      return;
    _handle = value;
    handle._markNeedsLayout();
  }

  @override
  void markNeedsLayout() {
    handle._markNeedsLayout();
    super.markNeedsLayout();
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DiagnosticsProperty<SliverOverlapAbsorberHandle>('handle', handle));
  }
}