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

// Copyright 2014 The Flutter 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 'dart:ui';

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

import 'actions.dart';
import 'basic.dart';
import 'focus_manager.dart';
import 'framework.dart';
import 'gesture_detector.dart';
import 'notification_listener.dart';
import 'scroll_configuration.dart';
import 'scroll_context.dart';
import 'scroll_controller.dart';
import 'scroll_metrics.dart';
import 'scroll_physics.dart';
import 'scroll_position.dart';
import 'scroll_position_with_single_context.dart';
import 'ticker_provider.dart';
import 'viewport.dart';

export 'package:flutter/physics.dart' show Tolerance;

/// Signature used by [Scrollable] to build the viewport through which the
/// scrollable content is displayed.
typedef ViewportBuilder = Widget Function(BuildContext context, ViewportOffset position);

/// A widget that scrolls.
///
/// [Scrollable] implements the interaction model for a scrollable widget,
/// including gesture recognition, but does not have an opinion about how the
/// viewport, which actually displays the children, is constructed.
///
/// It's rare to construct a [Scrollable] directly. Instead, consider [ListView]
/// or [GridView], which combine scrolling, viewporting, and a layout model. To
/// combine layout models (or to use a custom layout mode), consider using
/// [CustomScrollView].
///
/// The static [Scrollable.of] and [Scrollable.ensureVisible] functions are
/// often used to interact with the [Scrollable] widget inside a [ListView] or
/// a [GridView].
///
/// To further customize scrolling behavior with a [Scrollable]:
///
/// 1. You can provide a [viewportBuilder] to customize the child model. For
///    example, [SingleChildScrollView] uses a viewport that displays a single
///    box child whereas [CustomScrollView] uses a [Viewport] or a
///    [ShrinkWrappingViewport], both of which display a list of slivers.
///
/// 2. You can provide a custom [ScrollController] that creates a custom
///    [ScrollPosition] subclass. For example, [PageView] uses a
///    [PageController], which creates a page-oriented scroll position subclass
///    that keeps the same page visible when the [Scrollable] resizes.
///
/// See also:
///
///  * [ListView], which is a commonly used [ScrollView] that displays a
///    scrolling, linear list of child widgets.
///  * [PageView], which is a scrolling list of child widgets that are each the
///    size of the viewport.
///  * [GridView], which is a [ScrollView] that displays a scrolling, 2D array
///    of child widgets.
///  * [CustomScrollView], which is a [ScrollView] that creates custom scroll
///    effects using slivers.
///  * [SingleChildScrollView], which is a scrollable widget that has a single
///    child.
///  * [ScrollNotification] and [NotificationListener], which can be used to watch
///    the scroll position without using a [ScrollController].
class Scrollable extends StatefulWidget {
  /// Creates a widget that scrolls.
  ///
  /// The [axisDirection] and [viewportBuilder] arguments must not be null.
  const Scrollable({
    Key key,
    this.axisDirection = AxisDirection.down,
    this.controller,
    this.physics,
    @required this.viewportBuilder,
    this.incrementCalculator,
    this.excludeFromSemantics = false,
    this.semanticChildCount,
    this.dragStartBehavior = DragStartBehavior.start,
  }) : assert(axisDirection != null),
       assert(dragStartBehavior != null),
       assert(viewportBuilder != null),
       assert(excludeFromSemantics != null),
       assert(semanticChildCount == null || semanticChildCount >= 0),
       super (key: key);

  /// The direction in which this widget scrolls.
  ///
  /// For example, if the [axisDirection] is [AxisDirection.down], increasing
  /// the scroll position will cause content below the bottom of the viewport to
  /// become visible through the viewport. Similarly, if [axisDirection] is
  /// [AxisDirection.right], increasing the scroll position will cause content
  /// beyond the right edge of the viewport to become visible through the
  /// viewport.
  ///
  /// Defaults to [AxisDirection.down].
  final AxisDirection axisDirection;

  /// An object that can be used to control the position to which this widget is
  /// scrolled.
  ///
  /// A [ScrollController] serves several purposes. It can be used to control
  /// the initial scroll position (see [ScrollController.initialScrollOffset]).
  /// It can be used to control whether the scroll view should automatically
  /// save and restore its scroll position in the [PageStorage] (see
  /// [ScrollController.keepScrollOffset]). It can be used to read the current
  /// scroll position (see [ScrollController.offset]), or change it (see
  /// [ScrollController.animateTo]).
  ///
  /// See also:
  ///
  ///  * [ensureVisible], which animates the scroll position to reveal a given
  ///    [BuildContext].
  final ScrollController controller;

  /// How the widgets should respond to user input.
  ///
  /// For example, determines how the widget continues to animate after the
  /// user stops dragging the scroll view.
  ///
  /// Defaults to matching platform conventions via the physics provided from
  /// the ambient [ScrollConfiguration].
  ///
  /// The physics can be changed dynamically, but new physics will only take
  /// effect if the _class_ of the provided object changes. Merely constructing
  /// a new instance with a different configuration is insufficient to cause the
  /// physics to be reapplied. (This is because the final object used is
  /// generated dynamically, which can be relatively expensive, and it would be
  /// inefficient to speculatively create this object each frame to see if the
  /// physics should be updated.)
  ///
  /// See also:
  ///
  ///  * [AlwaysScrollableScrollPhysics], which can be used to indicate that the
  ///    scrollable should react to scroll requests (and possible overscroll)
  ///    even if the scrollable's contents fit without scrolling being necessary.
  final ScrollPhysics physics;

  /// Builds the viewport through which the scrollable content is displayed.
  ///
  /// A typical viewport uses the given [ViewportOffset] to determine which part
  /// of its content is actually visible through the viewport.
  ///
  /// See also:
  ///
  ///  * [Viewport], which is a viewport that displays a list of slivers.
  ///  * [ShrinkWrappingViewport], which is a viewport that displays a list of
  ///    slivers and sizes itself based on the size of the slivers.
  final ViewportBuilder viewportBuilder;

  /// An optional function that will be called to calculate the distance to
  /// scroll when the scrollable is asked to scroll via the keyboard using a
  /// [ScrollAction].
  ///
  /// If not supplied, the [Scrollable] will scroll a default amount when a
  /// keyboard navigation key is pressed (e.g. pageUp/pageDown, control-upArrow,
  /// etc.), or otherwise invoked by a [ScrollAction].
  ///
  /// If [incrementCalculator] is null, the default for
  /// [ScrollIncrementType.page] is 80% of the size of the scroll window, and
  /// for [ScrollIncrementType.line], 50 logical pixels.
  final ScrollIncrementCalculator incrementCalculator;

  /// Whether the scroll actions introduced by this [Scrollable] are exposed
  /// in the semantics tree.
  ///
  /// Text fields with an overflow are usually scrollable to make sure that the
  /// user can get to the beginning/end of the entered text. However, these
  /// scrolling actions are generally not exposed to the semantics layer.
  ///
  /// See also:
  ///
  ///  * [GestureDetector.excludeFromSemantics], which is used to accomplish the
  ///    exclusion.
  final bool excludeFromSemantics;

  /// The number of children that will contribute semantic information.
  ///
  /// The value will be null if the number of children is unknown or unbounded.
  ///
  /// Some subtypes of [ScrollView] can infer this value automatically. For
  /// example [ListView] will use the number of widgets in the child list,
  /// while the [new ListView.separated] constructor will use half that amount.
  ///
  /// For [CustomScrollView] and other types which do not receive a builder
  /// or list of widgets, the child count must be explicitly provided.
  ///
  /// See also:
  ///
  ///  * [CustomScrollView], for an explanation of scroll semantics.
  ///  * [SemanticsConfiguration.scrollChildCount], the corresponding semantics property.
  final int semanticChildCount;

  // TODO(jslavitz): Set the DragStartBehavior default to be start across all widgets.
  /// {@template flutter.widgets.scrollable.dragStartBehavior}
  /// Determines the way that drag start behavior is handled.
  ///
  /// If set to [DragStartBehavior.start], scrolling drag behavior will
  /// begin upon the detection of a drag gesture. If set to
  /// [DragStartBehavior.down] it will begin when a down event is first detected.
  ///
  /// In general, setting this to [DragStartBehavior.start] will make drag
  /// animation smoother and setting it to [DragStartBehavior.down] will make
  /// drag behavior feel slightly more reactive.
  ///
  /// By default, the drag start behavior is [DragStartBehavior.start].
  ///
  /// See also:
  ///
  ///  * [DragGestureRecognizer.dragStartBehavior], which gives an example for
  ///    the different behaviors.
  ///
  /// {@endtemplate}
  final DragStartBehavior dragStartBehavior;

  /// The axis along which the scroll view scrolls.
  ///
  /// Determined by the [axisDirection].
  Axis get axis => axisDirectionToAxis(axisDirection);

  @override
  ScrollableState createState() => ScrollableState();

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
    properties.add(DiagnosticsProperty<ScrollPhysics>('physics', physics));
  }

  /// The state from the closest instance of this class that encloses the given context.
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// ScrollableState scrollable = Scrollable.of(context);
  /// ```
  ///
  /// Calling this method will create a dependency on the closest [Scrollable]
  /// in the [context], if there is one.
  static ScrollableState of(BuildContext context) {
    final _ScrollableScope widget = context.dependOnInheritedWidgetOfExactType<_ScrollableScope>();
    return widget?.scrollable;
  }

  /// Provides a heuristic to determine if expensive frame-bound tasks should be
  /// deferred for the [context] at a specific point in time.
  ///
  /// Calling this method does _not_ create a dependency on any other widget.
  /// This also means that the value returned is only good for the point in time
  /// when it is called, and callers will not get updated if the value changes.
  ///
  /// The heuristic used is determined by the [physics] of this [Scrollable]
  /// via [ScrollPhysics.recommendDeferredScrolling]. That method is called with
  /// the current [activity]'s [ScrollActivity.velocity].
  ///
  /// If there is no [Scrollable] in the widget tree above the [context], this
  /// method returns false.
  static bool recommendDeferredLoadingForContext(BuildContext context) {
    final _ScrollableScope widget = context.getElementForInheritedWidgetOfExactType<_ScrollableScope>()?.widget as _ScrollableScope;
    if (widget == null) {
      return false;
    }
    return widget.position.recommendDeferredLoading(context);
  }

  /// Scrolls the scrollables that enclose the given context so as to make the
  /// given context visible.
  static Future<void> ensureVisible(
    BuildContext context, {
    double alignment = 0.0,
    Duration duration = Duration.zero,
    Curve curve = Curves.ease,
    ScrollPositionAlignmentPolicy alignmentPolicy = ScrollPositionAlignmentPolicy.explicit,
  }) {
    final List<Future<void>> futures = <Future<void>>[];

    ScrollableState scrollable = Scrollable.of(context);
    while (scrollable != null) {
      futures.add(scrollable.position.ensureVisible(
        context.findRenderObject(),
        alignment: alignment,
        duration: duration,
        curve: curve,
        alignmentPolicy: alignmentPolicy,
      ));
      context = scrollable.context;
      scrollable = Scrollable.of(context);
    }

    if (futures.isEmpty || duration == Duration.zero)
      return Future<void>.value();
    if (futures.length == 1)
      return futures.single;
    return Future.wait<void>(futures).then<void>((List<void> _) => null);
  }
}

// Enable Scrollable.of() to work as if ScrollableState was an inherited widget.
// ScrollableState.build() always rebuilds its _ScrollableScope.
class _ScrollableScope extends InheritedWidget {
  const _ScrollableScope({
    Key key,
    @required this.scrollable,
    @required this.position,
    @required Widget child,
  }) : assert(scrollable != null),
       assert(child != null),
       super(key: key, child: child);

  final ScrollableState scrollable;
  final ScrollPosition position;

  @override
  bool updateShouldNotify(_ScrollableScope old) {
    return position != old.position;
  }
}

/// State object for a [Scrollable] widget.
///
/// To manipulate a [Scrollable] widget's scroll position, use the object
/// obtained from the [position] property.
///
/// To be informed of when a [Scrollable] widget is scrolling, use a
/// [NotificationListener] to listen for [ScrollNotification] notifications.
///
/// This class is not intended to be subclassed. To specialize the behavior of a
/// [Scrollable], provide it with a [ScrollPhysics].
class ScrollableState extends State<Scrollable> with TickerProviderStateMixin
    implements ScrollContext {
  /// The manager for this [Scrollable] widget's viewport position.
  ///
  /// To control what kind of [ScrollPosition] is created for a [Scrollable],
  /// provide it with custom [ScrollController] that creates the appropriate
  /// [ScrollPosition] in its [ScrollController.createScrollPosition] method.
  ScrollPosition get position => _position;
  ScrollPosition _position;

  @override
  AxisDirection get axisDirection => widget.axisDirection;

  ScrollBehavior _configuration;
  ScrollPhysics _physics;

  // Only call this from places that will definitely trigger a rebuild.
  void _updatePosition() {
    _configuration = ScrollConfiguration.of(context);
    _physics = _configuration.getScrollPhysics(context);
    if (widget.physics != null)
      _physics = widget.physics.applyTo(_physics);
    final ScrollController controller = widget.controller;
    final ScrollPosition oldPosition = position;
    if (oldPosition != null) {
      controller?.detach(oldPosition);
      // It's important that we not dispose the old position until after the
      // viewport has had a chance to unregister its listeners from the old
      // position. So, schedule a microtask to do it.
      scheduleMicrotask(oldPosition.dispose);
    }

    _position = controller?.createScrollPosition(_physics, this, oldPosition)
      ?? ScrollPositionWithSingleContext(physics: _physics, context: this, oldPosition: oldPosition);
    assert(position != null);
    controller?.attach(position);
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    _updatePosition();
  }

  bool _shouldUpdatePosition(Scrollable oldWidget) {
    ScrollPhysics newPhysics = widget.physics;
    ScrollPhysics oldPhysics = oldWidget.physics;
    do {
      if (newPhysics?.runtimeType != oldPhysics?.runtimeType)
        return true;
      newPhysics = newPhysics?.parent;
      oldPhysics = oldPhysics?.parent;
    } while (newPhysics != null || oldPhysics != null);

    return widget.controller?.runtimeType != oldWidget.controller?.runtimeType;
  }

  @override
  void didUpdateWidget(Scrollable oldWidget) {
    super.didUpdateWidget(oldWidget);

    if (widget.controller != oldWidget.controller) {
      oldWidget.controller?.detach(position);
      widget.controller?.attach(position);
    }

    if (_shouldUpdatePosition(oldWidget))
      _updatePosition();
  }

  @override
  void dispose() {
    widget.controller?.detach(position);
    position.dispose();
    super.dispose();
  }


  // SEMANTICS

  final GlobalKey _scrollSemanticsKey = GlobalKey();

  @override
  @protected
  void setSemanticsActions(Set<SemanticsAction> actions) {
    if (_gestureDetectorKey.currentState != null)
      _gestureDetectorKey.currentState.replaceSemanticsActions(actions);
  }


  // GESTURE RECOGNITION AND POINTER IGNORING

  final GlobalKey<RawGestureDetectorState> _gestureDetectorKey = GlobalKey<RawGestureDetectorState>();
  final GlobalKey _ignorePointerKey = GlobalKey();

  // This field is set during layout, and then reused until the next time it is set.
  Map<Type, GestureRecognizerFactory> _gestureRecognizers = const <Type, GestureRecognizerFactory>{};
  bool _shouldIgnorePointer = false;

  bool _lastCanDrag;
  Axis _lastAxisDirection;

  @override
  @protected
  void setCanDrag(bool canDrag) {
    if (canDrag == _lastCanDrag && (!canDrag || widget.axis == _lastAxisDirection))
      return;
    if (!canDrag) {
      _gestureRecognizers = const <Type, GestureRecognizerFactory>{};
    } else {
      switch (widget.axis) {
        case Axis.vertical:
          _gestureRecognizers = <Type, GestureRecognizerFactory>{
            VerticalDragGestureRecognizer: GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
              () => VerticalDragGestureRecognizer(),
              (VerticalDragGestureRecognizer instance) {
                instance
                  ..onDown = _handleDragDown
                  ..onStart = _handleDragStart
                  ..onUpdate = _handleDragUpdate
                  ..onEnd = _handleDragEnd
                  ..onCancel = _handleDragCancel
                  ..minFlingDistance = _physics?.minFlingDistance
                  ..minFlingVelocity = _physics?.minFlingVelocity
                  ..maxFlingVelocity = _physics?.maxFlingVelocity
                  ..dragStartBehavior = widget.dragStartBehavior;
              },
            ),
          };
          break;
        case Axis.horizontal:
          _gestureRecognizers = <Type, GestureRecognizerFactory>{
            HorizontalDragGestureRecognizer: GestureRecognizerFactoryWithHandlers<HorizontalDragGestureRecognizer>(
              () => HorizontalDragGestureRecognizer(),
              (HorizontalDragGestureRecognizer instance) {
                instance
                  ..onDown = _handleDragDown
                  ..onStart = _handleDragStart
                  ..onUpdate = _handleDragUpdate
                  ..onEnd = _handleDragEnd
                  ..onCancel = _handleDragCancel
                  ..minFlingDistance = _physics?.minFlingDistance
                  ..minFlingVelocity = _physics?.minFlingVelocity
                  ..maxFlingVelocity = _physics?.maxFlingVelocity
                  ..dragStartBehavior = widget.dragStartBehavior;
              },
            ),
          };
          break;
      }
    }
    _lastCanDrag = canDrag;
    _lastAxisDirection = widget.axis;
    if (_gestureDetectorKey.currentState != null)
      _gestureDetectorKey.currentState.replaceGestureRecognizers(_gestureRecognizers);
  }

  @override
  TickerProvider get vsync => this;

  @override
  @protected
  void setIgnorePointer(bool value) {
    if (_shouldIgnorePointer == value)
      return;
    _shouldIgnorePointer = value;
    if (_ignorePointerKey.currentContext != null) {
      final RenderIgnorePointer renderBox = _ignorePointerKey.currentContext.findRenderObject() as RenderIgnorePointer;
      renderBox.ignoring = _shouldIgnorePointer;
    }
  }

  @override
  BuildContext get notificationContext => _gestureDetectorKey.currentContext;

  @override
  BuildContext get storageContext => context;

  // TOUCH HANDLERS

  Drag _drag;
  ScrollHoldController _hold;

  void _handleDragDown(DragDownDetails details) {
    assert(_drag == null);
    assert(_hold == null);
    _hold = position.hold(_disposeHold);
  }

  void _handleDragStart(DragStartDetails details) {
    // It's possible for _hold to become null between _handleDragDown and
    // _handleDragStart, for example if some user code calls jumpTo or otherwise
    // triggers a new activity to begin.
    assert(_drag == null);
    _drag = position.drag(details, _disposeDrag);
    assert(_drag != null);
    assert(_hold == null);
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    _drag?.update(details);
  }

  void _handleDragEnd(DragEndDetails details) {
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    _drag?.end(details);
    assert(_drag == null);
  }

  void _handleDragCancel() {
    // _hold might be null if the drag started.
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    _hold?.cancel();
    _drag?.cancel();
    assert(_hold == null);
    assert(_drag == null);
  }

  void _disposeHold() {
    _hold = null;
  }

  void _disposeDrag() {
    _drag = null;
  }

  // SCROLL WHEEL

  // Returns the offset that should result from applying [event] to the current
  // position, taking min/max scroll extent into account.
  double _targetScrollOffsetForPointerScroll(PointerScrollEvent event) {
    double delta = widget.axis == Axis.horizontal
        ? event.scrollDelta.dx
        : event.scrollDelta.dy;

    if (axisDirectionIsReversed(widget.axisDirection)) {
      delta *= -1;
    }

    return math.min(math.max(position.pixels + delta, position.minScrollExtent),
        position.maxScrollExtent);
  }

  void _receivedPointerSignal(PointerSignalEvent event) {
    if (event is PointerScrollEvent && position != null) {
      final double targetScrollOffset = _targetScrollOffsetForPointerScroll(event);
      // Only express interest in the event if it would actually result in a scroll.
      if (targetScrollOffset != position.pixels) {
        GestureBinding.instance.pointerSignalResolver.register(event, _handlePointerScroll);
      }
    }
  }

  void _handlePointerScroll(PointerEvent event) {
    assert(event is PointerScrollEvent);
    if (_physics != null && !_physics.shouldAcceptUserOffset(position)) {
      return;
    }
    final double targetScrollOffset = _targetScrollOffsetForPointerScroll(event as PointerScrollEvent);
    if (targetScrollOffset != position.pixels) {
      position.jumpTo(targetScrollOffset);
    }
  }

  // DESCRIPTION

  @override
  Widget build(BuildContext context) {
    assert(position != null);
    // _ScrollableScope must be placed above the BuildContext returned by notificationContext
    // so that we can get this ScrollableState by doing the following:
    //
    // ScrollNotification notification;
    // Scrollable.of(notification.context)
    //
    // Since notificationContext is pointing to _gestureDetectorKey.context, _ScrollableScope
    // must be placed above the widget using it: RawGestureDetector
    Widget result = _ScrollableScope(
      scrollable: this,
      position: position,
      // TODO(ianh): Having all these global keys is sad.
      child: Listener(
        onPointerSignal: _receivedPointerSignal,
        child: RawGestureDetector(
          key: _gestureDetectorKey,
          gestures: _gestureRecognizers,
          behavior: HitTestBehavior.opaque,
          excludeFromSemantics: widget.excludeFromSemantics,
          child: Semantics(
            explicitChildNodes: !widget.excludeFromSemantics,
            child: IgnorePointer(
              key: _ignorePointerKey,
              ignoring: _shouldIgnorePointer,
              ignoringSemantics: false,
              child: widget.viewportBuilder(context, position),
            ),
          ),
        ),
      ),
    );

    if (!widget.excludeFromSemantics) {
      result = _ScrollSemantics(
        key: _scrollSemanticsKey,
        child: result,
        position: position,
        allowImplicitScrolling: widget?.physics?.allowImplicitScrolling ?? _physics.allowImplicitScrolling,
        semanticChildCount: widget.semanticChildCount,
      );
    }

    return _configuration.buildViewportChrome(context, result, widget.axisDirection);
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<ScrollPosition>('position', position));
  }
}

/// With [_ScrollSemantics] certain child [SemanticsNode]s can be
/// excluded from the scrollable area for semantics purposes.
///
/// Nodes, that are to be excluded, have to be tagged with
/// [RenderViewport.excludeFromScrolling] and the [RenderAbstractViewport] in
/// use has to add the [RenderViewport.useTwoPaneSemantics] tag to its
/// [SemanticsConfiguration] by overriding
/// [RenderObject.describeSemanticsConfiguration].
///
/// If the tag [RenderViewport.useTwoPaneSemantics] is present on the viewport,
/// two semantics nodes will be used to represent the [Scrollable]: The outer
/// node will contain all children, that are excluded from scrolling. The inner
/// node, which is annotated with the scrolling actions, will house the
/// scrollable children.
class _ScrollSemantics extends SingleChildRenderObjectWidget {
  const _ScrollSemantics({
    Key key,
    @required this.position,
    @required this.allowImplicitScrolling,
    @required this.semanticChildCount,
    Widget child,
  }) : assert(position != null),
       assert(semanticChildCount == null || semanticChildCount >= 0),
       super(key: key, child: child);

  final ScrollPosition position;
  final bool allowImplicitScrolling;
  final int semanticChildCount;

  @override
  _RenderScrollSemantics createRenderObject(BuildContext context) {
    return _RenderScrollSemantics(
      position: position,
      allowImplicitScrolling: allowImplicitScrolling,
      semanticChildCount: semanticChildCount,
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderScrollSemantics renderObject) {
    renderObject
      ..allowImplicitScrolling = allowImplicitScrolling
      ..position = position
      ..semanticChildCount = semanticChildCount;
  }
}

class _RenderScrollSemantics extends RenderProxyBox {
  _RenderScrollSemantics({
    @required ScrollPosition position,
    @required bool allowImplicitScrolling,
    @required int semanticChildCount,
    RenderBox child,
  }) : _position = position,
       _allowImplicitScrolling = allowImplicitScrolling,
       _semanticChildCount = semanticChildCount,
       assert(position != null),
       super(child) {
    position.addListener(markNeedsSemanticsUpdate);
  }

  /// Whether this render object is excluded from the semantic tree.
  ScrollPosition get position => _position;
  ScrollPosition _position;
  set position(ScrollPosition value) {
    assert(value != null);
    if (value == _position)
      return;
    _position.removeListener(markNeedsSemanticsUpdate);
    _position = value;
    _position.addListener(markNeedsSemanticsUpdate);
    markNeedsSemanticsUpdate();
  }

  /// Whether this node can be scrolled implicitly.
  bool get allowImplicitScrolling => _allowImplicitScrolling;
  bool _allowImplicitScrolling;
  set allowImplicitScrolling(bool value) {
    if (value == _allowImplicitScrolling)
      return;
    _allowImplicitScrolling = value;
    markNeedsSemanticsUpdate();
  }

  int get semanticChildCount => _semanticChildCount;
  int _semanticChildCount;
  set semanticChildCount(int value) {
    if (value == semanticChildCount)
      return;
    _semanticChildCount = value;
    markNeedsSemanticsUpdate();
  }

  @override
  void describeSemanticsConfiguration(SemanticsConfiguration config) {
    super.describeSemanticsConfiguration(config);
    config.isSemanticBoundary = true;
    if (position.haveDimensions) {
      config
          ..hasImplicitScrolling = allowImplicitScrolling
          ..scrollPosition = _position.pixels
          ..scrollExtentMax = _position.maxScrollExtent
          ..scrollExtentMin = _position.minScrollExtent
          ..scrollChildCount = semanticChildCount;
    }
  }

  SemanticsNode _innerNode;

  @override
  void assembleSemanticsNode(SemanticsNode node, SemanticsConfiguration config, Iterable<SemanticsNode> children) {
    if (children.isEmpty || !children.first.isTagged(RenderViewport.useTwoPaneSemantics)) {
      super.assembleSemanticsNode(node, config, children);
      return;
    }

    _innerNode ??= SemanticsNode(showOnScreen: showOnScreen);
    _innerNode
      ..isMergedIntoParent = node.isPartOfNodeMerging
      ..rect = Offset.zero & node.rect.size;

    int firstVisibleIndex;
    final List<SemanticsNode> excluded = <SemanticsNode>[_innerNode];
    final List<SemanticsNode> included = <SemanticsNode>[];
    for (final SemanticsNode child in children) {
      assert(child.isTagged(RenderViewport.useTwoPaneSemantics));
      if (child.isTagged(RenderViewport.excludeFromScrolling)) {
        excluded.add(child);
      } else {
        if (!child.hasFlag(SemanticsFlag.isHidden))
          firstVisibleIndex ??= child.indexInParent;
        included.add(child);
      }
    }
    config.scrollIndex = firstVisibleIndex;
    node.updateWith(config: null, childrenInInversePaintOrder: excluded);
    _innerNode.updateWith(config: config, childrenInInversePaintOrder: included);
  }

  @override
  void clearSemantics() {
    super.clearSemantics();
    _innerNode = null;
  }
}

/// A typedef for a function that can calculate the offset for a type of scroll
/// increment given a [ScrollIncrementDetails].
///
/// This function is used as the type for [Scrollable.incrementCalculator],
/// which is called from a [ScrollAction].
typedef ScrollIncrementCalculator = double Function(ScrollIncrementDetails details);

/// Describes the type of scroll increment that will be performed by a
/// [ScrollAction] on a [Scrollable].
///
/// This is used to configure a [ScrollIncrementDetails] object to pass to a
/// [ScrollIncrementCalculator] function on a [Scrollable].
///
/// {@template flutter.widgets.scrollable.scroll_increment_type.intent}
/// This indicates the *intent* of the scroll, not necessarily the size. Not all
/// scrollable areas will have the concept of a "line" or "page", but they can
/// respond to the different standard key bindings that cause scrolling, which
/// are bound to keys that people use to indicate a "line" scroll (e.g.
/// control-arrowDown keys) or a "page" scroll (e.g. pageDown key). It is
/// recommended that at least the relative magnitudes of the scrolls match
/// expectations.
/// {@endtemplate}
enum ScrollIncrementType {
  /// Indicates that the [ScrollIncrementCalculator] should return the scroll
  /// distance it should move when the user requests to scroll by a "line".
  ///
  /// The distance a "line" scrolls refers to what should happen when the key
  /// binding for "scroll down/up by a line" is triggered. It's up to the
  /// [ScrollIncrementCalculator] function to decide what that means for a
  /// particular scrollable.
  line,

  /// Indicates that the [ScrollIncrementCalculator] should return the scroll
  /// distance it should move when the user requests to scroll by a "page".
  ///
  /// The distance a "page" scrolls refers to what should happen when the key
  /// binding for "scroll down/up by a page" is triggered. It's up to the
  /// [ScrollIncrementCalculator] function to decide what that means for a
  /// particular scrollable.
  page,
}

/// A details object that describes the type of scroll increment being requested
/// of a [ScrollIncrementCalculator] function, as well as the current metrics
/// for the scrollable.
class ScrollIncrementDetails {
  /// A const constructor for a [ScrollIncrementDetails].
  ///
  /// All of the arguments must not be null, and are required.
  const ScrollIncrementDetails({
    @required this.type,
    @required this.metrics,
  })  : assert(type != null),
        assert(metrics != null);

  /// The type of scroll this is (e.g. line, page, etc.).
  ///
  /// {@macro flutter.widgets.scrollable.scroll_increment_type.intent}
  final ScrollIncrementType type;

  /// The current metrics of the scrollable that is being scrolled.
  final ScrollMetrics metrics;
}

/// An [Intent] that represents scrolling the nearest scrollable by an amount
/// appropriate for the [type] specified.
///
/// The actual amount of the scroll is determined by the
/// [Scrollable.incrementCalculator], or by its defaults if that is not
/// specified.
class ScrollIntent extends Intent {
  /// Creates a const [ScrollIntent] that requests scrolling in the given
  /// [direction], with the given [type].
  ///
  /// If [reversed] is specified, then the scroll will happen in the opposite
  /// direction from the normal scroll direction.
  const ScrollIntent({
    @required this.direction,
    this.type = ScrollIncrementType.line,
  })  : assert(direction != null),
        assert(type != null);

  /// The direction in which to scroll the scrollable containing the focused
  /// widget.
  final AxisDirection direction;

  /// The type of scrolling that is intended.
  final ScrollIncrementType type;
}

/// An [Action] that scrolls the [Scrollable] that encloses the current
/// [primaryFocus] by the amount configured in the [ScrollIntent] given to it.
///
/// If [Scrollable.incrementCalculator] is null for the scrollable, the default
/// for a [ScrollIntent.type] set to [ScrollIncrementType.page] is 80% of the
/// size of the scroll window, and for [ScrollIncrementType.line], 50 logical
/// pixels.
class ScrollAction extends Action<ScrollIntent> {
  /// The [LocalKey] that uniquely connects this action to a [ScrollIntent].
  static const LocalKey key = ValueKey<Type>(ScrollAction);

  @override
  bool isEnabled(ScrollIntent intent) {
    final FocusNode focus = primaryFocus;
    return focus != null && focus.context != null && Scrollable.of(focus.context) != null;
  }

  // Returns the scroll increment for a single scroll request, for use when
  // scrolling using a hardware keyboard.
  //
  // Must not be called when the position is null, or when any of the position
  // metrics (pixels, viewportDimension, maxScrollExtent, minScrollExtent) are
  // null. The type and state arguments must not be null, and the widget must
  // have already been laid out so that the position fields are valid.
  double _calculateScrollIncrement(ScrollableState state, { ScrollIncrementType type = ScrollIncrementType.line }) {
    assert(type != null);
    assert(state.position != null);
    assert(state.position.pixels != null);
    assert(state.position.viewportDimension != null);
    assert(state.position.maxScrollExtent != null);
    assert(state.position.minScrollExtent != null);
    assert(state.widget.physics == null || state.widget.physics.shouldAcceptUserOffset(state.position));
    if (state.widget.incrementCalculator != null) {
      return state.widget.incrementCalculator(
        ScrollIncrementDetails(
          type: type,
          metrics: state.position,
        ),
      );
    }
    switch (type) {
      case ScrollIncrementType.line:
        return 50.0;
      case ScrollIncrementType.page:
        return 0.8 * state.position.viewportDimension;
    }
    return 0.0;
  }

  // Find out how much of an increment to move by, taking the different
  // directions into account.
  double _getIncrement(ScrollableState state, ScrollIntent intent) {
    final double increment = _calculateScrollIncrement(state, type: intent.type);
    switch (intent.direction) {
      case AxisDirection.down:
        switch (state.axisDirection) {
          case AxisDirection.up:
            return -increment;
            break;
          case AxisDirection.down:
            return increment;
            break;
          case AxisDirection.right:
          case AxisDirection.left:
            return 0.0;
        }
        break;
      case AxisDirection.up:
        switch (state.axisDirection) {
          case AxisDirection.up:
            return increment;
            break;
          case AxisDirection.down:
            return -increment;
            break;
          case AxisDirection.right:
          case AxisDirection.left:
            return 0.0;
        }
        break;
      case AxisDirection.left:
        switch (state.axisDirection) {
          case AxisDirection.right:
            return -increment;
            break;
          case AxisDirection.left:
            return increment;
            break;
          case AxisDirection.up:
          case AxisDirection.down:
            return 0.0;
        }
        break;
      case AxisDirection.right:
        switch (state.axisDirection) {
          case AxisDirection.right:
            return increment;
            break;
          case AxisDirection.left:
            return -increment;
            break;
          case AxisDirection.up:
          case AxisDirection.down:
            return 0.0;
        }
        break;
    }
    return 0.0;
  }

  @override
  void invoke(ScrollIntent intent) {
    final ScrollableState state = Scrollable.of(primaryFocus.context);
    assert(state != null, '$ScrollAction was invoked on a context that has no scrollable parent');
    assert(state.position.pixels != null, 'Scrollable must be laid out before it can be scrolled via a ScrollAction');
    assert(state.position.viewportDimension != null);
    assert(state.position.maxScrollExtent != null);
    assert(state.position.minScrollExtent != null);

    // Don't do anything if the user isn't allowed to scroll.
    if (state.widget.physics != null && !state.widget.physics.shouldAcceptUserOffset(state.position)) {
      return;
    }
    final double increment = _getIncrement(state, intent);
    if (increment == 0.0) {
      return;
    }
    state.position.moveTo(
      state.position.pixels + increment,
      duration: const Duration(milliseconds: 100),
      curve: Curves.easeInOut,
    );
  }
}