packages/flutter/lib/src/widgets/scrollable.dart - mirrors/flutter - Git at Google

 // Copyright 2015 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 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';

 import 'basic.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_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 Widget ViewportBuilder(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.excludeFromSemantics: false,
   }) : assert(axisDirection != null),
        assert(viewportBuilder != null),
        assert(excludeFromSemantics != null),
        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;

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

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

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(new EnumProperty<AxisDirection>('axisDirection', axisDirection));
     properties.add(new 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);
   /// ```
   static ScrollableState of(BuildContext context) {
     final _ScrollableScope widget = context.inheritFromWidgetOfExactType(_ScrollableScope);
     return widget?.scrollable;
   }

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

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

     if (futures.isEmpty || duration == Duration.zero)
       return new Future<Null>.value();
     if (futures.length == 1)
       return futures.single;
     return Future.wait<Null>(futures).then((List<Null> _) => 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)
       ?? new 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 _excludableScrollSemanticsKey = new 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 = new GlobalKey<RawGestureDetectorState>();
   final GlobalKey _ignorePointerKey = new 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: new GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
               () => new VerticalDragGestureRecognizer(),
               (VerticalDragGestureRecognizer instance) {
                 instance
                   ..onDown = _handleDragDown
                   ..onStart = _handleDragStart
                   ..onUpdate = _handleDragUpdate
                   ..onEnd = _handleDragEnd
                   ..onCancel = _handleDragCancel
                   ..minFlingDistance = _physics?.minFlingDistance
                   ..minFlingVelocity = _physics?.minFlingVelocity
                   ..maxFlingVelocity = _physics?.maxFlingVelocity;
               },
             ),
           };
           break;
         case Axis.horizontal:
           _gestureRecognizers = <Type, GestureRecognizerFactory>{
             HorizontalDragGestureRecognizer: new GestureRecognizerFactoryWithHandlers<HorizontalDragGestureRecognizer>(
               () => new HorizontalDragGestureRecognizer(),
               (HorizontalDragGestureRecognizer instance) {
                 instance
                   ..onDown = _handleDragDown
                   ..onStart = _handleDragStart
                   ..onUpdate = _handleDragUpdate
                   ..onEnd = _handleDragEnd
                   ..onCancel = _handleDragCancel
                   ..minFlingDistance = _physics?.minFlingDistance
                   ..minFlingVelocity = _physics?.minFlingVelocity
                   ..maxFlingVelocity = _physics?.maxFlingVelocity;
               },
             ),
           };
           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();
       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;
   }


   // DESCRIPTION

   @override
   Widget build(BuildContext context) {
     assert(position != null);
     // TODO(ianh): Having all these global keys is sad.
     Widget result = new RawGestureDetector(
       key: _gestureDetectorKey,
       gestures: _gestureRecognizers,
       behavior: HitTestBehavior.opaque,
       excludeFromSemantics: widget.excludeFromSemantics,
       child: new Semantics(
         explicitChildNodes: !widget.excludeFromSemantics,
         child: new IgnorePointer(
           key: _ignorePointerKey,
           ignoring: _shouldIgnorePointer,
           ignoringSemantics: false,
           child: new _ScrollableScope(
             scrollable: this,
             position: position,
             child: widget.viewportBuilder(context, position),
           ),
         ),
       ),
     );

     if (!widget.excludeFromSemantics) {
       result = new _ExcludableScrollSemantics(
         key: _excludableScrollSemanticsKey,
         child: result,
         position: position,
       );
     }

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

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

 /// With [_ExcludableScrollSemantics] 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 _ExcludableScrollSemantics extends SingleChildRenderObjectWidget {
   const _ExcludableScrollSemantics({
     Key key,
     @required this.position,
     Widget child
   }) : assert(position != null), super(key: key, child: child);

   final ScrollPosition position;

   @override
   _RenderExcludableScrollSemantics createRenderObject(BuildContext context) => new _RenderExcludableScrollSemantics(position: position);

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

 class _RenderExcludableScrollSemantics extends RenderProxyBox {
   _RenderExcludableScrollSemantics({
     @required ScrollPosition position,
     RenderBox child,
   }) : _position = position, 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();
   }

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

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

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

   @override
   void clearSemantics() {
     super.clearSemantics();
     _innerNode = null;
   }
 }
	// Copyright 2015 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 'package:flutter/gestures.dart';
	import 'package:flutter/rendering.dart';
	import 'package:flutter/scheduler.dart';

	import 'basic.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_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 Widget ViewportBuilder(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.excludeFromSemantics: false,
	}) : assert(axisDirection != null),
	assert(viewportBuilder != null),
	assert(excludeFromSemantics != null),
	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;

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

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

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(new EnumProperty<AxisDirection>('axisDirection', axisDirection));
	properties.add(new 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);
	/// ```
	static ScrollableState of(BuildContext context) {
	final _ScrollableScope widget = context.inheritFromWidgetOfExactType(_ScrollableScope);
	return widget?.scrollable;
	}

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

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

	if (futures.isEmpty \|\| duration == Duration.zero)
	return new Future<Null>.value();
	if (futures.length == 1)
	return futures.single;
	return Future.wait<Null>(futures).then((List<Null> _) => 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)
	?? new 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 _excludableScrollSemanticsKey = new 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 = new GlobalKey<RawGestureDetectorState>();
	final GlobalKey _ignorePointerKey = new 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: new GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
	() => new VerticalDragGestureRecognizer(),
	(VerticalDragGestureRecognizer instance) {
	instance
	..onDown = _handleDragDown
	..onStart = _handleDragStart
	..onUpdate = _handleDragUpdate
	..onEnd = _handleDragEnd
	..onCancel = _handleDragCancel
	..minFlingDistance = _physics?.minFlingDistance
	..minFlingVelocity = _physics?.minFlingVelocity
	..maxFlingVelocity = _physics?.maxFlingVelocity;
	},
	),
	};
	break;
	case Axis.horizontal:
	_gestureRecognizers = <Type, GestureRecognizerFactory>{
	HorizontalDragGestureRecognizer: new GestureRecognizerFactoryWithHandlers<HorizontalDragGestureRecognizer>(
	() => new HorizontalDragGestureRecognizer(),
	(HorizontalDragGestureRecognizer instance) {
	instance
	..onDown = _handleDragDown
	..onStart = _handleDragStart
	..onUpdate = _handleDragUpdate
	..onEnd = _handleDragEnd
	..onCancel = _handleDragCancel
	..minFlingDistance = _physics?.minFlingDistance
	..minFlingVelocity = _physics?.minFlingVelocity
	..maxFlingVelocity = _physics?.maxFlingVelocity;
	},
	),
	};
	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();
	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;
	}


	// DESCRIPTION

	@override
	Widget build(BuildContext context) {
	assert(position != null);
	// TODO(ianh): Having all these global keys is sad.
	Widget result = new RawGestureDetector(
	key: _gestureDetectorKey,
	gestures: _gestureRecognizers,
	behavior: HitTestBehavior.opaque,
	excludeFromSemantics: widget.excludeFromSemantics,
	child: new Semantics(
	explicitChildNodes: !widget.excludeFromSemantics,
	child: new IgnorePointer(
	key: _ignorePointerKey,
	ignoring: _shouldIgnorePointer,
	ignoringSemantics: false,
	child: new _ScrollableScope(
	scrollable: this,
	position: position,
	child: widget.viewportBuilder(context, position),
	),
	),
	),
	);

	if (!widget.excludeFromSemantics) {
	result = new _ExcludableScrollSemantics(
	key: _excludableScrollSemanticsKey,
	child: result,
	position: position,
	);
	}

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

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

	/// With [_ExcludableScrollSemantics] 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 _ExcludableScrollSemantics extends SingleChildRenderObjectWidget {
	const _ExcludableScrollSemantics({
	Key key,
	@required this.position,
	Widget child
	}) : assert(position != null), super(key: key, child: child);

	final ScrollPosition position;

	@override
	_RenderExcludableScrollSemantics createRenderObject(BuildContext context) => new _RenderExcludableScrollSemantics(position: position);

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

	class _RenderExcludableScrollSemantics extends RenderProxyBox {
	_RenderExcludableScrollSemantics({
	@required ScrollPosition position,
	RenderBox child,
	}) : _position = position, 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();
	}

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

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

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

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