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

 // 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 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';

 import 'basic.dart';
 import 'framework.dart';
 import 'inherited_notifier.dart';
 import 'layout_builder.dart';
 import 'notification_listener.dart';
 import 'scroll_context.dart';
 import 'scroll_controller.dart';
 import 'scroll_notification.dart';
 import 'scroll_physics.dart';
 import 'scroll_position.dart';
 import 'scroll_position_with_single_context.dart';
 import 'scroll_simulation.dart';

 /// The signature of a method that provides a [BuildContext] and
 /// [ScrollController] for building a widget that may overflow the draggable
 /// [Axis] of the containing [DraggableScrollSheet].
 ///
 /// Users should apply the [scrollController] to a [ScrollView] subclass, such
 /// as a [SingleChildScrollView], [ListView] or [GridView], to have the whole
 /// sheet be draggable.
 typedef ScrollableWidgetBuilder = Widget Function(
   BuildContext context,
   ScrollController scrollController,
 );

 /// A container for a [Scrollable] that responds to drag gestures by resizing
 /// the scrollable until a limit is reached, and then scrolling.
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=Hgw819mL_78}
 ///
 /// This widget can be dragged along the vertical axis between its
 /// [minChildSize], which defaults to `0.25` and [maxChildSize], which defaults
 /// to `1.0`. These sizes are percentages of the height of the parent container.
 ///
 /// The widget coordinates resizing and scrolling of the widget returned by
 /// builder as the user drags along the horizontal axis.
 ///
 /// The widget will initially be displayed at its initialChildSize which
 /// defaults to `0.5`, meaning half the height of its parent. Dragging will work
 /// between the range of minChildSize and maxChildSize (as percentages of the
 /// parent container's height) as long as the builder creates a widget which
 /// uses the provided [ScrollController]. If the widget created by the
 /// [ScrollableWidgetBuilder] does not use the provided [ScrollController], the
 /// sheet will remain at the initialChildSize.
 ///
 /// By default, the widget will expand its non-occupied area to fill available
 /// space in the parent. If this is not desired, e.g. because the parent wants
 /// to position sheet based on the space it is taking, the [expand] property
 /// may be set to false.
 ///
 /// {@tool snippet}
 ///
 /// This is a sample widget which shows a [ListView] that has 25 [ListTile]s.
 /// It starts out as taking up half the body of the [Scaffold], and can be
 /// dragged up to the full height of the scaffold or down to 25% of the height
 /// of the scaffold. Upon reaching full height, the list contents will be
 /// scrolled up or down, until they reach the top of the list again and the user
 /// drags the sheet back down.
 ///
 /// ```dart
 /// class HomePage extends StatelessWidget {
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return Scaffold(
 ///       appBar: AppBar(
 ///         title: const Text('DraggableScrollableSheet'),
 ///       ),
 ///       body: SizedBox.expand(
 ///         child: DraggableScrollableSheet(
 ///           builder: (BuildContext context, ScrollController scrollController) {
 ///             return Container(
 ///               color: Colors.blue[100],
 ///               child: ListView.builder(
 ///                 controller: scrollController,
 ///                 itemCount: 25,
 ///                 itemBuilder: (BuildContext context, int index) {
 ///                   return ListTile(title: Text('Item $index'));
 ///                 },
 ///               ),
 ///             );
 ///           },
 ///         ),
 ///       ),
 ///     );
 ///   }
 /// }
 /// ```
 /// {@end-tool}
 class DraggableScrollableSheet extends StatefulWidget {
   /// Creates a widget that can be dragged and scrolled in a single gesture.
   ///
   /// The [builder], [initialChildSize], [minChildSize], [maxChildSize] and
   /// [expand] parameters must not be null.
   const DraggableScrollableSheet({
     Key key,
     this.initialChildSize = 0.5,
     this.minChildSize = 0.25,
     this.maxChildSize = 1.0,
     this.expand = true,
     @required this.builder,
   })  : assert(initialChildSize != null),
         assert(minChildSize != null),
         assert(maxChildSize != null),
         assert(minChildSize >= 0.0),
         assert(maxChildSize <= 1.0),
         assert(minChildSize <= initialChildSize),
         assert(initialChildSize <= maxChildSize),
         assert(expand != null),
         assert(builder != null),
         super(key: key);

   /// The initial fractional value of the parent container's height to use when
   /// displaying the widget.
   ///
   /// The default value is `0.5`.
   final double initialChildSize;

   /// The minimum fractional value of the parent container's height to use when
   /// displaying the widget.
   ///
   /// The default value is `0.25`.
   final double minChildSize;

   /// The maximum fractional value of the parent container's height to use when
   /// displaying the widget.
   ///
   /// The default value is `1.0`.
   final double maxChildSize;

   /// Whether the widget should expand to fill the available space in its parent
   /// or not.
   ///
   /// In most cases, this should be true. However, in the case of a parent
   /// widget that will position this one based on its desired size (such as a
   /// [Center]), this should be set to false.
   ///
   /// The default value is true.
   final bool expand;

   /// The builder that creates a child to display in this widget, which will
   /// use the provided [ScrollController] to enable dragging and scrolling
   /// of the contents.
   final ScrollableWidgetBuilder builder;

   @override
   _DraggableScrollableSheetState createState() => _DraggableScrollableSheetState();
 }

 /// A [Notification] related to the extent, which is the size, and scroll
 /// offset, which is the position of the child list, of the
 /// [DraggableScrollableSheet].
 ///
 /// [DraggableScrollableSheet] widgets notify their ancestors when the size of
 /// the sheet changes. When the extent of the sheet changes via a drag,
 /// this notification bubbles up through the tree, which means a given
 /// [NotificationListener] will receive notifications for all descendant
 /// [DraggableScrollableSheet] widgets. To focus on notifications from the
 /// nearest [DraggableScorllableSheet] descendant, check that the [depth]
 /// property of the notification is zero.
 ///
 /// When an extent notification is received by a [NotificationListener], the
 /// listener will already have completed build and layout, and it is therefore
 /// too late for that widget to call [State.setState]. Any attempt to adjust the
 /// build or layout based on an extent notification would result in a layout
 /// that lagged one frame behind, which is a poor user experience. Extent
 /// notifications are used primarily to drive animations. The [Scaffold] widget
 /// listens for extent notifications and responds by driving animations for the
 /// [FloatingActionButton] as the bottom sheet scrolls up.
 class DraggableScrollableNotification extends Notification with ViewportNotificationMixin {
   /// Creates a notification that the extent of a [DraggableScrollableSheet] has
   /// changed.
   ///
   /// All parameters are required. The [minExtent] must be >= 0.  The [maxExtent]
   /// must be <= 1.0.  The [extent] must be between [minExtent] and [maxExtent].
   DraggableScrollableNotification({
     @required this.extent,
     @required this.minExtent,
     @required this.maxExtent,
     @required this.initialExtent,
     @required this.context,
   }) : assert(extent != null),
        assert(initialExtent != null),
        assert(minExtent != null),
        assert(maxExtent != null),
        assert(0.0 <= minExtent),
        assert(maxExtent <= 1.0),
        assert(minExtent <= extent),
        assert(minExtent <= initialExtent),
        assert(extent <= maxExtent),
        assert(initialExtent <= maxExtent),
        assert(context != null);

   /// The current value of the extent, between [minExtent] and [maxExtent].
   final double extent;

   /// The minimum value of [extent], which is >= 0.
   final double minExtent;

   /// The maximum value of [extent].
   final double maxExtent;

   /// The initially requested value for [extent].
   final double initialExtent;

   /// The build context of the widget that fired this notification.
   ///
   /// This can be used to find the sheet's render objects to determine the size
   /// of the viewport, for instance. A listener can only assume this context
   /// is live when it first gets the notification.
   final BuildContext context;

   @override
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
     description.add('minExtent: $minExtent, extent: $extent, maxExtent: $maxExtent, initialExtent: $initialExtent');
   }
 }

 /// Manages state between [_DraggableScrollableSheetState],
 /// [_DraggableScrollableSheetScrollController], and
 /// [_DraggableScrollableSheetScrollPosition].
 ///
 /// The State knows the pixels available along the axis the widget wants to
 /// scroll, but expects to get a fraction of those pixels to render the sheet.
 ///
 /// The ScrollPosition knows the number of pixels a user wants to move the sheet.
 ///
 /// The [currentExtent] will never be null.
 /// The [availablePixels] will never be null, but may be `double.infinity`.
 class _DraggableSheetExtent {
   _DraggableSheetExtent({
     @required this.minExtent,
     @required this.maxExtent,
     @required this.initialExtent,
     @required VoidCallback listener,
   }) : assert(minExtent != null),
        assert(maxExtent != null),
        assert(initialExtent != null),
        assert(minExtent >= 0),
        assert(maxExtent <= 1),
        assert(minExtent <= initialExtent),
        assert(initialExtent <= maxExtent),
        _currentExtent = ValueNotifier<double>(initialExtent)..addListener(listener),
        availablePixels = double.infinity;

   final double minExtent;
   final double maxExtent;
   final double initialExtent;
   final ValueNotifier<double> _currentExtent;
   double availablePixels;

   bool get isAtMin => minExtent >= _currentExtent.value;
   bool get isAtMax => maxExtent <= _currentExtent.value;

   set currentExtent(double value) {
     assert(value != null);
     _currentExtent.value = value.clamp(minExtent, maxExtent) as double;
   }
   double get currentExtent => _currentExtent.value;

   double get additionalMinExtent => isAtMin ? 0.0 : 1.0;
   double get additionalMaxExtent => isAtMax ? 0.0 : 1.0;

   /// The scroll position gets inputs in terms of pixels, but the extent is
   /// expected to be expressed as a number between 0..1.
   void addPixelDelta(double delta, BuildContext context) {
     if (availablePixels == 0) {
       return;
     }
     currentExtent += delta / availablePixels * maxExtent;
     DraggableScrollableNotification(
       minExtent: minExtent,
       maxExtent: maxExtent,
       extent: currentExtent,
       initialExtent: initialExtent,
       context: context,
     ).dispatch(context);
   }
 }

 class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
   _DraggableScrollableSheetScrollController _scrollController;
   _DraggableSheetExtent _extent;

   @override
   void initState() {
     super.initState();
     _extent = _DraggableSheetExtent(
       minExtent: widget.minChildSize,
       maxExtent: widget.maxChildSize,
       initialExtent: widget.initialChildSize,
       listener: _setExtent,
     );
     _scrollController = _DraggableScrollableSheetScrollController(extent: _extent);
   }

   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
     if (_InheritedResetNotifier.shouldReset(context)) {
       // jumpTo can result in trying to replace semantics during build.
       // Just animate really fast.
       // Avoid doing it at all if the offset is already 0.0.
       if (_scrollController.offset != 0.0) {
         _scrollController.animateTo(
           0.0,
           duration: const Duration(milliseconds: 1),
           curve: Curves.linear,
         );
       }
       _extent._currentExtent.value = _extent.initialExtent;
     }
   }

   void _setExtent() {
     setState(() {
       // _extent has been updated when this is called.
     });

   }

   @override
   Widget build(BuildContext context) {
     return LayoutBuilder(
       builder: (BuildContext context, BoxConstraints constraints) {
         _extent.availablePixels = widget.maxChildSize * constraints.biggest.height;
         final Widget sheet = FractionallySizedBox(
           heightFactor: _extent.currentExtent,
           child: widget.builder(context, _scrollController),
           alignment: Alignment.bottomCenter,
         );
         return widget.expand ? SizedBox.expand(child: sheet) : sheet;
       },
     );
   }

   @override
   void dispose() {
     _scrollController.dispose();
     super.dispose();
   }
 }

 /// A [ScrollController] suitable for use in a [ScrollableWidgetBuilder] created
 /// by a [DraggableScrollableSheet].
 ///
 /// If a [DraggableScrollableSheet] contains content that is exceeds the height
 /// of its container, this controller will allow the sheet to both be dragged to
 /// fill the container and then scroll the child content.
 ///
 /// See also:
 ///
 ///  * [_DraggableScrollableSheetScrollPosition], which manages the positioning logic for
 ///    this controller.
 ///  * [PrimaryScrollController], which can be used to establish a
 ///    [_DraggableScrollableSheetScrollController] as the primary controller for
 ///    descendants.
 class _DraggableScrollableSheetScrollController extends ScrollController {
   _DraggableScrollableSheetScrollController({
     double initialScrollOffset = 0.0,
     String debugLabel,
     @required this.extent,
   }) : assert(extent != null),
        super(
          debugLabel: debugLabel,
          initialScrollOffset: initialScrollOffset,
        );

   final _DraggableSheetExtent extent;

   @override
   _DraggableScrollableSheetScrollPosition createScrollPosition(
     ScrollPhysics physics,
     ScrollContext context,
     ScrollPosition oldPosition,
   ) {
     return _DraggableScrollableSheetScrollPosition(
       physics: physics,
       context: context,
       oldPosition: oldPosition,
       extent: extent,
     );
   }

   @override
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
     description.add('extent: $extent');
   }
 }

 /// A scroll position that manages scroll activities for
 /// [_DraggableScrollableSheetScrollController].
 ///
 /// This class is a concrete subclass of [ScrollPosition] logic that handles a
 /// single [ScrollContext], such as a [Scrollable]. An instance of this class
 /// manages [ScrollActivity] instances, which changes the
 /// [_DraggableSheetExtent.currentExtent] or visible content offset in the
 /// [Scrollable]'s [Viewport]
 ///
 /// See also:
 ///
 ///  * [_DraggableScrollableSheetScrollController], which uses this as its [ScrollPosition].
 class _DraggableScrollableSheetScrollPosition
     extends ScrollPositionWithSingleContext {
   _DraggableScrollableSheetScrollPosition({
     @required ScrollPhysics physics,
     @required ScrollContext context,
     double initialPixels = 0.0,
     bool keepScrollOffset = true,
     ScrollPosition oldPosition,
     String debugLabel,
     @required this.extent,
   })  : assert(extent != null),
         super(
           physics: physics,
           context: context,
           initialPixels: initialPixels,
           keepScrollOffset: keepScrollOffset,
           oldPosition: oldPosition,
           debugLabel: debugLabel,
         );

   VoidCallback _dragCancelCallback;
   final _DraggableSheetExtent extent;
   bool get listShouldScroll => pixels > 0.0;

   @override
   bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
     // We need to provide some extra extent if we haven't yet reached the max or
     // min extents. Otherwise, a list with fewer children than the extent of
     // the available space will get stuck.
     return super.applyContentDimensions(
       minScrollExtent - extent.additionalMinExtent,
       maxScrollExtent + extent.additionalMaxExtent,
     );
   }

   @override
   void applyUserOffset(double delta) {
     if (!listShouldScroll &&
         (!(extent.isAtMin || extent.isAtMax) ||
           (extent.isAtMin && delta < 0) ||
           (extent.isAtMax && delta > 0))) {
       extent.addPixelDelta(-delta, context.notificationContext);
     } else {
       super.applyUserOffset(delta);
     }
   }

   @override
   void goBallistic(double velocity) {
     if (velocity == 0.0 ||
        (velocity < 0.0 && listShouldScroll) ||
        (velocity > 0.0 && extent.isAtMax)) {
       super.goBallistic(velocity);
       return;
     }
     // Scrollable expects that we will dispose of its current _dragCancelCallback
     _dragCancelCallback?.call();
     _dragCancelCallback = null;

     // The iOS bouncing simulation just isn't right here - once we delegate
     // the ballistic back to the ScrollView, it will use the right simulation.
     final Simulation simulation = ClampingScrollSimulation(
       position: extent.currentExtent,
       velocity: velocity,
       tolerance: physics.tolerance,
     );

     final AnimationController ballisticController = AnimationController.unbounded(
       debugLabel: '${objectRuntimeType(this, '_DraggableScrollableSheetPosition')}',
       vsync: context.vsync,
     );
     double lastDelta = 0;
     void _tick() {
       final double delta = ballisticController.value - lastDelta;
       lastDelta = ballisticController.value;
       extent.addPixelDelta(delta, context.notificationContext);
       if ((velocity > 0 && extent.isAtMax) || (velocity < 0 && extent.isAtMin)) {
         // Make sure we pass along enough velocity to keep scrolling - otherwise
         // we just "bounce" off the top making it look like the list doesn't
         // have more to scroll.
         velocity = ballisticController.velocity + (physics.tolerance.velocity * ballisticController.velocity.sign);
         super.goBallistic(velocity);
         ballisticController.stop();
       } else if (ballisticController.isCompleted) {
         super.goBallistic(0);
       }
     }

     ballisticController
       ..addListener(_tick)
       ..animateWith(simulation).whenCompleteOrCancel(
         ballisticController.dispose,
       );
   }

   @override
   Drag drag(DragStartDetails details, VoidCallback dragCancelCallback) {
     // Save this so we can call it later if we have to [goBallistic] on our own.
     _dragCancelCallback = dragCancelCallback;
     return super.drag(details, dragCancelCallback);
   }
 }

 /// A widget that can notify a descendent [DraggableScrollableSheet] that it
 /// should reset its position to the initial state.
 ///
 /// The [Scaffold] uses this widget to notify a persistent bottom sheet that
 /// the user has tapped back if the sheet has started to cover more of the body
 /// than when at its initial position. This is important for users of assistive
 /// technology, where dragging may be difficult to communicate.
 class DraggableScrollableActuator extends StatelessWidget {
   /// Creates a widget that can notify descendent [DraggableScrollableSheet]s
   /// to reset to their initial position.
   ///
   /// The [child] parameter is required.
   DraggableScrollableActuator({
     Key key,
     @required this.child,
   }) : super(key: key);

   /// This child's [DraggableScrollableSheet] descendant will be reset when the
   /// [reset] method is applied to a context that includes it.
   ///
   /// Must not be null.
   final Widget child;

   final _ResetNotifier _notifier = _ResetNotifier();

   /// Notifies any descendant [DraggableScrollableSheet] that it should reset
   /// to its initial position.
   ///
   /// Returns `true` if a [DraggableScrollableActuator] is available and
   /// some [DraggableScrollableSheet] is listening for updates, `false`
   /// otherwise.
   static bool reset(BuildContext context) {
     final _InheritedResetNotifier notifier = context.dependOnInheritedWidgetOfExactType<_InheritedResetNotifier>();
     if (notifier == null) {
       return false;
     }
     return notifier._sendReset();
   }

   @override
   Widget build(BuildContext context) {
     return _InheritedResetNotifier(child: child, notifier: _notifier);
   }
 }

 /// A [ChangeNotifier] to use with [InheritedResetNotifer] to notify
 /// descendants that they should reset to initial state.
 class _ResetNotifier extends ChangeNotifier {
   /// Whether someone called [sendReset] or not.
   ///
   /// This flag should be reset after checking it.
   bool _wasCalled = false;

   /// Fires a reset notification to descendants.
   ///
   /// Returns false if there are no listeners.
   bool sendReset() {
     if (!hasListeners) {
       return false;
     }
     _wasCalled = true;
     notifyListeners();
     return true;
   }
 }

 class _InheritedResetNotifier extends InheritedNotifier<_ResetNotifier> {
   /// Creates an [InheritedNotifier] that the [DraggableScrollableSheet] will
   /// listen to for an indication that it should change its extent.
   ///
   /// The [child] and [notifier] properties must not be null.
   const _InheritedResetNotifier({
     Key key,
     @required Widget child,
     @required _ResetNotifier notifier,
   }) : super(key: key, child: child, notifier: notifier);

   bool _sendReset() => notifier.sendReset();

   /// Specifies whether the [DraggableScrollableSheet] should reset to its
   /// initial position.
   ///
   /// Returns true if the notifier requested a reset, false otherwise.
   static bool shouldReset(BuildContext context) {
     final InheritedWidget widget = context.dependOnInheritedWidgetOfExactType<_InheritedResetNotifier>();
     if (widget == null) {
       return false;
     }
     assert(widget is _InheritedResetNotifier);
     final _InheritedResetNotifier inheritedNotifier = widget as _InheritedResetNotifier;
     final bool wasCalled = inheritedNotifier.notifier._wasCalled;
     inheritedNotifier.notifier._wasCalled = false;
     return wasCalled;
   }
 }
	// 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 'package:flutter/foundation.dart';
	import 'package:flutter/gestures.dart';

	import 'basic.dart';
	import 'framework.dart';
	import 'inherited_notifier.dart';
	import 'layout_builder.dart';
	import 'notification_listener.dart';
	import 'scroll_context.dart';
	import 'scroll_controller.dart';
	import 'scroll_notification.dart';
	import 'scroll_physics.dart';
	import 'scroll_position.dart';
	import 'scroll_position_with_single_context.dart';
	import 'scroll_simulation.dart';

	/// The signature of a method that provides a [BuildContext] and
	/// [ScrollController] for building a widget that may overflow the draggable
	/// [Axis] of the containing [DraggableScrollSheet].
	///
	/// Users should apply the [scrollController] to a [ScrollView] subclass, such
	/// as a [SingleChildScrollView], [ListView] or [GridView], to have the whole
	/// sheet be draggable.
	typedef ScrollableWidgetBuilder = Widget Function(
	BuildContext context,
	ScrollController scrollController,
	);

	/// A container for a [Scrollable] that responds to drag gestures by resizing
	/// the scrollable until a limit is reached, and then scrolling.
	///
	/// {@youtube 560 315 https://www.youtube.com/watch?v=Hgw819mL_78}
	///
	/// This widget can be dragged along the vertical axis between its
	/// [minChildSize], which defaults to `0.25` and [maxChildSize], which defaults
	/// to `1.0`. These sizes are percentages of the height of the parent container.
	///
	/// The widget coordinates resizing and scrolling of the widget returned by
	/// builder as the user drags along the horizontal axis.
	///
	/// The widget will initially be displayed at its initialChildSize which
	/// defaults to `0.5`, meaning half the height of its parent. Dragging will work
	/// between the range of minChildSize and maxChildSize (as percentages of the
	/// parent container's height) as long as the builder creates a widget which
	/// uses the provided [ScrollController]. If the widget created by the
	/// [ScrollableWidgetBuilder] does not use the provided [ScrollController], the
	/// sheet will remain at the initialChildSize.
	///
	/// By default, the widget will expand its non-occupied area to fill available
	/// space in the parent. If this is not desired, e.g. because the parent wants
	/// to position sheet based on the space it is taking, the [expand] property
	/// may be set to false.
	///
	/// {@tool snippet}
	///
	/// This is a sample widget which shows a [ListView] that has 25 [ListTile]s.
	/// It starts out as taking up half the body of the [Scaffold], and can be
	/// dragged up to the full height of the scaffold or down to 25% of the height
	/// of the scaffold. Upon reaching full height, the list contents will be
	/// scrolled up or down, until they reach the top of the list again and the user
	/// drags the sheet back down.
	///
	/// ```dart
	/// class HomePage extends StatelessWidget {
	/// @override
	/// Widget build(BuildContext context) {
	/// return Scaffold(
	/// appBar: AppBar(
	/// title: const Text('DraggableScrollableSheet'),
	/// ),
	/// body: SizedBox.expand(
	/// child: DraggableScrollableSheet(
	/// builder: (BuildContext context, ScrollController scrollController) {
	/// return Container(
	/// color: Colors.blue[100],
	/// child: ListView.builder(
	/// controller: scrollController,
	/// itemCount: 25,
	/// itemBuilder: (BuildContext context, int index) {
	/// return ListTile(title: Text('Item $index'));
	/// },
	/// ),
	/// );
	/// },
	/// ),
	/// ),
	/// );
	/// }
	/// }
	/// ```
	/// {@end-tool}
	class DraggableScrollableSheet extends StatefulWidget {
	/// Creates a widget that can be dragged and scrolled in a single gesture.
	///
	/// The [builder], [initialChildSize], [minChildSize], [maxChildSize] and
	/// [expand] parameters must not be null.
	const DraggableScrollableSheet({
	Key key,
	this.initialChildSize = 0.5,
	this.minChildSize = 0.25,
	this.maxChildSize = 1.0,
	this.expand = true,
	@required this.builder,
	}) : assert(initialChildSize != null),
	assert(minChildSize != null),
	assert(maxChildSize != null),
	assert(minChildSize >= 0.0),
	assert(maxChildSize <= 1.0),
	assert(minChildSize <= initialChildSize),
	assert(initialChildSize <= maxChildSize),
	assert(expand != null),
	assert(builder != null),
	super(key: key);

	/// The initial fractional value of the parent container's height to use when
	/// displaying the widget.
	///
	/// The default value is `0.5`.
	final double initialChildSize;

	/// The minimum fractional value of the parent container's height to use when
	/// displaying the widget.
	///
	/// The default value is `0.25`.
	final double minChildSize;

	/// The maximum fractional value of the parent container's height to use when
	/// displaying the widget.
	///
	/// The default value is `1.0`.
	final double maxChildSize;

	/// Whether the widget should expand to fill the available space in its parent
	/// or not.
	///
	/// In most cases, this should be true. However, in the case of a parent
	/// widget that will position this one based on its desired size (such as a
	/// [Center]), this should be set to false.
	///
	/// The default value is true.
	final bool expand;

	/// The builder that creates a child to display in this widget, which will
	/// use the provided [ScrollController] to enable dragging and scrolling
	/// of the contents.
	final ScrollableWidgetBuilder builder;

	@override
	_DraggableScrollableSheetState createState() => _DraggableScrollableSheetState();
	}

	/// A [Notification] related to the extent, which is the size, and scroll
	/// offset, which is the position of the child list, of the
	/// [DraggableScrollableSheet].
	///
	/// [DraggableScrollableSheet] widgets notify their ancestors when the size of
	/// the sheet changes. When the extent of the sheet changes via a drag,
	/// this notification bubbles up through the tree, which means a given
	/// [NotificationListener] will receive notifications for all descendant
	/// [DraggableScrollableSheet] widgets. To focus on notifications from the
	/// nearest [DraggableScorllableSheet] descendant, check that the [depth]
	/// property of the notification is zero.
	///
	/// When an extent notification is received by a [NotificationListener], the
	/// listener will already have completed build and layout, and it is therefore
	/// too late for that widget to call [State.setState]. Any attempt to adjust the
	/// build or layout based on an extent notification would result in a layout
	/// that lagged one frame behind, which is a poor user experience. Extent
	/// notifications are used primarily to drive animations. The [Scaffold] widget
	/// listens for extent notifications and responds by driving animations for the
	/// [FloatingActionButton] as the bottom sheet scrolls up.
	class DraggableScrollableNotification extends Notification with ViewportNotificationMixin {
	/// Creates a notification that the extent of a [DraggableScrollableSheet] has
	/// changed.
	///
	/// All parameters are required. The [minExtent] must be >= 0. The [maxExtent]
	/// must be <= 1.0. The [extent] must be between [minExtent] and [maxExtent].
	DraggableScrollableNotification({
	@required this.extent,
	@required this.minExtent,
	@required this.maxExtent,
	@required this.initialExtent,
	@required this.context,
	}) : assert(extent != null),
	assert(initialExtent != null),
	assert(minExtent != null),
	assert(maxExtent != null),
	assert(0.0 <= minExtent),
	assert(maxExtent <= 1.0),
	assert(minExtent <= extent),
	assert(minExtent <= initialExtent),
	assert(extent <= maxExtent),
	assert(initialExtent <= maxExtent),
	assert(context != null);

	/// The current value of the extent, between [minExtent] and [maxExtent].
	final double extent;

	/// The minimum value of [extent], which is >= 0.
	final double minExtent;

	/// The maximum value of [extent].
	final double maxExtent;

	/// The initially requested value for [extent].
	final double initialExtent;

	/// The build context of the widget that fired this notification.
	///
	/// This can be used to find the sheet's render objects to determine the size
	/// of the viewport, for instance. A listener can only assume this context
	/// is live when it first gets the notification.
	final BuildContext context;

	@override
	void debugFillDescription(List<String> description) {
	super.debugFillDescription(description);
	description.add('minExtent: $minExtent, extent: $extent, maxExtent: $maxExtent, initialExtent: $initialExtent');
	}
	}

	/// Manages state between [_DraggableScrollableSheetState],
	/// [_DraggableScrollableSheetScrollController], and
	/// [_DraggableScrollableSheetScrollPosition].
	///
	/// The State knows the pixels available along the axis the widget wants to
	/// scroll, but expects to get a fraction of those pixels to render the sheet.
	///
	/// The ScrollPosition knows the number of pixels a user wants to move the sheet.
	///
	/// The [currentExtent] will never be null.
	/// The [availablePixels] will never be null, but may be `double.infinity`.
	class _DraggableSheetExtent {
	_DraggableSheetExtent({
	@required this.minExtent,
	@required this.maxExtent,
	@required this.initialExtent,
	@required VoidCallback listener,
	}) : assert(minExtent != null),
	assert(maxExtent != null),
	assert(initialExtent != null),
	assert(minExtent >= 0),
	assert(maxExtent <= 1),
	assert(minExtent <= initialExtent),
	assert(initialExtent <= maxExtent),
	_currentExtent = ValueNotifier<double>(initialExtent)..addListener(listener),
	availablePixels = double.infinity;

	final double minExtent;
	final double maxExtent;
	final double initialExtent;
	final ValueNotifier<double> _currentExtent;
	double availablePixels;

	bool get isAtMin => minExtent >= _currentExtent.value;
	bool get isAtMax => maxExtent <= _currentExtent.value;

	set currentExtent(double value) {
	assert(value != null);
	_currentExtent.value = value.clamp(minExtent, maxExtent) as double;
	}
	double get currentExtent => _currentExtent.value;

	double get additionalMinExtent => isAtMin ? 0.0 : 1.0;
	double get additionalMaxExtent => isAtMax ? 0.0 : 1.0;

	/// The scroll position gets inputs in terms of pixels, but the extent is
	/// expected to be expressed as a number between 0..1.
	void addPixelDelta(double delta, BuildContext context) {
	if (availablePixels == 0) {
	return;
	}
	currentExtent += delta / availablePixels * maxExtent;
	DraggableScrollableNotification(
	minExtent: minExtent,
	maxExtent: maxExtent,
	extent: currentExtent,
	initialExtent: initialExtent,
	context: context,
	).dispatch(context);
	}
	}

	class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
	_DraggableScrollableSheetScrollController _scrollController;
	_DraggableSheetExtent _extent;

	@override
	void initState() {
	super.initState();
	_extent = _DraggableSheetExtent(
	minExtent: widget.minChildSize,
	maxExtent: widget.maxChildSize,
	initialExtent: widget.initialChildSize,
	listener: _setExtent,
	);
	_scrollController = _DraggableScrollableSheetScrollController(extent: _extent);
	}

	@override
	void didChangeDependencies() {
	super.didChangeDependencies();
	if (_InheritedResetNotifier.shouldReset(context)) {
	// jumpTo can result in trying to replace semantics during build.
	// Just animate really fast.
	// Avoid doing it at all if the offset is already 0.0.
	if (_scrollController.offset != 0.0) {
	_scrollController.animateTo(
	0.0,
	duration: const Duration(milliseconds: 1),
	curve: Curves.linear,
	);
	}
	_extent._currentExtent.value = _extent.initialExtent;
	}
	}

	void _setExtent() {
	setState(() {
	// _extent has been updated when this is called.
	});

	}

	@override
	Widget build(BuildContext context) {
	return LayoutBuilder(
	builder: (BuildContext context, BoxConstraints constraints) {
	_extent.availablePixels = widget.maxChildSize * constraints.biggest.height;
	final Widget sheet = FractionallySizedBox(
	heightFactor: _extent.currentExtent,
	child: widget.builder(context, _scrollController),
	alignment: Alignment.bottomCenter,
	);
	return widget.expand ? SizedBox.expand(child: sheet) : sheet;
	},
	);
	}

	@override
	void dispose() {
	_scrollController.dispose();
	super.dispose();
	}
	}

	/// A [ScrollController] suitable for use in a [ScrollableWidgetBuilder] created
	/// by a [DraggableScrollableSheet].
	///
	/// If a [DraggableScrollableSheet] contains content that is exceeds the height
	/// of its container, this controller will allow the sheet to both be dragged to
	/// fill the container and then scroll the child content.
	///
	/// See also:
	///
	/// * [_DraggableScrollableSheetScrollPosition], which manages the positioning logic for
	/// this controller.
	/// * [PrimaryScrollController], which can be used to establish a
	/// [_DraggableScrollableSheetScrollController] as the primary controller for
	/// descendants.
	class _DraggableScrollableSheetScrollController extends ScrollController {
	_DraggableScrollableSheetScrollController({
	double initialScrollOffset = 0.0,
	String debugLabel,
	@required this.extent,
	}) : assert(extent != null),
	super(
	debugLabel: debugLabel,
	initialScrollOffset: initialScrollOffset,
	);

	final _DraggableSheetExtent extent;

	@override
	_DraggableScrollableSheetScrollPosition createScrollPosition(
	ScrollPhysics physics,
	ScrollContext context,
	ScrollPosition oldPosition,
	) {
	return _DraggableScrollableSheetScrollPosition(
	physics: physics,
	context: context,
	oldPosition: oldPosition,
	extent: extent,
	);
	}

	@override
	void debugFillDescription(List<String> description) {
	super.debugFillDescription(description);
	description.add('extent: $extent');
	}
	}

	/// A scroll position that manages scroll activities for
	/// [_DraggableScrollableSheetScrollController].
	///
	/// This class is a concrete subclass of [ScrollPosition] logic that handles a
	/// single [ScrollContext], such as a [Scrollable]. An instance of this class
	/// manages [ScrollActivity] instances, which changes the
	/// [_DraggableSheetExtent.currentExtent] or visible content offset in the
	/// [Scrollable]'s [Viewport]
	///
	/// See also:
	///
	/// * [_DraggableScrollableSheetScrollController], which uses this as its [ScrollPosition].
	class _DraggableScrollableSheetScrollPosition
	extends ScrollPositionWithSingleContext {
	_DraggableScrollableSheetScrollPosition({
	@required ScrollPhysics physics,
	@required ScrollContext context,
	double initialPixels = 0.0,
	bool keepScrollOffset = true,
	ScrollPosition oldPosition,
	String debugLabel,
	@required this.extent,
	}) : assert(extent != null),
	super(
	physics: physics,
	context: context,
	initialPixels: initialPixels,
	keepScrollOffset: keepScrollOffset,
	oldPosition: oldPosition,
	debugLabel: debugLabel,
	);

	VoidCallback _dragCancelCallback;
	final _DraggableSheetExtent extent;
	bool get listShouldScroll => pixels > 0.0;

	@override
	bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
	// We need to provide some extra extent if we haven't yet reached the max or
	// min extents. Otherwise, a list with fewer children than the extent of
	// the available space will get stuck.
	return super.applyContentDimensions(
	minScrollExtent - extent.additionalMinExtent,
	maxScrollExtent + extent.additionalMaxExtent,
	);
	}

	@override
	void applyUserOffset(double delta) {
	if (!listShouldScroll &&
	(!(extent.isAtMin \|\| extent.isAtMax) \|\|
	(extent.isAtMin && delta < 0) \|\|
	(extent.isAtMax && delta > 0))) {
	extent.addPixelDelta(-delta, context.notificationContext);
	} else {
	super.applyUserOffset(delta);
	}
	}

	@override
	void goBallistic(double velocity) {
	if (velocity == 0.0 \|\|
	(velocity < 0.0 && listShouldScroll) \|\|
	(velocity > 0.0 && extent.isAtMax)) {
	super.goBallistic(velocity);
	return;
	}
	// Scrollable expects that we will dispose of its current _dragCancelCallback
	_dragCancelCallback?.call();
	_dragCancelCallback = null;

	// The iOS bouncing simulation just isn't right here - once we delegate
	// the ballistic back to the ScrollView, it will use the right simulation.
	final Simulation simulation = ClampingScrollSimulation(
	position: extent.currentExtent,
	velocity: velocity,
	tolerance: physics.tolerance,
	);

	final AnimationController ballisticController = AnimationController.unbounded(
	debugLabel: '${objectRuntimeType(this, '_DraggableScrollableSheetPosition')}',
	vsync: context.vsync,
	);
	double lastDelta = 0;
	void _tick() {
	final double delta = ballisticController.value - lastDelta;
	lastDelta = ballisticController.value;
	extent.addPixelDelta(delta, context.notificationContext);
	if ((velocity > 0 && extent.isAtMax) \|\| (velocity < 0 && extent.isAtMin)) {
	// Make sure we pass along enough velocity to keep scrolling - otherwise
	// we just "bounce" off the top making it look like the list doesn't
	// have more to scroll.
	velocity = ballisticController.velocity + (physics.tolerance.velocity * ballisticController.velocity.sign);
	super.goBallistic(velocity);
	ballisticController.stop();
	} else if (ballisticController.isCompleted) {
	super.goBallistic(0);
	}
	}

	ballisticController
	..addListener(_tick)
	..animateWith(simulation).whenCompleteOrCancel(
	ballisticController.dispose,
	);
	}

	@override
	Drag drag(DragStartDetails details, VoidCallback dragCancelCallback) {
	// Save this so we can call it later if we have to [goBallistic] on our own.
	_dragCancelCallback = dragCancelCallback;
	return super.drag(details, dragCancelCallback);
	}
	}

	/// A widget that can notify a descendent [DraggableScrollableSheet] that it
	/// should reset its position to the initial state.
	///
	/// The [Scaffold] uses this widget to notify a persistent bottom sheet that
	/// the user has tapped back if the sheet has started to cover more of the body
	/// than when at its initial position. This is important for users of assistive
	/// technology, where dragging may be difficult to communicate.
	class DraggableScrollableActuator extends StatelessWidget {
	/// Creates a widget that can notify descendent [DraggableScrollableSheet]s
	/// to reset to their initial position.
	///
	/// The [child] parameter is required.
	DraggableScrollableActuator({
	Key key,
	@required this.child,
	}) : super(key: key);

	/// This child's [DraggableScrollableSheet] descendant will be reset when the
	/// [reset] method is applied to a context that includes it.
	///
	/// Must not be null.
	final Widget child;

	final _ResetNotifier _notifier = _ResetNotifier();

	/// Notifies any descendant [DraggableScrollableSheet] that it should reset
	/// to its initial position.
	///
	/// Returns `true` if a [DraggableScrollableActuator] is available and
	/// some [DraggableScrollableSheet] is listening for updates, `false`
	/// otherwise.
	static bool reset(BuildContext context) {
	final _InheritedResetNotifier notifier = context.dependOnInheritedWidgetOfExactType<_InheritedResetNotifier>();
	if (notifier == null) {
	return false;
	}
	return notifier._sendReset();
	}

	@override
	Widget build(BuildContext context) {
	return _InheritedResetNotifier(child: child, notifier: _notifier);
	}
	}

	/// A [ChangeNotifier] to use with [InheritedResetNotifer] to notify
	/// descendants that they should reset to initial state.
	class _ResetNotifier extends ChangeNotifier {
	/// Whether someone called [sendReset] or not.
	///
	/// This flag should be reset after checking it.
	bool _wasCalled = false;

	/// Fires a reset notification to descendants.
	///
	/// Returns false if there are no listeners.
	bool sendReset() {
	if (!hasListeners) {
	return false;
	}
	_wasCalled = true;
	notifyListeners();
	return true;
	}
	}

	class _InheritedResetNotifier extends InheritedNotifier<_ResetNotifier> {
	/// Creates an [InheritedNotifier] that the [DraggableScrollableSheet] will
	/// listen to for an indication that it should change its extent.
	///
	/// The [child] and [notifier] properties must not be null.
	const _InheritedResetNotifier({
	Key key,
	@required Widget child,
	@required _ResetNotifier notifier,
	}) : super(key: key, child: child, notifier: notifier);

	bool _sendReset() => notifier.sendReset();

	/// Specifies whether the [DraggableScrollableSheet] should reset to its
	/// initial position.
	///
	/// Returns true if the notifier requested a reset, false otherwise.
	static bool shouldReset(BuildContext context) {
	final InheritedWidget widget = context.dependOnInheritedWidgetOfExactType<_InheritedResetNotifier>();
	if (widget == null) {
	return false;
	}
	assert(widget is _InheritedResetNotifier);
	final _InheritedResetNotifier inheritedNotifier = widget as _InheritedResetNotifier;
	final bool wasCalled = inheritedNotifier.notifier._wasCalled;
	inheritedNotifier.notifier._wasCalled = false;
	return wasCalled;
	}
	}