packages/flutter/lib/src/cupertino/picker.dart - external/github.com/flutter/flutter - Git at Google

 // Copyright 2017 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 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';

 import 'theme.dart';

 /// Color of the 'magnifier' lens border.
 const Color _kHighlighterBorder = Color(0xFF7F7F7F);
 const Color _kDefaultBackground = Color(0xFFD2D4DB);
 // Eyeballed values comparing with a native picker to produce the right
 // curvatures and densities.
 const double _kDefaultDiameterRatio = 1.07;
 const double _kDefaultPerspective = 0.003;
 const double _kSqueeze = 1.45;
 /// Opacity fraction value that hides the wheel above and below the 'magnifier'
 /// lens with the same color as the background.
 const double _kForegroundScreenOpacityFraction = 0.7;

 /// An iOS-styled picker.
 ///
 /// Displays its children widgets on a wheel for selection and
 /// calls back when the currently selected item changes.
 ///
 /// Can be used with [showCupertinoModalPopup] to display the picker modally at the
 /// bottom of the screen.
 ///
 /// Sizes itself to its parent. All children are sized to the same size based
 /// on [itemExtent].
 ///
 /// By default, descendent texts are shown with [CupertinoTextThemeData.pickerTextStyle].
 ///
 /// See also:
 ///
 ///  * [ListWheelScrollView], the generic widget backing this picker without
 ///    the iOS design specific chrome.
 ///  * <https://developer.apple.com/ios/human-interface-guidelines/controls/pickers/>
 class CupertinoPicker extends StatefulWidget {
   /// Creates a picker from a concrete list of children.
   ///
   /// The [diameterRatio] and [itemExtent] arguments must not be null. The
   /// [itemExtent] must be greater than zero.
   ///
   /// The [backgroundColor] defaults to light gray. It can be set to null to
   /// disable the background painting entirely; this is mildly more efficient
   /// than using [Colors.transparent]. Also, if it has transparency, no gradient
   /// effect will be rendered.
   ///
   /// The [scrollController] argument can be used to specify a custom
   /// [FixedExtentScrollController] for programmatically reading or changing
   /// the current picker index or for selecting an initial index value.
   ///
   /// The [looping] argument decides whether the child list loops and can be
   /// scrolled infinitely.  If set to true, scrolling past the end of the list
   /// will loop the list back to the beginning.  If set to false, the list will
   /// stop scrolling when you reach the end or the beginning.
   CupertinoPicker({
     Key key,
     this.diameterRatio = _kDefaultDiameterRatio,
     this.backgroundColor = _kDefaultBackground,
     this.offAxisFraction = 0.0,
     this.useMagnifier = false,
     this.magnification = 1.0,
     this.scrollController,
     this.squeeze = _kSqueeze,
     @required this.itemExtent,
     @required this.onSelectedItemChanged,
     @required List<Widget> children,
     bool looping = false,
   }) : assert(children != null),
        assert(diameterRatio != null),
        assert(diameterRatio > 0.0, RenderListWheelViewport.diameterRatioZeroMessage),
        assert(magnification > 0),
        assert(itemExtent != null),
        assert(itemExtent > 0),
        assert(squeeze != null),
        assert(squeeze > 0),
        childDelegate = looping
                        ? ListWheelChildLoopingListDelegate(children: children)
                        : ListWheelChildListDelegate(children: children),
        super(key: key);

   /// Creates a picker from an [IndexedWidgetBuilder] callback where the builder
   /// is dynamically invoked during layout.
   ///
   /// A child is lazily created when it starts becoming visible in the viewport.
   /// All of the children provided by the builder are cached and reused, so
   /// normally the builder is only called once for each index (except when
   /// rebuilding - the cache is cleared).
   ///
   /// The [itemBuilder] argument must not be null. The [childCount] argument
   /// reflects the number of children that will be provided by the [itemBuilder].
   /// {@macro flutter.widgets.wheelList.childCount}
   ///
   /// The [itemExtent] argument must be non-null and positive.
   ///
   /// The [backgroundColor] defaults to light gray. It can be set to null to
   /// disable the background painting entirely; this is mildly more efficient
   /// than using [Colors.transparent].
   CupertinoPicker.builder({
     Key key,
     this.diameterRatio = _kDefaultDiameterRatio,
     this.backgroundColor = _kDefaultBackground,
     this.offAxisFraction = 0.0,
     this.useMagnifier = false,
     this.magnification = 1.0,
     this.scrollController,
     this.squeeze = _kSqueeze,
     @required this.itemExtent,
     @required this.onSelectedItemChanged,
     @required IndexedWidgetBuilder itemBuilder,
     int childCount,
   }) : assert(itemBuilder != null),
        assert(diameterRatio != null),
        assert(diameterRatio > 0.0, RenderListWheelViewport.diameterRatioZeroMessage),
        assert(magnification > 0),
        assert(itemExtent != null),
        assert(itemExtent > 0),
        assert(squeeze != null),
        assert(squeeze > 0),
        childDelegate = ListWheelChildBuilderDelegate(builder: itemBuilder, childCount: childCount),
        super(key: key);

   /// Relative ratio between this picker's height and the simulated cylinder's diameter.
   ///
   /// Smaller values creates more pronounced curvatures in the scrollable wheel.
   ///
   /// For more details, see [ListWheelScrollView.diameterRatio].
   ///
   /// Must not be null and defaults to `1.1` to visually mimic iOS.
   final double diameterRatio;

   /// Background color behind the children.
   ///
   /// Defaults to a gray color in the iOS color palette.
   ///
   /// This can be set to null to disable the background painting entirely; this
   /// is mildly more efficient than using [Colors.transparent].
   ///
   /// Any alpha value less 255 (fully opaque) will cause the removal of the
   /// wheel list edge fade gradient from rendering of the widget.
   final Color backgroundColor;

   /// {@macro flutter.rendering.wheelList.offAxisFraction}
   final double offAxisFraction;

   /// {@macro flutter.rendering.wheelList.useMagnifier}
   final bool useMagnifier;

   /// {@macro flutter.rendering.wheelList.magnification}
   final double magnification;

   /// A [FixedExtentScrollController] to read and control the current item.
   ///
   /// If null, an implicit one will be created internally.
   final FixedExtentScrollController scrollController;

   /// The uniform height of all children.
   ///
   /// All children will be given the [BoxConstraints] to match this exact
   /// height. Must not be null and must be positive.
   final double itemExtent;

   /// {@macro flutter.rendering.wheelList.squeeze}
   ///
   /// Defaults to `1.45` fo visually mimic iOS.
   final double squeeze;

   /// An option callback when the currently centered item changes.
   ///
   /// Value changes when the item closest to the center changes.
   ///
   /// This can be called during scrolls and during ballistic flings. To get the
   /// value only when the scrolling settles, use a [NotificationListener],
   /// listen for [ScrollEndNotification] and read its [FixedExtentMetrics].
   final ValueChanged<int> onSelectedItemChanged;

   /// A delegate that lazily instantiates children.
   final ListWheelChildDelegate childDelegate;

   @override
   State<StatefulWidget> createState() => _CupertinoPickerState();
 }

 class _CupertinoPickerState extends State<CupertinoPicker> {
   int _lastHapticIndex;
   FixedExtentScrollController _controller;

   @override
   void initState() {
     super.initState();
     if (widget.scrollController == null) {
       _controller = FixedExtentScrollController();
     }
   }

   @override
   void didUpdateWidget(CupertinoPicker oldWidget) {
     if (widget.scrollController != null && oldWidget.scrollController == null) {
       _controller = null;
     } else if (widget.scrollController == null && oldWidget.scrollController != null) {
       assert(_controller == null);
       _controller = FixedExtentScrollController();
     }
     super.didUpdateWidget(oldWidget);
   }

   @override
   void dispose() {
     _controller?.dispose();
     super.dispose();
   }

   void _handleSelectedItemChanged(int index) {
     // Only the haptic engine hardware on iOS devices would produce the
     // intended effects.
     bool hasSuitableHapticHardware;
     switch (defaultTargetPlatform) {
       case TargetPlatform.iOS:
         hasSuitableHapticHardware = true;
         break;
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
         hasSuitableHapticHardware = false;
         break;
     }
     assert(hasSuitableHapticHardware != null);
     if (hasSuitableHapticHardware && index != _lastHapticIndex) {
       _lastHapticIndex = index;
       HapticFeedback.selectionClick();
     }

     if (widget.onSelectedItemChanged != null) {
       widget.onSelectedItemChanged(index);
     }
   }

   /// Makes the fade to [CupertinoPicker.backgroundColor] edge gradients.
   Widget _buildGradientScreen() {
     // Because BlendMode.dstOut doesn't work correctly with BoxDecoration we
     // have to just do a color blend. And a due to the way we are layering
     // the magnifier and the gradient on the background, using a transparent
     // background color makes the picker look odd.
     if (widget.backgroundColor != null && widget.backgroundColor.alpha < 255)
       return Container();

     final Color widgetBackgroundColor = widget.backgroundColor ?? const Color(0xFFFFFFFF);
     return Positioned.fill(
       child: IgnorePointer(
         child: Container(
           decoration: BoxDecoration(
             gradient: LinearGradient(
               colors: <Color>[
                 widgetBackgroundColor,
                 widgetBackgroundColor.withAlpha(0xF2),
                 widgetBackgroundColor.withAlpha(0xDD),
                 widgetBackgroundColor.withAlpha(0),
                 widgetBackgroundColor.withAlpha(0),
                 widgetBackgroundColor.withAlpha(0xDD),
                 widgetBackgroundColor.withAlpha(0xF2),
                 widgetBackgroundColor,
               ],
               stops: const <double>[
                 0.0, 0.05, 0.09, 0.22, 0.78, 0.91, 0.95, 1.0,
               ],
               begin: Alignment.topCenter,
               end: Alignment.bottomCenter,
             ),
           ),
         ),
       ),
     );
   }

   /// Makes the magnifier lens look so that the colors are normal through
   /// the lens and partially grayed out around it.
   Widget _buildMagnifierScreen() {
     final Color foreground = widget.backgroundColor?.withAlpha(
       (widget.backgroundColor.alpha * _kForegroundScreenOpacityFraction).toInt()
     );

     return IgnorePointer(
       child: Column(
         children: <Widget>[
           Expanded(
             child: Container(
               color: foreground,
             ),
           ),
           Container(
             decoration: const BoxDecoration(
               border: Border(
                 top: BorderSide(width: 0.0, color: _kHighlighterBorder),
                 bottom: BorderSide(width: 0.0, color: _kHighlighterBorder),
               ),
             ),
             constraints: BoxConstraints.expand(
                 height: widget.itemExtent * widget.magnification,
             ),
           ),
           Expanded(
             child: Container(
               color: foreground,
             ),
           ),
         ],
       ),
     );
   }

   Widget _buildUnderMagnifierScreen() {
     final Color foreground = widget.backgroundColor?.withAlpha(
         (widget.backgroundColor.alpha * _kForegroundScreenOpacityFraction).toInt()
     );

     return Column(
       children: <Widget>[
         Expanded(child: Container()),
         Container(
           color: foreground,
           constraints: BoxConstraints.expand(
             height: widget.itemExtent * widget.magnification,
           ),
         ),
         Expanded(child: Container()),
       ],
     );
   }

   Widget _addBackgroundToChild(Widget child) {
     return DecoratedBox(
       decoration: BoxDecoration(
         color: widget.backgroundColor,
       ),
       child: child,
     );
   }

   @override
   Widget build(BuildContext context) {
     Widget result = DefaultTextStyle(
       style: CupertinoTheme.of(context).textTheme.pickerTextStyle,
       child: Stack(
         children: <Widget>[
           Positioned.fill(
             child: _CupertinoPickerSemantics(
               scrollController: widget.scrollController ?? _controller,
               child: ListWheelScrollView.useDelegate(
                 controller: widget.scrollController ?? _controller,
                 physics: const FixedExtentScrollPhysics(),
                 diameterRatio: widget.diameterRatio,
                 perspective: _kDefaultPerspective,
                 offAxisFraction: widget.offAxisFraction,
                 useMagnifier: widget.useMagnifier,
                 magnification: widget.magnification,
                 itemExtent: widget.itemExtent,
                 squeeze: widget.squeeze,
                 onSelectedItemChanged: _handleSelectedItemChanged,
                 childDelegate: widget.childDelegate,
               ),
             ),
           ),
           _buildGradientScreen(),
           _buildMagnifierScreen(),
         ],
       ),
     );
     // Adds the appropriate opacity under the magnifier if the background
     // color is transparent.
     if (widget.backgroundColor != null && widget.backgroundColor.alpha < 255) {
       result = Stack(
         children: <Widget> [
           _buildUnderMagnifierScreen(),
           _addBackgroundToChild(result),
         ],
       );
     } else {
       result = _addBackgroundToChild(result);
     }
     return result;
   }
 }

 // Turns the scroll semantics of the ListView into a single adjustable semantics
 // node. This is done by removing all of the child semantics of the scroll
 // wheel and using the scroll indexes to look up the current, previous, and
 // next semantic label. This label is then turned into the value of a new
 // adjustable semantic node, with adjustment callbacks wired to move the
 // scroll controller.
 class _CupertinoPickerSemantics extends SingleChildRenderObjectWidget {
   const _CupertinoPickerSemantics({
     Key key,
     Widget child,
     @required this.scrollController,
   }) : super(key: key, child: child);

   final FixedExtentScrollController scrollController;

   @override
   RenderObject createRenderObject(BuildContext context) => _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));

   @override
   void updateRenderObject(BuildContext context, covariant _RenderCupertinoPickerSemantics renderObject) {
     renderObject
       ..textDirection = Directionality.of(context)
       ..controller = scrollController;
   }
 }

 class _RenderCupertinoPickerSemantics extends RenderProxyBox {
   _RenderCupertinoPickerSemantics(FixedExtentScrollController controller, this._textDirection) {
     this.controller = controller;
   }

   FixedExtentScrollController get controller => _controller;
   FixedExtentScrollController _controller;
   set controller(FixedExtentScrollController value) {
     if (value == _controller)
       return;
     if (_controller != null)
       _controller.removeListener(_handleScrollUpdate);
     else
       _currentIndex = value.initialItem ?? 0;
     value.addListener(_handleScrollUpdate);
     _controller = value;
   }

   TextDirection get textDirection => _textDirection;
   TextDirection _textDirection;
   set textDirection(TextDirection value) {
     if (textDirection == value)
       return;
     _textDirection = value;
     markNeedsSemanticsUpdate();
   }

   int _currentIndex = 0;

   void _handleIncrease() {
     controller.jumpToItem(_currentIndex + 1);
   }

   void _handleDecrease() {
     if (_currentIndex == 0)
       return;
     controller.jumpToItem(_currentIndex - 1);
   }

   void _handleScrollUpdate() {
     if (controller.selectedItem == _currentIndex)
       return;
     _currentIndex = controller.selectedItem;
     markNeedsSemanticsUpdate();
   }
   @override
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     super.describeSemanticsConfiguration(config);
     config.isSemanticBoundary = true;
     config.textDirection = textDirection;
   }

   @override
   void assembleSemanticsNode(SemanticsNode node, SemanticsConfiguration config, Iterable<SemanticsNode> children) {
     if (children.isEmpty)
       return super.assembleSemanticsNode(node, config, children);
     final SemanticsNode scrollable = children.first;
     final Map<int, SemanticsNode> indexedChildren = <int, SemanticsNode>{};
     scrollable.visitChildren((SemanticsNode child) {
       assert(child.indexInParent != null);
       indexedChildren[child.indexInParent] = child;
       return true;
     });
     if (indexedChildren[_currentIndex] == null) {
       return node.updateWith(config: config);
     }
     config.value = indexedChildren[_currentIndex].label;
     final SemanticsNode previousChild = indexedChildren[_currentIndex - 1];
     final SemanticsNode nextChild = indexedChildren[_currentIndex + 1];
     if (nextChild != null) {
       config.increasedValue = nextChild.label;
       config.onIncrease = _handleIncrease;
     }
     if (previousChild != null) {
       config.decreasedValue = previousChild.label;
       config.onDecrease = _handleDecrease;
     }
     node.updateWith(config: config);
   }
 }
	// Copyright 2017 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 'package:flutter/foundation.dart';
	import 'package:flutter/rendering.dart';
	import 'package:flutter/services.dart';
	import 'package:flutter/widgets.dart';

	import 'theme.dart';

	/// Color of the 'magnifier' lens border.
	const Color _kHighlighterBorder = Color(0xFF7F7F7F);
	const Color _kDefaultBackground = Color(0xFFD2D4DB);
	// Eyeballed values comparing with a native picker to produce the right
	// curvatures and densities.
	const double _kDefaultDiameterRatio = 1.07;
	const double _kDefaultPerspective = 0.003;
	const double _kSqueeze = 1.45;
	/// Opacity fraction value that hides the wheel above and below the 'magnifier'
	/// lens with the same color as the background.
	const double _kForegroundScreenOpacityFraction = 0.7;

	/// An iOS-styled picker.
	///
	/// Displays its children widgets on a wheel for selection and
	/// calls back when the currently selected item changes.
	///
	/// Can be used with [showCupertinoModalPopup] to display the picker modally at the
	/// bottom of the screen.
	///
	/// Sizes itself to its parent. All children are sized to the same size based
	/// on [itemExtent].
	///
	/// By default, descendent texts are shown with [CupertinoTextThemeData.pickerTextStyle].
	///
	/// See also:
	///
	/// * [ListWheelScrollView], the generic widget backing this picker without
	/// the iOS design specific chrome.
	/// * <https://developer.apple.com/ios/human-interface-guidelines/controls/pickers/>
	class CupertinoPicker extends StatefulWidget {
	/// Creates a picker from a concrete list of children.
	///
	/// The [diameterRatio] and [itemExtent] arguments must not be null. The
	/// [itemExtent] must be greater than zero.
	///
	/// The [backgroundColor] defaults to light gray. It can be set to null to
	/// disable the background painting entirely; this is mildly more efficient
	/// than using [Colors.transparent]. Also, if it has transparency, no gradient
	/// effect will be rendered.
	///
	/// The [scrollController] argument can be used to specify a custom
	/// [FixedExtentScrollController] for programmatically reading or changing
	/// the current picker index or for selecting an initial index value.
	///
	/// The [looping] argument decides whether the child list loops and can be
	/// scrolled infinitely. If set to true, scrolling past the end of the list
	/// will loop the list back to the beginning. If set to false, the list will
	/// stop scrolling when you reach the end or the beginning.
	CupertinoPicker({
	Key key,
	this.diameterRatio = _kDefaultDiameterRatio,
	this.backgroundColor = _kDefaultBackground,
	this.offAxisFraction = 0.0,
	this.useMagnifier = false,
	this.magnification = 1.0,
	this.scrollController,
	this.squeeze = _kSqueeze,
	@required this.itemExtent,
	@required this.onSelectedItemChanged,
	@required List<Widget> children,
	bool looping = false,
	}) : assert(children != null),
	assert(diameterRatio != null),
	assert(diameterRatio > 0.0, RenderListWheelViewport.diameterRatioZeroMessage),
	assert(magnification > 0),
	assert(itemExtent != null),
	assert(itemExtent > 0),
	assert(squeeze != null),
	assert(squeeze > 0),
	childDelegate = looping
	? ListWheelChildLoopingListDelegate(children: children)
	: ListWheelChildListDelegate(children: children),
	super(key: key);

	/// Creates a picker from an [IndexedWidgetBuilder] callback where the builder
	/// is dynamically invoked during layout.
	///
	/// A child is lazily created when it starts becoming visible in the viewport.
	/// All of the children provided by the builder are cached and reused, so
	/// normally the builder is only called once for each index (except when
	/// rebuilding - the cache is cleared).
	///
	/// The [itemBuilder] argument must not be null. The [childCount] argument
	/// reflects the number of children that will be provided by the [itemBuilder].
	/// {@macro flutter.widgets.wheelList.childCount}
	///
	/// The [itemExtent] argument must be non-null and positive.
	///
	/// The [backgroundColor] defaults to light gray. It can be set to null to
	/// disable the background painting entirely; this is mildly more efficient
	/// than using [Colors.transparent].
	CupertinoPicker.builder({
	Key key,
	this.diameterRatio = _kDefaultDiameterRatio,
	this.backgroundColor = _kDefaultBackground,
	this.offAxisFraction = 0.0,
	this.useMagnifier = false,
	this.magnification = 1.0,
	this.scrollController,
	this.squeeze = _kSqueeze,
	@required this.itemExtent,
	@required this.onSelectedItemChanged,
	@required IndexedWidgetBuilder itemBuilder,
	int childCount,
	}) : assert(itemBuilder != null),
	assert(diameterRatio != null),
	assert(diameterRatio > 0.0, RenderListWheelViewport.diameterRatioZeroMessage),
	assert(magnification > 0),
	assert(itemExtent != null),
	assert(itemExtent > 0),
	assert(squeeze != null),
	assert(squeeze > 0),
	childDelegate = ListWheelChildBuilderDelegate(builder: itemBuilder, childCount: childCount),
	super(key: key);

	/// Relative ratio between this picker's height and the simulated cylinder's diameter.
	///
	/// Smaller values creates more pronounced curvatures in the scrollable wheel.
	///
	/// For more details, see [ListWheelScrollView.diameterRatio].
	///
	/// Must not be null and defaults to `1.1` to visually mimic iOS.
	final double diameterRatio;

	/// Background color behind the children.
	///
	/// Defaults to a gray color in the iOS color palette.
	///
	/// This can be set to null to disable the background painting entirely; this
	/// is mildly more efficient than using [Colors.transparent].
	///
	/// Any alpha value less 255 (fully opaque) will cause the removal of the
	/// wheel list edge fade gradient from rendering of the widget.
	final Color backgroundColor;

	/// {@macro flutter.rendering.wheelList.offAxisFraction}
	final double offAxisFraction;

	/// {@macro flutter.rendering.wheelList.useMagnifier}
	final bool useMagnifier;

	/// {@macro flutter.rendering.wheelList.magnification}
	final double magnification;

	/// A [FixedExtentScrollController] to read and control the current item.
	///
	/// If null, an implicit one will be created internally.
	final FixedExtentScrollController scrollController;

	/// The uniform height of all children.
	///
	/// All children will be given the [BoxConstraints] to match this exact
	/// height. Must not be null and must be positive.
	final double itemExtent;

	/// {@macro flutter.rendering.wheelList.squeeze}
	///
	/// Defaults to `1.45` fo visually mimic iOS.
	final double squeeze;

	/// An option callback when the currently centered item changes.
	///
	/// Value changes when the item closest to the center changes.
	///
	/// This can be called during scrolls and during ballistic flings. To get the
	/// value only when the scrolling settles, use a [NotificationListener],
	/// listen for [ScrollEndNotification] and read its [FixedExtentMetrics].
	final ValueChanged<int> onSelectedItemChanged;

	/// A delegate that lazily instantiates children.
	final ListWheelChildDelegate childDelegate;

	@override
	State<StatefulWidget> createState() => _CupertinoPickerState();
	}

	class _CupertinoPickerState extends State<CupertinoPicker> {
	int _lastHapticIndex;
	FixedExtentScrollController _controller;

	@override
	void initState() {
	super.initState();
	if (widget.scrollController == null) {
	_controller = FixedExtentScrollController();
	}
	}

	@override
	void didUpdateWidget(CupertinoPicker oldWidget) {
	if (widget.scrollController != null && oldWidget.scrollController == null) {
	_controller = null;
	} else if (widget.scrollController == null && oldWidget.scrollController != null) {
	assert(_controller == null);
	_controller = FixedExtentScrollController();
	}
	super.didUpdateWidget(oldWidget);
	}

	@override
	void dispose() {
	_controller?.dispose();
	super.dispose();
	}

	void _handleSelectedItemChanged(int index) {
	// Only the haptic engine hardware on iOS devices would produce the
	// intended effects.
	bool hasSuitableHapticHardware;
	switch (defaultTargetPlatform) {
	case TargetPlatform.iOS:
	hasSuitableHapticHardware = true;
	break;
	case TargetPlatform.android:
	case TargetPlatform.fuchsia:
	hasSuitableHapticHardware = false;
	break;
	}
	assert(hasSuitableHapticHardware != null);
	if (hasSuitableHapticHardware && index != _lastHapticIndex) {
	_lastHapticIndex = index;
	HapticFeedback.selectionClick();
	}

	if (widget.onSelectedItemChanged != null) {
	widget.onSelectedItemChanged(index);
	}
	}

	/// Makes the fade to [CupertinoPicker.backgroundColor] edge gradients.
	Widget _buildGradientScreen() {
	// Because BlendMode.dstOut doesn't work correctly with BoxDecoration we
	// have to just do a color blend. And a due to the way we are layering
	// the magnifier and the gradient on the background, using a transparent
	// background color makes the picker look odd.
	if (widget.backgroundColor != null && widget.backgroundColor.alpha < 255)
	return Container();

	final Color widgetBackgroundColor = widget.backgroundColor ?? const Color(0xFFFFFFFF);
	return Positioned.fill(
	child: IgnorePointer(
	child: Container(
	decoration: BoxDecoration(
	gradient: LinearGradient(
	colors: <Color>[
	widgetBackgroundColor,
	widgetBackgroundColor.withAlpha(0xF2),
	widgetBackgroundColor.withAlpha(0xDD),
	widgetBackgroundColor.withAlpha(0),
	widgetBackgroundColor.withAlpha(0),
	widgetBackgroundColor.withAlpha(0xDD),
	widgetBackgroundColor.withAlpha(0xF2),
	widgetBackgroundColor,
	],
	stops: const <double>[
	0.0, 0.05, 0.09, 0.22, 0.78, 0.91, 0.95, 1.0,
	],
	begin: Alignment.topCenter,
	end: Alignment.bottomCenter,
	),
	),
	),
	),
	);
	}

	/// Makes the magnifier lens look so that the colors are normal through
	/// the lens and partially grayed out around it.
	Widget _buildMagnifierScreen() {
	final Color foreground = widget.backgroundColor?.withAlpha(
	(widget.backgroundColor.alpha * _kForegroundScreenOpacityFraction).toInt()
	);

	return IgnorePointer(
	child: Column(
	children: <Widget>[
	Expanded(
	child: Container(
	color: foreground,
	),
	),
	Container(
	decoration: const BoxDecoration(
	border: Border(
	top: BorderSide(width: 0.0, color: _kHighlighterBorder),
	bottom: BorderSide(width: 0.0, color: _kHighlighterBorder),
	),
	),
	constraints: BoxConstraints.expand(
	height: widget.itemExtent * widget.magnification,
	),
	),
	Expanded(
	child: Container(
	color: foreground,
	),
	),
	],
	),
	);
	}

	Widget _buildUnderMagnifierScreen() {
	final Color foreground = widget.backgroundColor?.withAlpha(
	(widget.backgroundColor.alpha * _kForegroundScreenOpacityFraction).toInt()
	);

	return Column(
	children: <Widget>[
	Expanded(child: Container()),
	Container(
	color: foreground,
	constraints: BoxConstraints.expand(
	height: widget.itemExtent * widget.magnification,
	),
	),
	Expanded(child: Container()),
	],
	);
	}

	Widget _addBackgroundToChild(Widget child) {
	return DecoratedBox(
	decoration: BoxDecoration(
	color: widget.backgroundColor,
	),
	child: child,
	);
	}

	@override
	Widget build(BuildContext context) {
	Widget result = DefaultTextStyle(
	style: CupertinoTheme.of(context).textTheme.pickerTextStyle,
	child: Stack(
	children: <Widget>[
	Positioned.fill(
	child: _CupertinoPickerSemantics(
	scrollController: widget.scrollController ?? _controller,
	child: ListWheelScrollView.useDelegate(
	controller: widget.scrollController ?? _controller,
	physics: const FixedExtentScrollPhysics(),
	diameterRatio: widget.diameterRatio,
	perspective: _kDefaultPerspective,
	offAxisFraction: widget.offAxisFraction,
	useMagnifier: widget.useMagnifier,
	magnification: widget.magnification,
	itemExtent: widget.itemExtent,
	squeeze: widget.squeeze,
	onSelectedItemChanged: _handleSelectedItemChanged,
	childDelegate: widget.childDelegate,
	),
	),
	),
	_buildGradientScreen(),
	_buildMagnifierScreen(),
	],
	),
	);
	// Adds the appropriate opacity under the magnifier if the background
	// color is transparent.
	if (widget.backgroundColor != null && widget.backgroundColor.alpha < 255) {
	result = Stack(
	children: <Widget> [
	_buildUnderMagnifierScreen(),
	_addBackgroundToChild(result),
	],
	);
	} else {
	result = _addBackgroundToChild(result);
	}
	return result;
	}
	}

	// Turns the scroll semantics of the ListView into a single adjustable semantics
	// node. This is done by removing all of the child semantics of the scroll
	// wheel and using the scroll indexes to look up the current, previous, and
	// next semantic label. This label is then turned into the value of a new
	// adjustable semantic node, with adjustment callbacks wired to move the
	// scroll controller.
	class _CupertinoPickerSemantics extends SingleChildRenderObjectWidget {
	const _CupertinoPickerSemantics({
	Key key,
	Widget child,
	@required this.scrollController,
	}) : super(key: key, child: child);

	final FixedExtentScrollController scrollController;

	@override
	RenderObject createRenderObject(BuildContext context) => _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));

	@override
	void updateRenderObject(BuildContext context, covariant _RenderCupertinoPickerSemantics renderObject) {
	renderObject
	..textDirection = Directionality.of(context)
	..controller = scrollController;
	}
	}

	class _RenderCupertinoPickerSemantics extends RenderProxyBox {
	_RenderCupertinoPickerSemantics(FixedExtentScrollController controller, this._textDirection) {
	this.controller = controller;
	}

	FixedExtentScrollController get controller => _controller;
	FixedExtentScrollController _controller;
	set controller(FixedExtentScrollController value) {
	if (value == _controller)
	return;
	if (_controller != null)
	_controller.removeListener(_handleScrollUpdate);
	else
	_currentIndex = value.initialItem ?? 0;
	value.addListener(_handleScrollUpdate);
	_controller = value;
	}

	TextDirection get textDirection => _textDirection;
	TextDirection _textDirection;
	set textDirection(TextDirection value) {
	if (textDirection == value)
	return;
	_textDirection = value;
	markNeedsSemanticsUpdate();
	}

	int _currentIndex = 0;

	void _handleIncrease() {
	controller.jumpToItem(_currentIndex + 1);
	}

	void _handleDecrease() {
	if (_currentIndex == 0)
	return;
	controller.jumpToItem(_currentIndex - 1);
	}

	void _handleScrollUpdate() {
	if (controller.selectedItem == _currentIndex)
	return;
	_currentIndex = controller.selectedItem;
	markNeedsSemanticsUpdate();
	}
	@override
	void describeSemanticsConfiguration(SemanticsConfiguration config) {
	super.describeSemanticsConfiguration(config);
	config.isSemanticBoundary = true;
	config.textDirection = textDirection;
	}

	@override
	void assembleSemanticsNode(SemanticsNode node, SemanticsConfiguration config, Iterable<SemanticsNode> children) {
	if (children.isEmpty)
	return super.assembleSemanticsNode(node, config, children);
	final SemanticsNode scrollable = children.first;
	final Map<int, SemanticsNode> indexedChildren = <int, SemanticsNode>{};
	scrollable.visitChildren((SemanticsNode child) {
	assert(child.indexInParent != null);
	indexedChildren[child.indexInParent] = child;
	return true;
	});
	if (indexedChildren[_currentIndex] == null) {
	return node.updateWith(config: config);
	}
	config.value = indexedChildren[_currentIndex].label;
	final SemanticsNode previousChild = indexedChildren[_currentIndex - 1];
	final SemanticsNode nextChild = indexedChildren[_currentIndex + 1];
	if (nextChild != null) {
	config.increasedValue = nextChild.label;
	config.onIncrease = _handleIncrease;
	}
	if (previousChild != null) {
	config.decreasedValue = previousChild.label;
	config.onDecrease = _handleDecrease;
	}
	node.updateWith(config: config);
	}
	}