packages/flutter/lib/src/cupertino/picker.dart - external/github.com/flutter/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/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';

 import 'colors.dart';
 import 'theme.dart';

 // 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 dims the wheel above and below the "magnifier"
 // lens.
 const double _kOverAndUnderCenterOpacity = 0.447;

 /// An iOS-styled picker.
 ///
 /// Displays its children widgets on a wheel for selection and
 /// calls back when the currently selected item changes.
 ///
 /// By default, the first child in `children` will be the initially selected child.
 /// The index of a different child can be specified in [scrollController], to make
 /// that child the initially selected child.
 ///
 /// Can be used with [showCupertinoModalPopup] to display the picker modally at the
 /// bottom of the screen. When calling [showCupertinoModalPopup], be sure to set
 /// `semanticsDismissible` to true to enable dismissing the modal via semantics.
 ///
 /// 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 null, which disables background painting entirely.
   /// (i.e. the picker is going to have a completely transparent background), to match
   /// the native UIPicker and UIDatePicker. 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,
     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,
     this.selectionOverlay = const CupertinoPickerDefaultSelectionOverlay(),
     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.ListWheelChildBuilderDelegate.childCount}
   ///
   /// The [itemExtent] argument must be non-null and positive.
   ///
   /// The [backgroundColor] defaults to null, which disables background painting entirely.
   /// (i.e. the picker is going to have a completely transparent background), to match
   /// the native UIPicker and UIDatePicker.
   CupertinoPicker.builder({
     Key? key,
     this.diameterRatio = _kDefaultDiameterRatio,
     this.backgroundColor,
     this.offAxisFraction = 0.0,
     this.useMagnifier = false,
     this.magnification = 1.0,
     this.scrollController,
     this.squeeze = _kSqueeze,
     required this.itemExtent,
     required this.onSelectedItemChanged,
     required NullableIndexedWidgetBuilder itemBuilder,
     int? childCount,
     this.selectionOverlay = const CupertinoPickerDefaultSelectionOverlay(),
   }) : 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 null, which disables background painting entirely.
   /// (i.e. the picker is going to have a completely transparent background), to match
   /// the native UIPicker and UIDatePicker.
   ///
   /// 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.RenderListWheelViewport.offAxisFraction}
   final double offAxisFraction;

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

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

   /// A [FixedExtentScrollController] to read and control the current item, and
   /// to set the initial 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.RenderListWheelViewport.squeeze}
   ///
   /// Defaults to `1.45` to 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;

   /// A widget overlaid on the picker to highlight the currently selected entry.
   ///
   /// The [selectionOverlay] widget drawn above the [CupertinoPicker]'s picker
   /// wheel.
   /// It is vertically centered in the picker and is constrained to have the
   /// same height as the center row.
   ///
   /// If unspecified, it defaults to a [CupertinoPickerDefaultSelectionOverlay]
   /// which is a gray rounded rectangle overlay in iOS 14 style.
   /// This property can be set to null to remove the overlay.
   final Widget? selectionOverlay;

   @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.
     final bool hasSuitableHapticHardware;
     switch (defaultTargetPlatform) {
       case TargetPlatform.iOS:
         hasSuitableHapticHardware = true;
         break;
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
       case TargetPlatform.linux:
       case TargetPlatform.macOS:
       case TargetPlatform.windows:
         hasSuitableHapticHardware = false;
         break;
     }
     assert(hasSuitableHapticHardware != null);
     if (hasSuitableHapticHardware && index != _lastHapticIndex) {
       _lastHapticIndex = index;
       HapticFeedback.selectionClick();
     }

     widget.onSelectedItemChanged?.call(index);
   }

   /// Draws the selectionOverlay.
   Widget _buildSelectionOverlay(Widget selectionOverlay) {
     final double height = widget.itemExtent * widget.magnification;

     return IgnorePointer(
       child: Center(
         child: ConstrainedBox(
           constraints: BoxConstraints.expand(
             height: height,
           ),
           child: selectionOverlay,
         ),
       ),
     );
   }

   @override
   Widget build(BuildContext context) {
     final Color? resolvedBackgroundColor = CupertinoDynamicColor.maybeResolve(widget.backgroundColor, context);

     assert(RenderListWheelViewport.defaultPerspective == _kDefaultPerspective);
     final 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,
                 offAxisFraction: widget.offAxisFraction,
                 useMagnifier: widget.useMagnifier,
                 magnification: widget.magnification,
                 overAndUnderCenterOpacity: _kOverAndUnderCenterOpacity,
                 itemExtent: widget.itemExtent,
                 squeeze: widget.squeeze,
                 onSelectedItemChanged: _handleSelectedItemChanged,
                 childDelegate: widget.childDelegate,
               ),
             ),
           ),
           if (widget.selectionOverlay != null)
             _buildSelectionOverlay(widget.selectionOverlay!),
         ],
       ),
     );

     return DecoratedBox(
       decoration: BoxDecoration(color: resolvedBackgroundColor),
       child: result,
     );
   }
 }

 /// A default selection overlay for [CupertinoPicker]s.
 ///
 /// It draws a gray rounded rectangle to match the picker visuals introduced in
 /// iOS 14.
 ///
 /// This widget is typically only used in [CupertinoPicker.selectionOverlay].
 /// In an iOS 14 multi-column picker, the selection overlay is a single rounded
 /// rectangle that spans the entire multi-column picker.
 /// To achieve the same effect using [CupertinoPickerDefaultSelectionOverlay],
 /// the additional margin and corner radii on the left or the right side can be
 /// disabled by turning off [capLeftEdge] and [capRightEdge], so this selection
 /// overlay visually connects with selection overlays of adjoining
 /// [CupertinoPicker]s (i.e., other "column"s).
 ///
 /// See also:
 ///
 ///  * [CupertinoPicker], which uses this widget as its default [CupertinoPicker.selectionOverlay].
 class CupertinoPickerDefaultSelectionOverlay extends StatelessWidget {

   /// Creates an iOS 14 style selection overlay that highlights the magnified
   /// area (or the currently selected item, depending on how you described it
   /// elsewhere) of a [CupertinoPicker].
   ///
   /// The [background] argument default value is [CupertinoColors.tertiarySystemFill].
   /// It must be non-null.
   ///
   /// The [capLeftEdge] and [capRightEdge] arguments decide whether to add a
   /// default margin and use rounded corners on the left and right side of the
   /// rectangular overlay.
   /// Default to true and must not be null.
   const CupertinoPickerDefaultSelectionOverlay({
     Key? key,
     this.background = CupertinoColors.tertiarySystemFill,
     this.capLeftEdge = true,
     this.capRightEdge = true,
   }) : assert(background != null),
        assert(capLeftEdge != null),
        assert(capRightEdge != null),
        super(key: key);

   /// Whether to use the default use rounded corners and margin on the left side.
   final bool capLeftEdge;

   /// Whether to use the default use rounded corners and margin on the right side.
   final bool capRightEdge;

   /// The color to fill in the background of the [CupertinoPickerDefaultSelectionOverlay].
   /// It Support for use [CupertinoDynamicColor].
   ///
   /// Typically this should not be set to a fully opaque color, as the currently
   /// selected item of the underlying [CupertinoPicker] should remain visible.
   /// Defaults to [CupertinoColors.tertiarySystemFill].
   final Color background;

   /// Default margin of the 'SelectionOverlay'.
   static const double _defaultSelectionOverlayHorizontalMargin = 9;

   /// Default radius of the 'SelectionOverlay'.
   static const double _defaultSelectionOverlayRadius = 8;

   @override
   Widget build(BuildContext context) {
     const Radius radius = Radius.circular(_defaultSelectionOverlayRadius);

     return Container(
       margin: EdgeInsets.only(
         left: capLeftEdge ? _defaultSelectionOverlayHorizontalMargin : 0,
         right: capRightEdge ? _defaultSelectionOverlayHorizontalMargin : 0,
       ),
       decoration: BoxDecoration(
         borderRadius: BorderRadius.horizontal(
           left: capLeftEdge ? radius : Radius.zero,
           right: capRightEdge ? radius : Radius.zero,
         ),
         color: CupertinoDynamicColor.resolve(background, context),
       ),
     );
   }
 }

 // 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) {
     assert(debugCheckHasDirectionality(context));
     return _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));
   }

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

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

   FixedExtentScrollController get controller => _controller;
   late FixedExtentScrollController _controller;
   set controller(FixedExtentScrollController value) => _updateController(_controller, value);

   // This method exists to allow controller to be non-null. It is only called with a null oldValue from constructor.
   void _updateController(FixedExtentScrollController? oldValue, FixedExtentScrollController value) {
     if (value == oldValue)
       return;
     if (oldValue != null)
       oldValue.removeListener(_handleScrollUpdate);
     else
       _currentIndex = value.initialItem;
     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 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/rendering.dart';
	import 'package:flutter/services.dart';
	import 'package:flutter/widgets.dart';

	import 'colors.dart';
	import 'theme.dart';

	// 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 dims the wheel above and below the "magnifier"
	// lens.
	const double _kOverAndUnderCenterOpacity = 0.447;

	/// An iOS-styled picker.
	///
	/// Displays its children widgets on a wheel for selection and
	/// calls back when the currently selected item changes.
	///
	/// By default, the first child in `children` will be the initially selected child.
	/// The index of a different child can be specified in [scrollController], to make
	/// that child the initially selected child.
	///
	/// Can be used with [showCupertinoModalPopup] to display the picker modally at the
	/// bottom of the screen. When calling [showCupertinoModalPopup], be sure to set
	/// `semanticsDismissible` to true to enable dismissing the modal via semantics.
	///
	/// 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 null, which disables background painting entirely.
	/// (i.e. the picker is going to have a completely transparent background), to match
	/// the native UIPicker and UIDatePicker. 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,
	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,
	this.selectionOverlay = const CupertinoPickerDefaultSelectionOverlay(),
	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.ListWheelChildBuilderDelegate.childCount}
	///
	/// The [itemExtent] argument must be non-null and positive.
	///
	/// The [backgroundColor] defaults to null, which disables background painting entirely.
	/// (i.e. the picker is going to have a completely transparent background), to match
	/// the native UIPicker and UIDatePicker.
	CupertinoPicker.builder({
	Key? key,
	this.diameterRatio = _kDefaultDiameterRatio,
	this.backgroundColor,
	this.offAxisFraction = 0.0,
	this.useMagnifier = false,
	this.magnification = 1.0,
	this.scrollController,
	this.squeeze = _kSqueeze,
	required this.itemExtent,
	required this.onSelectedItemChanged,
	required NullableIndexedWidgetBuilder itemBuilder,
	int? childCount,
	this.selectionOverlay = const CupertinoPickerDefaultSelectionOverlay(),
	}) : 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 null, which disables background painting entirely.
	/// (i.e. the picker is going to have a completely transparent background), to match
	/// the native UIPicker and UIDatePicker.
	///
	/// 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.RenderListWheelViewport.offAxisFraction}
	final double offAxisFraction;

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

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

	/// A [FixedExtentScrollController] to read and control the current item, and
	/// to set the initial 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.RenderListWheelViewport.squeeze}
	///
	/// Defaults to `1.45` to 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;

	/// A widget overlaid on the picker to highlight the currently selected entry.
	///
	/// The [selectionOverlay] widget drawn above the [CupertinoPicker]'s picker
	/// wheel.
	/// It is vertically centered in the picker and is constrained to have the
	/// same height as the center row.
	///
	/// If unspecified, it defaults to a [CupertinoPickerDefaultSelectionOverlay]
	/// which is a gray rounded rectangle overlay in iOS 14 style.
	/// This property can be set to null to remove the overlay.
	final Widget? selectionOverlay;

	@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.
	final bool hasSuitableHapticHardware;
	switch (defaultTargetPlatform) {
	case TargetPlatform.iOS:
	hasSuitableHapticHardware = true;
	break;
	case TargetPlatform.android:
	case TargetPlatform.fuchsia:
	case TargetPlatform.linux:
	case TargetPlatform.macOS:
	case TargetPlatform.windows:
	hasSuitableHapticHardware = false;
	break;
	}
	assert(hasSuitableHapticHardware != null);
	if (hasSuitableHapticHardware && index != _lastHapticIndex) {
	_lastHapticIndex = index;
	HapticFeedback.selectionClick();
	}

	widget.onSelectedItemChanged?.call(index);
	}

	/// Draws the selectionOverlay.
	Widget _buildSelectionOverlay(Widget selectionOverlay) {
	final double height = widget.itemExtent * widget.magnification;

	return IgnorePointer(
	child: Center(
	child: ConstrainedBox(
	constraints: BoxConstraints.expand(
	height: height,
	),
	child: selectionOverlay,
	),
	),
	);
	}

	@override
	Widget build(BuildContext context) {
	final Color? resolvedBackgroundColor = CupertinoDynamicColor.maybeResolve(widget.backgroundColor, context);

	assert(RenderListWheelViewport.defaultPerspective == _kDefaultPerspective);
	final 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,
	offAxisFraction: widget.offAxisFraction,
	useMagnifier: widget.useMagnifier,
	magnification: widget.magnification,
	overAndUnderCenterOpacity: _kOverAndUnderCenterOpacity,
	itemExtent: widget.itemExtent,
	squeeze: widget.squeeze,
	onSelectedItemChanged: _handleSelectedItemChanged,
	childDelegate: widget.childDelegate,
	),
	),
	),
	if (widget.selectionOverlay != null)
	_buildSelectionOverlay(widget.selectionOverlay!),
	],
	),
	);

	return DecoratedBox(
	decoration: BoxDecoration(color: resolvedBackgroundColor),
	child: result,
	);
	}
	}

	/// A default selection overlay for [CupertinoPicker]s.
	///
	/// It draws a gray rounded rectangle to match the picker visuals introduced in
	/// iOS 14.
	///
	/// This widget is typically only used in [CupertinoPicker.selectionOverlay].
	/// In an iOS 14 multi-column picker, the selection overlay is a single rounded
	/// rectangle that spans the entire multi-column picker.
	/// To achieve the same effect using [CupertinoPickerDefaultSelectionOverlay],
	/// the additional margin and corner radii on the left or the right side can be
	/// disabled by turning off [capLeftEdge] and [capRightEdge], so this selection
	/// overlay visually connects with selection overlays of adjoining
	/// [CupertinoPicker]s (i.e., other "column"s).
	///
	/// See also:
	///
	/// * [CupertinoPicker], which uses this widget as its default [CupertinoPicker.selectionOverlay].
	class CupertinoPickerDefaultSelectionOverlay extends StatelessWidget {

	/// Creates an iOS 14 style selection overlay that highlights the magnified
	/// area (or the currently selected item, depending on how you described it
	/// elsewhere) of a [CupertinoPicker].
	///
	/// The [background] argument default value is [CupertinoColors.tertiarySystemFill].
	/// It must be non-null.
	///
	/// The [capLeftEdge] and [capRightEdge] arguments decide whether to add a
	/// default margin and use rounded corners on the left and right side of the
	/// rectangular overlay.
	/// Default to true and must not be null.
	const CupertinoPickerDefaultSelectionOverlay({
	Key? key,
	this.background = CupertinoColors.tertiarySystemFill,
	this.capLeftEdge = true,
	this.capRightEdge = true,
	}) : assert(background != null),
	assert(capLeftEdge != null),
	assert(capRightEdge != null),
	super(key: key);

	/// Whether to use the default use rounded corners and margin on the left side.
	final bool capLeftEdge;

	/// Whether to use the default use rounded corners and margin on the right side.
	final bool capRightEdge;

	/// The color to fill in the background of the [CupertinoPickerDefaultSelectionOverlay].
	/// It Support for use [CupertinoDynamicColor].
	///
	/// Typically this should not be set to a fully opaque color, as the currently
	/// selected item of the underlying [CupertinoPicker] should remain visible.
	/// Defaults to [CupertinoColors.tertiarySystemFill].
	final Color background;

	/// Default margin of the 'SelectionOverlay'.
	static const double _defaultSelectionOverlayHorizontalMargin = 9;

	/// Default radius of the 'SelectionOverlay'.
	static const double _defaultSelectionOverlayRadius = 8;

	@override
	Widget build(BuildContext context) {
	const Radius radius = Radius.circular(_defaultSelectionOverlayRadius);

	return Container(
	margin: EdgeInsets.only(
	left: capLeftEdge ? _defaultSelectionOverlayHorizontalMargin : 0,
	right: capRightEdge ? _defaultSelectionOverlayHorizontalMargin : 0,
	),
	decoration: BoxDecoration(
	borderRadius: BorderRadius.horizontal(
	left: capLeftEdge ? radius : Radius.zero,
	right: capRightEdge ? radius : Radius.zero,
	),
	color: CupertinoDynamicColor.resolve(background, context),
	),
	);
	}
	}

	// 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) {
	assert(debugCheckHasDirectionality(context));
	return _RenderCupertinoPickerSemantics(scrollController, Directionality.of(context));
	}

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

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

	FixedExtentScrollController get controller => _controller;
	late FixedExtentScrollController _controller;
	set controller(FixedExtentScrollController value) => _updateController(_controller, value);

	// This method exists to allow controller to be non-null. It is only called with a null oldValue from constructor.
	void _updateController(FixedExtentScrollController? oldValue, FixedExtentScrollController value) {
	if (value == oldValue)
	return;
	if (oldValue != null)
	oldValue.removeListener(_handleScrollUpdate);
	else
	_currentIndex = value.initialItem;
	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);
	}
	}