packages/flutter/lib/src/material/expansion_panel.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/widgets.dart';

 import 'constants.dart';
 import 'expand_icon.dart';
 import 'ink_well.dart';
 import 'material_localizations.dart';
 import 'mergeable_material.dart';
 import 'shadows.dart';
 import 'theme.dart';

 const double _kPanelHeaderCollapsedHeight = kMinInteractiveDimension;
 const EdgeInsets _kPanelHeaderExpandedDefaultPadding = EdgeInsets.symmetric(
     vertical: 64.0 - _kPanelHeaderCollapsedHeight,
 );
 const EdgeInsets _kExpandIconPadding = EdgeInsets.all(12.0);

 class _SaltedKey<S, V> extends LocalKey {
   const _SaltedKey(this.salt, this.value);

   final S salt;
   final V value;

   @override
   bool operator ==(Object other) {
     if (other.runtimeType != runtimeType) {
       return false;
     }
     return other is _SaltedKey<S, V>
         && other.salt == salt
         && other.value == value;
   }

   @override
   int get hashCode => Object.hash(runtimeType, salt, value);

   @override
   String toString() {
     final String saltString = S == String ? "<'$salt'>" : '<$salt>';
     final String valueString = V == String ? "<'$value'>" : '<$value>';
     return '[$saltString $valueString]';
   }
 }

 /// Signature for the callback that's called when an [ExpansionPanel] is
 /// expanded or collapsed.
 ///
 /// The position of the panel within an [ExpansionPanelList] is given by
 /// [panelIndex].
 typedef ExpansionPanelCallback = void Function(int panelIndex, bool isExpanded);

 /// Signature for the callback that's called when the header of the
 /// [ExpansionPanel] needs to rebuild.
 typedef ExpansionPanelHeaderBuilder = Widget Function(BuildContext context, bool isExpanded);

 /// A material expansion panel. It has a header and a body and can be either
 /// expanded or collapsed. The body of the panel is only visible when it is
 /// expanded.
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=2aJZzRMziJc}
 ///
 /// Expansion panels are only intended to be used as children for
 /// [ExpansionPanelList].
 ///
 /// See [ExpansionPanelList] for a sample implementation.
 ///
 /// See also:
 ///
 ///  * [ExpansionPanelList]
 ///  * <https://material.io/design/components/lists.html#types>
 class ExpansionPanel {
   /// Creates an expansion panel to be used as a child for [ExpansionPanelList].
   /// See [ExpansionPanelList] for an example on how to use this widget.
   ///
   /// The [headerBuilder], [body], and [isExpanded] arguments must not be null.
   ExpansionPanel({
     required this.headerBuilder,
     required this.body,
     this.isExpanded = false,
     this.canTapOnHeader = false,
     this.backgroundColor,
   });

   /// The widget builder that builds the expansion panels' header.
   final ExpansionPanelHeaderBuilder headerBuilder;

   /// The body of the expansion panel that's displayed below the header.
   ///
   /// This widget is visible only when the panel is expanded.
   final Widget body;

   /// Whether the panel is expanded.
   ///
   /// Defaults to false.
   final bool isExpanded;

   /// Whether tapping on the panel's header will expand/collapse it.
   ///
   /// Defaults to false.
   final bool canTapOnHeader;

   /// Defines the background color of the panel.
   ///
   /// Defaults to [ThemeData.cardColor].
   final Color? backgroundColor;
 }

 /// An expansion panel that allows for radio-like functionality.
 /// This means that at any given time, at most, one [ExpansionPanelRadio]
 /// can remain expanded.
 ///
 /// A unique identifier [value] must be assigned to each panel.
 /// This identifier allows the [ExpansionPanelList] to determine
 /// which [ExpansionPanelRadio] instance should be expanded.
 ///
 /// See [ExpansionPanelList.radio] for a sample implementation.
 class ExpansionPanelRadio extends ExpansionPanel {
   /// An expansion panel that allows for radio functionality.
   ///
   /// A unique [value] must be passed into the constructor. The
   /// [headerBuilder], [body], [value] must not be null.
   ExpansionPanelRadio({
     required this.value,
     required super.headerBuilder,
     required super.body,
     super.canTapOnHeader,
     super.backgroundColor,
   });

   /// The value that uniquely identifies a radio panel so that the currently
   /// selected radio panel can be identified.
   final Object value;
 }

 /// A material expansion panel list that lays out its children and animates
 /// expansions.
 ///
 /// The [expansionCallback] is called when the expansion state changes. For
 /// normal [ExpansionPanelList] widgets, it is the responsibility of the parent
 /// widget to rebuild the [ExpansionPanelList] with updated values for
 /// [ExpansionPanel.isExpanded]. For [ExpansionPanelList.radio] widgets, the
 /// open state is tracked internally and the callback is invoked both for the
 /// previously open panel, which is closing, and the previously closed panel,
 /// which is opening.
 ///
 /// {@tool dartpad}
 /// Here is a simple example of how to use [ExpansionPanelList].
 ///
 /// ** See code in examples/api/lib/material/expansion_panel/expansion_panel_list.0.dart **
 /// {@end-tool}
 ///
 /// See also:
 ///
 ///  * [ExpansionPanel], which is used in the [children] property.
 ///  * [ExpansionPanelList.radio], a variant of this widget where only one panel is open at a time.
 ///  * <https://material.io/design/components/lists.html#types>
 class ExpansionPanelList extends StatefulWidget {
   /// Creates an expansion panel list widget. The [expansionCallback] is
   /// triggered when an expansion panel expand/collapse button is pushed.
   ///
   /// The [children] and [animationDuration] arguments must not be null.
   const ExpansionPanelList({
     super.key,
     this.children = const <ExpansionPanel>[],
     this.expansionCallback,
     this.animationDuration = kThemeAnimationDuration,
     this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
     this.dividerColor,
     this.elevation = 2,
     this.expandIconColor,
     this.materialGapSize = 16.0,
   }) : _allowOnlyOnePanelOpen = false,
        initialOpenPanelValue = null;

   /// Creates a radio expansion panel list widget.
   ///
   /// This widget allows for at most one panel in the list to be open.
   /// The expansion panel callback is triggered when an expansion panel
   /// expand/collapse button is pushed. The [children] and [animationDuration]
   /// arguments must not be null. The [children] objects must be instances
   /// of [ExpansionPanelRadio].
   ///
   /// {@tool dartpad}
   /// Here is a simple example of how to implement ExpansionPanelList.radio.
   ///
   /// ** See code in examples/api/lib/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0.dart **
   /// {@end-tool}
   const ExpansionPanelList.radio({
     super.key,
     this.children = const <ExpansionPanelRadio>[],
     this.expansionCallback,
     this.animationDuration = kThemeAnimationDuration,
     this.initialOpenPanelValue,
     this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
     this.dividerColor,
     this.elevation = 2,
     this.expandIconColor,
     this.materialGapSize = 16.0,
   }) : _allowOnlyOnePanelOpen = true;

   /// The children of the expansion panel list. They are laid out in a similar
   /// fashion to [ListBody].
   final List<ExpansionPanel> children;

   /// The callback that gets called whenever one of the expand/collapse buttons
   /// is pressed. The arguments passed to the callback are the index of the
   /// pressed panel and whether the panel is currently expanded or not.
   ///
   /// If [ExpansionPanelList.radio] is used, the callback may be called a
   /// second time if a different panel was previously open. The arguments
   /// passed to the second callback are the index of the panel that will close
   /// and false, marking that it will be closed.
   ///
   /// For [ExpansionPanelList], the callback should call [State.setState] when
   /// it is notified about the closing/opening panel. On the other hand, the
   /// callback for [ExpansionPanelList.radio] is intended to inform the parent
   /// widget of changes, as the radio panels' open/close states are managed
   /// internally.
   ///
   /// This callback is useful in order to keep track of the expanded/collapsed
   /// panels in a parent widget that may need to react to these changes.
   final ExpansionPanelCallback? expansionCallback;

   /// The duration of the expansion animation.
   final Duration animationDuration;

   // Whether multiple panels can be open simultaneously
   final bool _allowOnlyOnePanelOpen;

   /// The value of the panel that initially begins open. (This value is
   /// only used when initializing with the [ExpansionPanelList.radio]
   /// constructor.)
   final Object? initialOpenPanelValue;

   /// The padding that surrounds the panel header when expanded.
   ///
   /// By default, 16px of space is added to the header vertically (above and below)
   /// during expansion.
   final EdgeInsets expandedHeaderPadding;

   /// Defines color for the divider when [ExpansionPanel.isExpanded] is false.
   ///
   /// If [dividerColor] is null, then [DividerThemeData.color] is used. If that
   /// is null, then [ThemeData.dividerColor] is used.
   final Color? dividerColor;

   /// Defines elevation for the [ExpansionPanel] while it's expanded.
   ///
   /// By default, the value of elevation is 2.
   final double elevation;

   /// {@macro flutter.material.ExpandIcon.color}
   final Color? expandIconColor;

   /// Defines the [MaterialGap.size] of the [MaterialGap] which is placed
   /// between the [ExpansionPanelList.children] when they're expanded.
   ///
   /// Defaults to `16.0`.
   final double materialGapSize;

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

 class _ExpansionPanelListState extends State<ExpansionPanelList> {
   ExpansionPanelRadio? _currentOpenPanel;

   @override
   void initState() {
     super.initState();
     if (widget._allowOnlyOnePanelOpen) {
       assert(_allIdentifiersUnique(), 'All ExpansionPanelRadio identifier values must be unique.');
       if (widget.initialOpenPanelValue != null) {
         _currentOpenPanel =
           searchPanelByValue(widget.children.cast<ExpansionPanelRadio>(), widget.initialOpenPanelValue);
       }
     }
   }

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

     if (widget._allowOnlyOnePanelOpen) {
       assert(_allIdentifiersUnique(), 'All ExpansionPanelRadio identifier values must be unique.');
       // If the previous widget was non-radio ExpansionPanelList, initialize the
       // open panel to widget.initialOpenPanelValue
       if (!oldWidget._allowOnlyOnePanelOpen) {
         _currentOpenPanel =
           searchPanelByValue(widget.children.cast<ExpansionPanelRadio>(), widget.initialOpenPanelValue);
       }
     } else {
       _currentOpenPanel = null;
     }
   }

   bool _allIdentifiersUnique() {
     final Map<Object, bool> identifierMap = <Object, bool>{};
     for (final ExpansionPanelRadio child in widget.children.cast<ExpansionPanelRadio>()) {
       identifierMap[child.value] = true;
     }
     return identifierMap.length == widget.children.length;
   }

   bool _isChildExpanded(int index) {
     if (widget._allowOnlyOnePanelOpen) {
       final ExpansionPanelRadio radioWidget = widget.children[index] as ExpansionPanelRadio;
       return _currentOpenPanel?.value == radioWidget.value;
     }
     return widget.children[index].isExpanded;
   }

  void _handlePressed(bool isExpanded, int index) {
     if (widget._allowOnlyOnePanelOpen) {
       final ExpansionPanelRadio pressedChild = widget.children[index] as ExpansionPanelRadio;

       // If another ExpansionPanelRadio was already open, apply its
       // expansionCallback (if any) to false, because it's closing.
       for (int childIndex = 0; childIndex < widget.children.length; childIndex += 1) {
         final ExpansionPanelRadio child = widget.children[childIndex] as ExpansionPanelRadio;
         if (widget.expansionCallback != null &&
             childIndex != index &&
             child.value == _currentOpenPanel?.value) {
           widget.expansionCallback!(childIndex, false);
         }
       }

       setState(() {
         _currentOpenPanel = isExpanded ? null : pressedChild;
       });
     }
     // !isExpanded is passed because, when _handlePressed, the state of the panel to expand is not yet expanded.
     widget.expansionCallback?.call(index, !isExpanded);
   }

   ExpansionPanelRadio? searchPanelByValue(List<ExpansionPanelRadio> panels, Object? value)  {
     for (final ExpansionPanelRadio panel in panels) {
       if (panel.value == value) {
         return panel;
       }
     }
     return null;
   }

   @override
   Widget build(BuildContext context) {
     assert(kElevationToShadow.containsKey(widget.elevation),
       'Invalid value for elevation. See the kElevationToShadow constant for'
       ' possible elevation values.',
     );

     final List<MergeableMaterialItem> items = <MergeableMaterialItem>[];

     for (int index = 0; index < widget.children.length; index += 1) {
       if (_isChildExpanded(index) && index != 0 && !_isChildExpanded(index - 1)) {
         items.add(MaterialGap(key: _SaltedKey<BuildContext, int>(context, index * 2 - 1), size: widget.materialGapSize));
       }

       final ExpansionPanel child = widget.children[index];
       final Widget headerWidget = child.headerBuilder(
         context,
         _isChildExpanded(index),
       );

       Widget expandIconContainer = Container(
         margin: const EdgeInsetsDirectional.only(end: 8.0),
         child: ExpandIcon(
           color: widget.expandIconColor,
           isExpanded: _isChildExpanded(index),
           padding: _kExpandIconPadding,
           onPressed: !child.canTapOnHeader
               ? (bool isExpanded) => _handlePressed(isExpanded, index)
               : null,
         ),
       );
       if (!child.canTapOnHeader) {
         final MaterialLocalizations localizations = MaterialLocalizations.of(context);
         expandIconContainer = Semantics(
           label: _isChildExpanded(index)? localizations.expandedIconTapHint : localizations.collapsedIconTapHint,
           container: true,
           child: expandIconContainer,
         );
       }
       Widget header = Row(
         children: <Widget>[
           Expanded(
             child: AnimatedContainer(
               duration: widget.animationDuration,
               curve: Curves.fastOutSlowIn,
               margin: _isChildExpanded(index) ? widget.expandedHeaderPadding : EdgeInsets.zero,
               child: ConstrainedBox(
                 constraints: const BoxConstraints(minHeight: _kPanelHeaderCollapsedHeight),
                 child: headerWidget,
               ),
             ),
           ),
           expandIconContainer,
         ],
       );
       if (child.canTapOnHeader) {
         header = MergeSemantics(
           child: InkWell(
             onTap: () => _handlePressed(_isChildExpanded(index), index),
             child: header,
           ),
         );
       }
       items.add(
         MaterialSlice(
           key: _SaltedKey<BuildContext, int>(context, index * 2),
           color: child.backgroundColor,
           child: Column(
             children: <Widget>[
               header,
               AnimatedCrossFade(
                 firstChild: Container(height: 0.0),
                 secondChild: child.body,
                 firstCurve: const Interval(0.0, 0.6, curve: Curves.fastOutSlowIn),
                 secondCurve: const Interval(0.4, 1.0, curve: Curves.fastOutSlowIn),
                 sizeCurve: Curves.fastOutSlowIn,
                 crossFadeState: _isChildExpanded(index) ? CrossFadeState.showSecond : CrossFadeState.showFirst,
                 duration: widget.animationDuration,
               ),
             ],
           ),
         ),
       );

       if (_isChildExpanded(index) && index != widget.children.length - 1) {
         items.add(MaterialGap(key: _SaltedKey<BuildContext, int>(context, index * 2 + 1), size: widget.materialGapSize));
       }
     }

     return MergeableMaterial(
       hasDividers: true,
       dividerColor: widget.dividerColor,
       elevation: widget.elevation,
       children: items,
     );
   }
 }
	// 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/widgets.dart';

	import 'constants.dart';
	import 'expand_icon.dart';
	import 'ink_well.dart';
	import 'material_localizations.dart';
	import 'mergeable_material.dart';
	import 'shadows.dart';
	import 'theme.dart';

	const double _kPanelHeaderCollapsedHeight = kMinInteractiveDimension;
	const EdgeInsets _kPanelHeaderExpandedDefaultPadding = EdgeInsets.symmetric(
	vertical: 64.0 - _kPanelHeaderCollapsedHeight,
	);
	const EdgeInsets _kExpandIconPadding = EdgeInsets.all(12.0);

	class _SaltedKey<S, V> extends LocalKey {
	const _SaltedKey(this.salt, this.value);

	final S salt;
	final V value;

	@override
	bool operator ==(Object other) {
	if (other.runtimeType != runtimeType) {
	return false;
	}
	return other is _SaltedKey<S, V>
	&& other.salt == salt
	&& other.value == value;
	}

	@override
	int get hashCode => Object.hash(runtimeType, salt, value);

	@override
	String toString() {
	final String saltString = S == String ? "<'$salt'>" : '<$salt>';
	final String valueString = V == String ? "<'$value'>" : '<$value>';
	return '[$saltString $valueString]';
	}
	}

	/// Signature for the callback that's called when an [ExpansionPanel] is
	/// expanded or collapsed.
	///
	/// The position of the panel within an [ExpansionPanelList] is given by
	/// [panelIndex].
	typedef ExpansionPanelCallback = void Function(int panelIndex, bool isExpanded);

	/// Signature for the callback that's called when the header of the
	/// [ExpansionPanel] needs to rebuild.
	typedef ExpansionPanelHeaderBuilder = Widget Function(BuildContext context, bool isExpanded);

	/// A material expansion panel. It has a header and a body and can be either
	/// expanded or collapsed. The body of the panel is only visible when it is
	/// expanded.
	///
	/// {@youtube 560 315 https://www.youtube.com/watch?v=2aJZzRMziJc}
	///
	/// Expansion panels are only intended to be used as children for
	/// [ExpansionPanelList].
	///
	/// See [ExpansionPanelList] for a sample implementation.
	///
	/// See also:
	///
	/// * [ExpansionPanelList]
	/// * <https://material.io/design/components/lists.html#types>
	class ExpansionPanel {
	/// Creates an expansion panel to be used as a child for [ExpansionPanelList].
	/// See [ExpansionPanelList] for an example on how to use this widget.
	///
	/// The [headerBuilder], [body], and [isExpanded] arguments must not be null.
	ExpansionPanel({
	required this.headerBuilder,
	required this.body,
	this.isExpanded = false,
	this.canTapOnHeader = false,
	this.backgroundColor,
	});

	/// The widget builder that builds the expansion panels' header.
	final ExpansionPanelHeaderBuilder headerBuilder;

	/// The body of the expansion panel that's displayed below the header.
	///
	/// This widget is visible only when the panel is expanded.
	final Widget body;

	/// Whether the panel is expanded.
	///
	/// Defaults to false.
	final bool isExpanded;

	/// Whether tapping on the panel's header will expand/collapse it.
	///
	/// Defaults to false.
	final bool canTapOnHeader;

	/// Defines the background color of the panel.
	///
	/// Defaults to [ThemeData.cardColor].
	final Color? backgroundColor;
	}

	/// An expansion panel that allows for radio-like functionality.
	/// This means that at any given time, at most, one [ExpansionPanelRadio]
	/// can remain expanded.
	///
	/// A unique identifier [value] must be assigned to each panel.
	/// This identifier allows the [ExpansionPanelList] to determine
	/// which [ExpansionPanelRadio] instance should be expanded.
	///
	/// See [ExpansionPanelList.radio] for a sample implementation.
	class ExpansionPanelRadio extends ExpansionPanel {
	/// An expansion panel that allows for radio functionality.
	///
	/// A unique [value] must be passed into the constructor. The
	/// [headerBuilder], [body], [value] must not be null.
	ExpansionPanelRadio({
	required this.value,
	required super.headerBuilder,
	required super.body,
	super.canTapOnHeader,
	super.backgroundColor,
	});

	/// The value that uniquely identifies a radio panel so that the currently
	/// selected radio panel can be identified.
	final Object value;
	}

	/// A material expansion panel list that lays out its children and animates
	/// expansions.
	///
	/// The [expansionCallback] is called when the expansion state changes. For
	/// normal [ExpansionPanelList] widgets, it is the responsibility of the parent
	/// widget to rebuild the [ExpansionPanelList] with updated values for
	/// [ExpansionPanel.isExpanded]. For [ExpansionPanelList.radio] widgets, the
	/// open state is tracked internally and the callback is invoked both for the
	/// previously open panel, which is closing, and the previously closed panel,
	/// which is opening.
	///
	/// {@tool dartpad}
	/// Here is a simple example of how to use [ExpansionPanelList].
	///
	/// See code in examples/api/lib/material/expansion_panel/expansion_panel_list.0.dart
	/// {@end-tool}
	///
	/// See also:
	///
	/// * [ExpansionPanel], which is used in the [children] property.
	/// * [ExpansionPanelList.radio], a variant of this widget where only one panel is open at a time.
	/// * <https://material.io/design/components/lists.html#types>
	class ExpansionPanelList extends StatefulWidget {
	/// Creates an expansion panel list widget. The [expansionCallback] is
	/// triggered when an expansion panel expand/collapse button is pushed.
	///
	/// The [children] and [animationDuration] arguments must not be null.
	const ExpansionPanelList({
	super.key,
	this.children = const <ExpansionPanel>[],
	this.expansionCallback,
	this.animationDuration = kThemeAnimationDuration,
	this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
	this.dividerColor,
	this.elevation = 2,
	this.expandIconColor,
	this.materialGapSize = 16.0,
	}) : _allowOnlyOnePanelOpen = false,
	initialOpenPanelValue = null;

	/// Creates a radio expansion panel list widget.
	///
	/// This widget allows for at most one panel in the list to be open.
	/// The expansion panel callback is triggered when an expansion panel
	/// expand/collapse button is pushed. The [children] and [animationDuration]
	/// arguments must not be null. The [children] objects must be instances
	/// of [ExpansionPanelRadio].
	///
	/// {@tool dartpad}
	/// Here is a simple example of how to implement ExpansionPanelList.radio.
	///
	/// See code in examples/api/lib/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0.dart
	/// {@end-tool}
	const ExpansionPanelList.radio({
	super.key,
	this.children = const <ExpansionPanelRadio>[],
	this.expansionCallback,
	this.animationDuration = kThemeAnimationDuration,
	this.initialOpenPanelValue,
	this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
	this.dividerColor,
	this.elevation = 2,
	this.expandIconColor,
	this.materialGapSize = 16.0,
	}) : _allowOnlyOnePanelOpen = true;

	/// The children of the expansion panel list. They are laid out in a similar
	/// fashion to [ListBody].
	final List<ExpansionPanel> children;

	/// The callback that gets called whenever one of the expand/collapse buttons
	/// is pressed. The arguments passed to the callback are the index of the
	/// pressed panel and whether the panel is currently expanded or not.
	///
	/// If [ExpansionPanelList.radio] is used, the callback may be called a
	/// second time if a different panel was previously open. The arguments
	/// passed to the second callback are the index of the panel that will close
	/// and false, marking that it will be closed.
	///
	/// For [ExpansionPanelList], the callback should call [State.setState] when
	/// it is notified about the closing/opening panel. On the other hand, the
	/// callback for [ExpansionPanelList.radio] is intended to inform the parent
	/// widget of changes, as the radio panels' open/close states are managed
	/// internally.
	///
	/// This callback is useful in order to keep track of the expanded/collapsed
	/// panels in a parent widget that may need to react to these changes.
	final ExpansionPanelCallback? expansionCallback;

	/// The duration of the expansion animation.
	final Duration animationDuration;

	// Whether multiple panels can be open simultaneously
	final bool _allowOnlyOnePanelOpen;

	/// The value of the panel that initially begins open. (This value is
	/// only used when initializing with the [ExpansionPanelList.radio]
	/// constructor.)
	final Object? initialOpenPanelValue;

	/// The padding that surrounds the panel header when expanded.
	///
	/// By default, 16px of space is added to the header vertically (above and below)
	/// during expansion.
	final EdgeInsets expandedHeaderPadding;

	/// Defines color for the divider when [ExpansionPanel.isExpanded] is false.
	///
	/// If [dividerColor] is null, then [DividerThemeData.color] is used. If that
	/// is null, then [ThemeData.dividerColor] is used.
	final Color? dividerColor;

	/// Defines elevation for the [ExpansionPanel] while it's expanded.
	///
	/// By default, the value of elevation is 2.
	final double elevation;

	/// {@macro flutter.material.ExpandIcon.color}
	final Color? expandIconColor;

	/// Defines the [MaterialGap.size] of the [MaterialGap] which is placed
	/// between the [ExpansionPanelList.children] when they're expanded.
	///
	/// Defaults to `16.0`.
	final double materialGapSize;

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

	class _ExpansionPanelListState extends State<ExpansionPanelList> {
	ExpansionPanelRadio? _currentOpenPanel;

	@override
	void initState() {
	super.initState();
	if (widget._allowOnlyOnePanelOpen) {
	assert(_allIdentifiersUnique(), 'All ExpansionPanelRadio identifier values must be unique.');
	if (widget.initialOpenPanelValue != null) {
	_currentOpenPanel =
	searchPanelByValue(widget.children.cast<ExpansionPanelRadio>(), widget.initialOpenPanelValue);
	}
	}
	}

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

	if (widget._allowOnlyOnePanelOpen) {
	assert(_allIdentifiersUnique(), 'All ExpansionPanelRadio identifier values must be unique.');
	// If the previous widget was non-radio ExpansionPanelList, initialize the
	// open panel to widget.initialOpenPanelValue
	if (!oldWidget._allowOnlyOnePanelOpen) {
	_currentOpenPanel =
	searchPanelByValue(widget.children.cast<ExpansionPanelRadio>(), widget.initialOpenPanelValue);
	}
	} else {
	_currentOpenPanel = null;
	}
	}

	bool _allIdentifiersUnique() {
	final Map<Object, bool> identifierMap = <Object, bool>{};
	for (final ExpansionPanelRadio child in widget.children.cast<ExpansionPanelRadio>()) {
	identifierMap[child.value] = true;
	}
	return identifierMap.length == widget.children.length;
	}

	bool _isChildExpanded(int index) {
	if (widget._allowOnlyOnePanelOpen) {
	final ExpansionPanelRadio radioWidget = widget.children[index] as ExpansionPanelRadio;
	return _currentOpenPanel?.value == radioWidget.value;
	}
	return widget.children[index].isExpanded;
	}

	void _handlePressed(bool isExpanded, int index) {
	if (widget._allowOnlyOnePanelOpen) {
	final ExpansionPanelRadio pressedChild = widget.children[index] as ExpansionPanelRadio;

	// If another ExpansionPanelRadio was already open, apply its
	// expansionCallback (if any) to false, because it's closing.
	for (int childIndex = 0; childIndex < widget.children.length; childIndex += 1) {
	final ExpansionPanelRadio child = widget.children[childIndex] as ExpansionPanelRadio;
	if (widget.expansionCallback != null &&
	childIndex != index &&
	child.value == _currentOpenPanel?.value) {
	widget.expansionCallback!(childIndex, false);
	}
	}

	setState(() {
	_currentOpenPanel = isExpanded ? null : pressedChild;
	});
	}
	// !isExpanded is passed because, when _handlePressed, the state of the panel to expand is not yet expanded.
	widget.expansionCallback?.call(index, !isExpanded);
	}

	ExpansionPanelRadio? searchPanelByValue(List<ExpansionPanelRadio> panels, Object? value) {
	for (final ExpansionPanelRadio panel in panels) {
	if (panel.value == value) {
	return panel;
	}
	}
	return null;
	}

	@override
	Widget build(BuildContext context) {
	assert(kElevationToShadow.containsKey(widget.elevation),
	'Invalid value for elevation. See the kElevationToShadow constant for'
	' possible elevation values.',
	);

	final List<MergeableMaterialItem> items = <MergeableMaterialItem>[];

	for (int index = 0; index < widget.children.length; index += 1) {
	if (_isChildExpanded(index) && index != 0 && !_isChildExpanded(index - 1)) {
	items.add(MaterialGap(key: _SaltedKey<BuildContext, int>(context, index * 2 - 1), size: widget.materialGapSize));
	}

	final ExpansionPanel child = widget.children[index];
	final Widget headerWidget = child.headerBuilder(
	context,
	_isChildExpanded(index),
	);

	Widget expandIconContainer = Container(
	margin: const EdgeInsetsDirectional.only(end: 8.0),
	child: ExpandIcon(
	color: widget.expandIconColor,
	isExpanded: _isChildExpanded(index),
	padding: _kExpandIconPadding,
	onPressed: !child.canTapOnHeader
	? (bool isExpanded) => _handlePressed(isExpanded, index)
	: null,
	),
	);
	if (!child.canTapOnHeader) {
	final MaterialLocalizations localizations = MaterialLocalizations.of(context);
	expandIconContainer = Semantics(
	label: _isChildExpanded(index)? localizations.expandedIconTapHint : localizations.collapsedIconTapHint,
	container: true,
	child: expandIconContainer,
	);
	}
	Widget header = Row(
	children: <Widget>[
	Expanded(
	child: AnimatedContainer(
	duration: widget.animationDuration,
	curve: Curves.fastOutSlowIn,
	margin: _isChildExpanded(index) ? widget.expandedHeaderPadding : EdgeInsets.zero,
	child: ConstrainedBox(
	constraints: const BoxConstraints(minHeight: _kPanelHeaderCollapsedHeight),
	child: headerWidget,
	),
	),
	),
	expandIconContainer,
	],
	);
	if (child.canTapOnHeader) {
	header = MergeSemantics(
	child: InkWell(
	onTap: () => _handlePressed(_isChildExpanded(index), index),
	child: header,
	),
	);
	}
	items.add(
	MaterialSlice(
	key: _SaltedKey<BuildContext, int>(context, index * 2),
	color: child.backgroundColor,
	child: Column(
	children: <Widget>[
	header,
	AnimatedCrossFade(
	firstChild: Container(height: 0.0),
	secondChild: child.body,
	firstCurve: const Interval(0.0, 0.6, curve: Curves.fastOutSlowIn),
	secondCurve: const Interval(0.4, 1.0, curve: Curves.fastOutSlowIn),
	sizeCurve: Curves.fastOutSlowIn,
	crossFadeState: _isChildExpanded(index) ? CrossFadeState.showSecond : CrossFadeState.showFirst,
	duration: widget.animationDuration,
	),
	],
	),
	),
	);

	if (_isChildExpanded(index) && index != widget.children.length - 1) {
	items.add(MaterialGap(key: _SaltedKey<BuildContext, int>(context, index * 2 + 1), size: widget.materialGapSize));
	}
	}

	return MergeableMaterial(
	hasDividers: true,
	dividerColor: widget.dividerColor,
	elevation: widget.elevation,
	children: items,
	);
	}
	}