packages/flutter/lib/src/widgets/expansible.dart - mirrors/flutter.git - 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.

 /// @docImport 'package:flutter/material.dart';
 library;

 import 'basic.dart';
 import 'framework.dart';
 import 'page_storage.dart';
 import 'ticker_provider.dart';
 import 'transitions.dart';

 /// The type of the callback that returns the header or body of an [Expansible].
 ///
 /// The `animation` property exposes the underlying expanding or collapsing
 /// animation, which has a value of 0 when the [Expansible] is completely
 /// collapsed and 1 when it is completely expanded. This can be used to drive
 /// animations that sync up with the expanding or collapsing animation, such as
 /// rotating an icon.
 ///
 /// See also:
 ///
 ///   * [Expansible.headerBuilder], which is of this type.
 ///   * [Expansible.bodyBuilder], which is also of this type.
 typedef ExpansibleComponentBuilder =
     Widget Function(BuildContext context, Animation<double> animation);

 /// The type of the callback that uses the header and body of an [Expansible]
 /// widget to build the widget.
 ///
 /// The `header` property is the header returned by [Expansible.headerBuilder].
 /// The `body` property is the body returned by [Expansible.bodyBuilder] wrapped
 /// in an [Offstage] to hide the body when the [Expansible] is collapsed.
 ///
 /// The `animation` property exposes the underlying expanding or collapsing
 /// animation, which has a value of 0 when the [Expansible] is completely
 /// collapsed and 1 when it is completely expanded. This can be used to drive
 /// animations that sync up with the expanding or collapsing animation, such as
 /// rotating an icon.
 ///
 /// See also:
 ///
 ///   * [Expansible.expansibleBuilder], which is of this type.
 typedef ExpansibleBuilder =
     Widget Function(BuildContext context, Widget header, Widget body, Animation<double> animation);

 /// A controller for managing the expansion state of an [Expansible].
 ///
 /// This class is a [ChangeNotifier] that notifies its listeners if the value of
 /// [isExpanded] changes.
 ///
 /// This controller provides methods to programmatically expand or collapse the
 /// widget, and it allows external components to query the current expansion
 /// state.
 ///
 /// The controller's [expand] and [collapse] methods cause the
 /// [Expansible] to rebuild, so they may not be called from
 /// a build method.
 ///
 /// Remember to [dispose] of the [ExpansibleController] when it is no longer
 /// needed. This will ensure all resources used by the object are discarded.
 class ExpansibleController extends ChangeNotifier {
   /// Creates a controller to be used with [Expansible.controller].
   ExpansibleController();

   bool _isExpanded = false;

   void _setExpansionState(bool newValue) {
     if (newValue != _isExpanded) {
       _isExpanded = newValue;
       notifyListeners();
     }
   }

   /// Whether the expansible widget built with this controller is in expanded
   /// state.
   ///
   /// This property doesn't take the animation into account. It reports `true`
   /// even if the expansion animation is not completed.
   ///
   /// To be notified when this property changes, add a listener to the
   /// controller using [ExpansibleController.addListener].
   ///
   /// See also:
   ///
   ///  * [expand], which expands the expansible widget.
   ///  * [collapse], which collapses the expansible widget.
   bool get isExpanded => _isExpanded;

   /// Expands the [Expansible] that was built with this controller.
   ///
   /// If the widget is already in the expanded state (see [isExpanded]), calling
   /// this method has no effect.
   ///
   /// Calling this method may cause the [Expansible] to rebuild, so it may
   /// not be called from a build method.
   ///
   /// Calling this method will notify registered listeners of this controller
   /// that the expansion state has changed.
   ///
   /// See also:
   ///
   ///  * [collapse], which collapses the expansible widget.
   ///  * [isExpanded] to check whether the expansible widget is expanded.
   void expand() {
     _setExpansionState(true);
   }

   /// Collapses the [Expansible] that was built with this controller.
   ///
   /// If the widget is already in the collapsed state (see [isExpanded]),
   /// calling this method has no effect.
   ///
   /// Calling this method may cause the [Expansible] to rebuild, so it may not
   /// be called from a build method.
   ///
   /// Calling this method will notify registered listeners of this controller
   /// that the expansion state has changed.
   ///
   /// See also:
   ///
   ///  * [expand], which expands the [Expansible].
   ///  * [isExpanded] to check whether the [Expansible] is expanded.
   void collapse() {
     _setExpansionState(false);
   }

   /// Finds the [ExpansibleController] for the closest [Expansible] instance
   /// that encloses the given context.
   ///
   /// If no [Expansible] encloses the given context, calling this
   /// method will cause an assert in debug mode, and throw an
   /// exception in release mode.
   ///
   /// To return null if there is no [Expansible] use [maybeOf] instead.
   ///
   /// Typical usage of the [ExpansibleController.of] function is to call it from
   /// within the `build` method of a descendant of an [Expansible].
   static ExpansibleController of(BuildContext context) {
     final _ExpansibleState? result = context.findAncestorStateOfType<_ExpansibleState>();
     assert(() {
       if (result == null) {
         throw FlutterError.fromParts(<DiagnosticsNode>[
           ErrorSummary(
             'ExpansibleController.of() called with a context that does not contain a Expansible.',
           ),
           ErrorDescription(
             'No Expansible ancestor could be found starting from the context that was passed to ExpansibleController.of(). '
             'This usually happens when the context provided is from the same StatefulWidget as that '
             'whose build function actually creates the Expansible widget being sought.',
           ),
           ErrorHint(
             'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
             'context that is "under" the Expansible. ',
           ),
           ErrorHint(
             'A more efficient solution is to split your build function into several widgets. This '
             'introduces a new context from which you can obtain the Expansible. In this solution, '
             'you would have an outer widget that creates the Expansible populated by instances of '
             'your new inner widgets, and then in these inner widgets you would use ExpansibleController.of().\n'
             'An other solution is assign a GlobalKey to the Expansible, '
             'then use the key.currentState property to obtain the Expansible rather than '
             'using the ExpansibleController.of() function.',
           ),
           context.describeElement('The context used was'),
         ]);
       }
       return true;
     }());
     return result!.widget.controller;
   }

   /// Finds the [Expansible] from the closest instance of this class that
   /// encloses the given context and returns its [ExpansibleController].
   ///
   /// If no [Expansible] encloses the given context then return null.
   /// To throw an exception instead, use [of] instead of this function.
   ///
   /// See also:
   ///
   ///  * [of], a similar function to this one that throws if no [Expansible]
   ///    encloses the given context.
   static ExpansibleController? maybeOf(BuildContext context) {
     return context.findAncestorStateOfType<_ExpansibleState>()?.widget.controller;
   }
 }

 /// A [StatefulWidget] that expands and collapses.
 ///
 /// An [Expansible] consists of a header, which is always shown, and a
 /// body, which is hidden in its collapsed state and shown in its expanded
 /// state.
 ///
 /// The [Expansible] is expanded or collapsed with an animation driven by an
 /// [AnimationController]. When the widget is expanded, the height of its body
 /// animates from 0 to its fully expanded height.
 ///
 /// This widget is typically used with [ListView] to create an "expand /
 /// collapse" list entry. When used with scrolling widgets like [ListView], a
 /// unique [PageStorageKey] must be specified as the [key], to enable the
 /// [Expansible] to save and restore its expanded state when it is scrolled
 /// in and out of view.
 ///
 /// Provide [headerBuilder] and [bodyBuilder] callbacks to
 /// build the header and body widgets. An additional [expansibleBuilder]
 /// callback can be provided to further customize the layout of the widget.
 ///
 /// The [Expansible] does not inherently toggle the expansion state. To toggle
 /// the expansion state, call [ExpansibleController.expand] and
 /// [ExpansibleController.collapse] as needed, most typically when the header
 /// returned in [headerBuilder] is tapped.
 ///
 /// See also:
 ///
 ///  * [ExpansionTile], a Material-styled widget that expands and collapses.
 class Expansible extends StatefulWidget {
   /// Creates an instance of [Expansible].
   const Expansible({
     super.key,
     required this.headerBuilder,
     required this.bodyBuilder,
     required this.controller,
     this.expansibleBuilder = _defaultExpansibleBuilder,
     this.animationStyle,
     @Deprecated(
       'Use animationStyle instead. '
       'This feature was deprecated after v3.38.0-0.2.pre.',
     )
     this.duration = const Duration(milliseconds: 200),
     @Deprecated(
       'Use animationStyle instead. '
       'This feature was deprecated after v3.38.0-0.2.pre.',
     )
     this.curve = Curves.ease,
     @Deprecated(
       'Use animationStyle instead. '
       'This feature was deprecated after v3.38.0-0.2.pre.',
     )
     this.reverseCurve,
     this.maintainState = true,
   });

   /// Expands and collapses the widget.
   ///
   /// The controller manages the expansion state and toggles the expansion.
   final ExpansibleController controller;

   /// Builds the always-displayed header.
   ///
   /// Many use cases involve toggling the expansion state when this header is
   /// tapped. To toggle the expansion state, call [ExpansibleController.expand]
   /// or [ExpansibleController.collapse].
   final ExpansibleComponentBuilder headerBuilder;

   /// Builds the collapsible body.
   ///
   /// When this widget is expanded, the height of its body animates from 0 to
   /// its fully extended height.
   final ExpansibleComponentBuilder bodyBuilder;

   /// Used to override the expansion animation curve and duration.
   ///
   /// If [AnimationStyle.duration] is provided, it will be used instead of
   /// [duration]. If not provided, [duration] is used, which defaults to
   /// 200ms.
   ///
   /// If [AnimationStyle.curve] is provided, it will be used to override
   /// [curve]. If it is null, then [curve] will be used. Otherwise, defaults
   /// to [Curves.ease].
   ///
   /// If [AnimationStyle.reverseCurve] is provided, it will be used to
   /// override [reverseCurve]. If it is null, then [reverseCurve] will be
   /// used.
   ///
   /// To disable the theme animation, use [AnimationStyle.noAnimation].
   final AnimationStyle? animationStyle;

   /// The duration of the expansion animation.
   ///
   /// Defaults to a duration of 200ms.
   ///
   /// This property is deprecated, use [animationStyle] instead.
   @Deprecated(
     'Use animationStyle instead. '
     'This feature was deprecated after v3.38.0-0.2.pre.',
   )
   final Duration duration;

   /// The curve of the expansion animation.
   ///
   /// Defaults to [Curves.ease].
   ///
   /// This property is deprecated, use [animationStyle] instead.
   @Deprecated(
     'Use animationStyle instead. '
     'This feature was deprecated after v3.38.0-0.2.pre.',
   )
   final Curve curve;

   /// The reverse curve of the expansion animation.
   ///
   /// If null, uses [curve] in both directions.
   ///
   /// This property is deprecated, use [animationStyle] instead.
   @Deprecated(
     'Use animationStyle instead. '
     'This feature was deprecated after v3.38.0-0.2.pre.',
   )
   final Curve? reverseCurve;

   /// Whether the state of the body is maintained when the widget expands or
   /// collapses.
   ///
   /// If true, the body is kept in the tree while the widget is
   /// collapsed. Otherwise, the body is removed from the tree when the
   /// widget is collapsed and recreated upon expansion.
   ///
   /// Defaults to true.
   final bool maintainState;

   /// Builds the widget with the results of [headerBuilder] and [bodyBuilder].
   ///
   /// Defaults to placing the header and body in a [Column].
   final ExpansibleBuilder expansibleBuilder;

   static Widget _defaultExpansibleBuilder(
     BuildContext context,
     Widget header,
     Widget body,
     Animation<double> animation,
   ) {
     return Column(mainAxisSize: MainAxisSize.min, children: <Widget>[header, body]);
   }

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

 class _ExpansibleState extends State<Expansible> with SingleTickerProviderStateMixin {
   late AnimationController _animationController;
   late CurvedAnimation _heightFactor;

   Duration get _duration {
     return widget.animationStyle?.duration ?? widget.duration;
   }

   Curve get _curve {
     return widget.animationStyle?.curve ?? widget.curve;
   }

   Curve? get _reverseCurve {
     return widget.animationStyle?.reverseCurve ?? widget.reverseCurve;
   }

   @override
   void initState() {
     super.initState();
     _animationController = AnimationController(duration: _duration, vsync: this);
     final bool initiallyExpanded =
         PageStorage.maybeOf(context)?.readState(context) as bool? ?? widget.controller.isExpanded;
     if (initiallyExpanded) {
       _animationController.value = 1.0;
       widget.controller.expand();
     } else {
       widget.controller.collapse();
     }
     final heightFactorTween = Tween<double>(begin: 0.0, end: 1.0);
     _heightFactor = CurvedAnimation(
       parent: _animationController.drive(heightFactorTween),
       curve: _curve,
       reverseCurve: _reverseCurve,
     );
     widget.controller.addListener(_toggleExpansion);
   }

   @override
   void didUpdateWidget(covariant Expansible oldWidget) {
     super.didUpdateWidget(oldWidget);
     final Duration oldDuration = oldWidget.animationStyle?.duration ?? oldWidget.duration;
     final Curve oldCurve = oldWidget.animationStyle?.curve ?? oldWidget.curve;
     final Curve? oldReverseCurve = oldWidget.animationStyle?.reverseCurve ?? oldWidget.reverseCurve;

     if (_curve != oldCurve) {
       _heightFactor.curve = _curve;
     }
     if (_reverseCurve != oldReverseCurve) {
       _heightFactor.reverseCurve = _reverseCurve;
     }
     if (_duration != oldDuration) {
       _animationController.duration = _duration;
     }
     if (widget.controller != oldWidget.controller) {
       oldWidget.controller.removeListener(_toggleExpansion);
       widget.controller.addListener(_toggleExpansion);
       if (oldWidget.controller.isExpanded != widget.controller.isExpanded) {
         _toggleExpansion();
       }
     }
   }

   @override
   void dispose() {
     widget.controller.removeListener(_toggleExpansion);
     _animationController.dispose();
     _heightFactor.dispose();
     super.dispose();
   }

   void _toggleExpansion() {
     setState(() {
       // Rebuild with the header and the animating body.
       if (widget.controller.isExpanded) {
         _animationController.forward();
       } else {
         _animationController.reverse().then<void>((void value) {
           if (!mounted) {
             return;
           }
           setState(() {
             // Rebuild without the body.
           });
         });
       }
       PageStorage.maybeOf(context)?.writeState(context, widget.controller.isExpanded);
     });
   }

   @override
   Widget build(BuildContext context) {
     assert(!_animationController.isDismissed || !widget.controller.isExpanded);
     final bool closed = !widget.controller.isExpanded && _animationController.isDismissed;
     final bool shouldRemoveBody = closed && !widget.maintainState;

     final Widget result = Offstage(
       offstage: closed,
       child: TickerMode(enabled: !closed, child: widget.bodyBuilder(context, _animationController)),
     );

     return AnimatedBuilder(
       animation: _animationController.view,
       builder: (BuildContext context, Widget? child) {
         final Widget header = widget.headerBuilder(context, _animationController);
         final Widget body = ClipRect(
           child: Align(heightFactor: _heightFactor.value, child: child),
         );
         return widget.expansibleBuilder(context, header, body, _animationController);
       },
       child: shouldRemoveBody ? null : result,
     );
   }
 }
	// 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.

	/// @docImport 'package:flutter/material.dart';
	library;

	import 'basic.dart';
	import 'framework.dart';
	import 'page_storage.dart';
	import 'ticker_provider.dart';
	import 'transitions.dart';

	/// The type of the callback that returns the header or body of an [Expansible].
	///
	/// The `animation` property exposes the underlying expanding or collapsing
	/// animation, which has a value of 0 when the [Expansible] is completely
	/// collapsed and 1 when it is completely expanded. This can be used to drive
	/// animations that sync up with the expanding or collapsing animation, such as
	/// rotating an icon.
	///
	/// See also:
	///
	/// * [Expansible.headerBuilder], which is of this type.
	/// * [Expansible.bodyBuilder], which is also of this type.
	typedef ExpansibleComponentBuilder =
	Widget Function(BuildContext context, Animation<double> animation);

	/// The type of the callback that uses the header and body of an [Expansible]
	/// widget to build the widget.
	///
	/// The `header` property is the header returned by [Expansible.headerBuilder].
	/// The `body` property is the body returned by [Expansible.bodyBuilder] wrapped
	/// in an [Offstage] to hide the body when the [Expansible] is collapsed.
	///
	/// The `animation` property exposes the underlying expanding or collapsing
	/// animation, which has a value of 0 when the [Expansible] is completely
	/// collapsed and 1 when it is completely expanded. This can be used to drive
	/// animations that sync up with the expanding or collapsing animation, such as
	/// rotating an icon.
	///
	/// See also:
	///
	/// * [Expansible.expansibleBuilder], which is of this type.
	typedef ExpansibleBuilder =
	Widget Function(BuildContext context, Widget header, Widget body, Animation<double> animation);

	/// A controller for managing the expansion state of an [Expansible].
	///
	/// This class is a [ChangeNotifier] that notifies its listeners if the value of
	/// [isExpanded] changes.
	///
	/// This controller provides methods to programmatically expand or collapse the
	/// widget, and it allows external components to query the current expansion
	/// state.
	///
	/// The controller's [expand] and [collapse] methods cause the
	/// [Expansible] to rebuild, so they may not be called from
	/// a build method.
	///
	/// Remember to [dispose] of the [ExpansibleController] when it is no longer
	/// needed. This will ensure all resources used by the object are discarded.
	class ExpansibleController extends ChangeNotifier {
	/// Creates a controller to be used with [Expansible.controller].
	ExpansibleController();

	bool _isExpanded = false;

	void _setExpansionState(bool newValue) {
	if (newValue != _isExpanded) {
	_isExpanded = newValue;
	notifyListeners();
	}
	}

	/// Whether the expansible widget built with this controller is in expanded
	/// state.
	///
	/// This property doesn't take the animation into account. It reports `true`
	/// even if the expansion animation is not completed.
	///
	/// To be notified when this property changes, add a listener to the
	/// controller using [ExpansibleController.addListener].
	///
	/// See also:
	///
	/// * [expand], which expands the expansible widget.
	/// * [collapse], which collapses the expansible widget.
	bool get isExpanded => _isExpanded;

	/// Expands the [Expansible] that was built with this controller.
	///
	/// If the widget is already in the expanded state (see [isExpanded]), calling
	/// this method has no effect.
	///
	/// Calling this method may cause the [Expansible] to rebuild, so it may
	/// not be called from a build method.
	///
	/// Calling this method will notify registered listeners of this controller
	/// that the expansion state has changed.
	///
	/// See also:
	///
	/// * [collapse], which collapses the expansible widget.
	/// * [isExpanded] to check whether the expansible widget is expanded.
	void expand() {
	_setExpansionState(true);
	}

	/// Collapses the [Expansible] that was built with this controller.
	///
	/// If the widget is already in the collapsed state (see [isExpanded]),
	/// calling this method has no effect.
	///
	/// Calling this method may cause the [Expansible] to rebuild, so it may not
	/// be called from a build method.
	///
	/// Calling this method will notify registered listeners of this controller
	/// that the expansion state has changed.
	///
	/// See also:
	///
	/// * [expand], which expands the [Expansible].
	/// * [isExpanded] to check whether the [Expansible] is expanded.
	void collapse() {
	_setExpansionState(false);
	}

	/// Finds the [ExpansibleController] for the closest [Expansible] instance
	/// that encloses the given context.
	///
	/// If no [Expansible] encloses the given context, calling this
	/// method will cause an assert in debug mode, and throw an
	/// exception in release mode.
	///
	/// To return null if there is no [Expansible] use [maybeOf] instead.
	///
	/// Typical usage of the [ExpansibleController.of] function is to call it from
	/// within the `build` method of a descendant of an [Expansible].
	static ExpansibleController of(BuildContext context) {
	final _ExpansibleState? result = context.findAncestorStateOfType<_ExpansibleState>();
	assert(() {
	if (result == null) {
	throw FlutterError.fromParts(<DiagnosticsNode>[
	ErrorSummary(
	'ExpansibleController.of() called with a context that does not contain a Expansible.',
	),
	ErrorDescription(
	'No Expansible ancestor could be found starting from the context that was passed to ExpansibleController.of(). '
	'This usually happens when the context provided is from the same StatefulWidget as that '
	'whose build function actually creates the Expansible widget being sought.',
	),
	ErrorHint(
	'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
	'context that is "under" the Expansible. ',
	),
	ErrorHint(
	'A more efficient solution is to split your build function into several widgets. This '
	'introduces a new context from which you can obtain the Expansible. In this solution, '
	'you would have an outer widget that creates the Expansible populated by instances of '
	'your new inner widgets, and then in these inner widgets you would use ExpansibleController.of().\n'
	'An other solution is assign a GlobalKey to the Expansible, '
	'then use the key.currentState property to obtain the Expansible rather than '
	'using the ExpansibleController.of() function.',
	),
	context.describeElement('The context used was'),
	]);
	}
	return true;
	}());
	return result!.widget.controller;
	}

	/// Finds the [Expansible] from the closest instance of this class that
	/// encloses the given context and returns its [ExpansibleController].
	///
	/// If no [Expansible] encloses the given context then return null.
	/// To throw an exception instead, use [of] instead of this function.
	///
	/// See also:
	///
	/// * [of], a similar function to this one that throws if no [Expansible]
	/// encloses the given context.
	static ExpansibleController? maybeOf(BuildContext context) {
	return context.findAncestorStateOfType<_ExpansibleState>()?.widget.controller;
	}
	}

	/// A [StatefulWidget] that expands and collapses.
	///
	/// An [Expansible] consists of a header, which is always shown, and a
	/// body, which is hidden in its collapsed state and shown in its expanded
	/// state.
	///
	/// The [Expansible] is expanded or collapsed with an animation driven by an
	/// [AnimationController]. When the widget is expanded, the height of its body
	/// animates from 0 to its fully expanded height.
	///
	/// This widget is typically used with [ListView] to create an "expand /
	/// collapse" list entry. When used with scrolling widgets like [ListView], a
	/// unique [PageStorageKey] must be specified as the [key], to enable the
	/// [Expansible] to save and restore its expanded state when it is scrolled
	/// in and out of view.
	///
	/// Provide [headerBuilder] and [bodyBuilder] callbacks to
	/// build the header and body widgets. An additional [expansibleBuilder]
	/// callback can be provided to further customize the layout of the widget.
	///
	/// The [Expansible] does not inherently toggle the expansion state. To toggle
	/// the expansion state, call [ExpansibleController.expand] and
	/// [ExpansibleController.collapse] as needed, most typically when the header
	/// returned in [headerBuilder] is tapped.
	///
	/// See also:
	///
	/// * [ExpansionTile], a Material-styled widget that expands and collapses.
	class Expansible extends StatefulWidget {
	/// Creates an instance of [Expansible].
	const Expansible({
	super.key,
	required this.headerBuilder,
	required this.bodyBuilder,
	required this.controller,
	this.expansibleBuilder = _defaultExpansibleBuilder,
	this.animationStyle,
	@Deprecated(
	'Use animationStyle instead. '
	'This feature was deprecated after v3.38.0-0.2.pre.',
	)
	this.duration = const Duration(milliseconds: 200),
	@Deprecated(
	'Use animationStyle instead. '
	'This feature was deprecated after v3.38.0-0.2.pre.',
	)
	this.curve = Curves.ease,
	@Deprecated(
	'Use animationStyle instead. '
	'This feature was deprecated after v3.38.0-0.2.pre.',
	)
	this.reverseCurve,
	this.maintainState = true,
	});

	/// Expands and collapses the widget.
	///
	/// The controller manages the expansion state and toggles the expansion.
	final ExpansibleController controller;

	/// Builds the always-displayed header.
	///
	/// Many use cases involve toggling the expansion state when this header is
	/// tapped. To toggle the expansion state, call [ExpansibleController.expand]
	/// or [ExpansibleController.collapse].
	final ExpansibleComponentBuilder headerBuilder;

	/// Builds the collapsible body.
	///
	/// When this widget is expanded, the height of its body animates from 0 to
	/// its fully extended height.
	final ExpansibleComponentBuilder bodyBuilder;

	/// Used to override the expansion animation curve and duration.
	///
	/// If [AnimationStyle.duration] is provided, it will be used instead of
	/// [duration]. If not provided, [duration] is used, which defaults to
	/// 200ms.
	///
	/// If [AnimationStyle.curve] is provided, it will be used to override
	/// [curve]. If it is null, then [curve] will be used. Otherwise, defaults
	/// to [Curves.ease].
	///
	/// If [AnimationStyle.reverseCurve] is provided, it will be used to
	/// override [reverseCurve]. If it is null, then [reverseCurve] will be
	/// used.
	///
	/// To disable the theme animation, use [AnimationStyle.noAnimation].
	final AnimationStyle? animationStyle;

	/// The duration of the expansion animation.
	///
	/// Defaults to a duration of 200ms.
	///
	/// This property is deprecated, use [animationStyle] instead.
	@Deprecated(
	'Use animationStyle instead. '
	'This feature was deprecated after v3.38.0-0.2.pre.',
	)
	final Duration duration;

	/// The curve of the expansion animation.
	///
	/// Defaults to [Curves.ease].
	///
	/// This property is deprecated, use [animationStyle] instead.
	@Deprecated(
	'Use animationStyle instead. '
	'This feature was deprecated after v3.38.0-0.2.pre.',
	)
	final Curve curve;

	/// The reverse curve of the expansion animation.
	///
	/// If null, uses [curve] in both directions.
	///
	/// This property is deprecated, use [animationStyle] instead.
	@Deprecated(
	'Use animationStyle instead. '
	'This feature was deprecated after v3.38.0-0.2.pre.',
	)
	final Curve? reverseCurve;

	/// Whether the state of the body is maintained when the widget expands or
	/// collapses.
	///
	/// If true, the body is kept in the tree while the widget is
	/// collapsed. Otherwise, the body is removed from the tree when the
	/// widget is collapsed and recreated upon expansion.
	///
	/// Defaults to true.
	final bool maintainState;

	/// Builds the widget with the results of [headerBuilder] and [bodyBuilder].
	///
	/// Defaults to placing the header and body in a [Column].
	final ExpansibleBuilder expansibleBuilder;

	static Widget _defaultExpansibleBuilder(
	BuildContext context,
	Widget header,
	Widget body,
	Animation<double> animation,
	) {
	return Column(mainAxisSize: MainAxisSize.min, children: <Widget>[header, body]);
	}

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

	class _ExpansibleState extends State<Expansible> with SingleTickerProviderStateMixin {
	late AnimationController _animationController;
	late CurvedAnimation _heightFactor;

	Duration get _duration {
	return widget.animationStyle?.duration ?? widget.duration;
	}

	Curve get _curve {
	return widget.animationStyle?.curve ?? widget.curve;
	}

	Curve? get _reverseCurve {
	return widget.animationStyle?.reverseCurve ?? widget.reverseCurve;
	}

	@override
	void initState() {
	super.initState();
	_animationController = AnimationController(duration: _duration, vsync: this);
	final bool initiallyExpanded =
	PageStorage.maybeOf(context)?.readState(context) as bool? ?? widget.controller.isExpanded;
	if (initiallyExpanded) {
	_animationController.value = 1.0;
	widget.controller.expand();
	} else {
	widget.controller.collapse();
	}
	final heightFactorTween = Tween<double>(begin: 0.0, end: 1.0);
	_heightFactor = CurvedAnimation(
	parent: _animationController.drive(heightFactorTween),
	curve: _curve,
	reverseCurve: _reverseCurve,
	);
	widget.controller.addListener(_toggleExpansion);
	}

	@override
	void didUpdateWidget(covariant Expansible oldWidget) {
	super.didUpdateWidget(oldWidget);
	final Duration oldDuration = oldWidget.animationStyle?.duration ?? oldWidget.duration;
	final Curve oldCurve = oldWidget.animationStyle?.curve ?? oldWidget.curve;
	final Curve? oldReverseCurve = oldWidget.animationStyle?.reverseCurve ?? oldWidget.reverseCurve;

	if (_curve != oldCurve) {
	_heightFactor.curve = _curve;
	}
	if (_reverseCurve != oldReverseCurve) {
	_heightFactor.reverseCurve = _reverseCurve;
	}
	if (_duration != oldDuration) {
	_animationController.duration = _duration;
	}
	if (widget.controller != oldWidget.controller) {
	oldWidget.controller.removeListener(_toggleExpansion);
	widget.controller.addListener(_toggleExpansion);
	if (oldWidget.controller.isExpanded != widget.controller.isExpanded) {
	_toggleExpansion();
	}
	}
	}

	@override
	void dispose() {
	widget.controller.removeListener(_toggleExpansion);
	_animationController.dispose();
	_heightFactor.dispose();
	super.dispose();
	}

	void _toggleExpansion() {
	setState(() {
	// Rebuild with the header and the animating body.
	if (widget.controller.isExpanded) {
	_animationController.forward();
	} else {
	_animationController.reverse().then<void>((void value) {
	if (!mounted) {
	return;
	}
	setState(() {
	// Rebuild without the body.
	});
	});
	}
	PageStorage.maybeOf(context)?.writeState(context, widget.controller.isExpanded);
	});
	}

	@override
	Widget build(BuildContext context) {
	assert(!_animationController.isDismissed \|\| !widget.controller.isExpanded);
	final bool closed = !widget.controller.isExpanded && _animationController.isDismissed;
	final bool shouldRemoveBody = closed && !widget.maintainState;

	final Widget result = Offstage(
	offstage: closed,
	child: TickerMode(enabled: !closed, child: widget.bodyBuilder(context, _animationController)),
	);

	return AnimatedBuilder(
	animation: _animationController.view,
	builder: (BuildContext context, Widget? child) {
	final Widget header = widget.headerBuilder(context, _animationController);
	final Widget body = ClipRect(
	child: Align(heightFactor: _heightFactor.value, child: child),
	);
	return widget.expansibleBuilder(context, header, body, _animationController);
	},
	child: shouldRemoveBody ? null : result,
	);
	}
	}