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

 import 'colors.dart';
 import 'icons.dart';
 import 'list_tile.dart';
 import 'theme.dart';
 import 'theme_data.dart';

 const Duration _kExpand = Duration(milliseconds: 200);

 /// A single-line [ListTile] with a trailing button that expands or collapses
 /// the tile to reveal or hide the [children].
 ///
 /// 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 to enable the
 /// [ExpansionTile] to save and restore its expanded state when it is scrolled
 /// in and out of view.
 ///
 /// See also:
 ///
 ///  * [ListTile], useful for creating expansion tile [children] when the
 ///    expansion tile represents a sublist.
 ///  * The "Expand/collapse" section of
 ///    <https://material.io/guidelines/components/lists-controls.html>.
 class ExpansionTile extends StatefulWidget {
   /// Creates a single-line [ListTile] with a trailing button that expands or collapses
   /// the tile to reveal or hide the [children]. The [initiallyExpanded] property must
   /// be non-null.
   const ExpansionTile({
     Key? key,
     this.leading,
     required this.title,
     this.subtitle,
     this.backgroundColor,
     this.onExpansionChanged,
     this.children = const <Widget>[],
     this.trailing,
     this.initiallyExpanded = false,
     this.maintainState = false,
     this.tilePadding,
     this.expandedCrossAxisAlignment,
     this.expandedAlignment,
     this.childrenPadding,
     this.collapsedBackgroundColor,
   }) : assert(initiallyExpanded != null),
        assert(maintainState != null),
        assert(
        expandedCrossAxisAlignment != CrossAxisAlignment.baseline,
        'CrossAxisAlignment.baseline is not supported since the expanded children '
            'are aligned in a column, not a row. Try to use another constant.',
        ),
        super(key: key);

   /// A widget to display before the title.
   ///
   /// Typically a [CircleAvatar] widget.
   final Widget? leading;

   /// The primary content of the list item.
   ///
   /// Typically a [Text] widget.
   final Widget title;

   /// Additional content displayed below the title.
   ///
   /// Typically a [Text] widget.
   final Widget? subtitle;

   /// Called when the tile expands or collapses.
   ///
   /// When the tile starts expanding, this function is called with the value
   /// true. When the tile starts collapsing, this function is called with
   /// the value false.
   final ValueChanged<bool>? onExpansionChanged;

   /// The widgets that are displayed when the tile expands.
   ///
   /// Typically [ListTile] widgets.
   final List<Widget> children;

   /// The color to display behind the sublist when expanded.
   final Color? backgroundColor;

   /// When not null, defines the background color of tile when the sublist is collapsed.
   final Color? collapsedBackgroundColor;

   /// A widget to display instead of a rotating arrow icon.
   final Widget? trailing;

   /// Specifies if the list tile is initially expanded (true) or collapsed (false, the default).
   final bool initiallyExpanded;

   /// Specifies whether the state of the children is maintained when the tile expands and collapses.
   ///
   /// When true, the children are kept in the tree while the tile is collapsed.
   /// When false (default), the children are removed from the tree when the tile is
   /// collapsed and recreated upon expansion.
   final bool maintainState;

   /// Specifies padding for the [ListTile].
   ///
   /// Analogous to [ListTile.contentPadding], this property defines the insets for
   /// the [leading], [title], [subtitle] and [trailing] widgets. It does not inset
   /// the expanded [children] widgets.
   ///
   /// When the value is null, the tile's padding is `EdgeInsets.symmetric(horizontal: 16.0)`.
   final EdgeInsetsGeometry? tilePadding;

   /// Specifies the alignment of [children], which are arranged in a column when
   /// the tile is expanded.
   ///
   /// The internals of the expanded tile make use of a [Column] widget for
   /// [children], and [Align] widget to align the column. The `expandedAlignment`
   /// parameter is passed directly into the [Align].
   ///
   /// Modifying this property controls the alignment of the column within the
   /// expanded tile, not the alignment of [children] widgets within the column.
   /// To align each child within [children], see [expandedCrossAxisAlignment].
   ///
   /// The width of the column is the width of the widest child widget in [children].
   ///
   /// When the value is null, the value of `expandedAlignment` is [Alignment.center].
   final Alignment? expandedAlignment;

   /// Specifies the alignment of each child within [children] when the tile is expanded.
   ///
   /// The internals of the expanded tile make use of a [Column] widget for
   /// [children], and the `crossAxisAlignment` parameter is passed directly into the [Column].
   ///
   /// Modifying this property controls the cross axis alignment of each child
   /// within its [Column]. Note that the width of the [Column] that houses
   /// [children] will be the same as the widest child widget in [children]. It is
   /// not necessarily the width of [Column] is equal to the width of expanded tile.
   ///
   /// To align the [Column] along the expanded tile, use the [expandedAlignment] property
   /// instead.
   ///
   /// When the value is null, the value of `expandedCrossAxisAlignment` is [CrossAxisAlignment.center].
   final CrossAxisAlignment? expandedCrossAxisAlignment;

   /// Specifies padding for [children].
   ///
   /// When the value is null, the value of `childrenPadding` is [EdgeInsets.zero].
   final EdgeInsetsGeometry? childrenPadding;

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

 class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProviderStateMixin {
   static final Animatable<double> _easeOutTween = CurveTween(curve: Curves.easeOut);
   static final Animatable<double> _easeInTween = CurveTween(curve: Curves.easeIn);
   static final Animatable<double> _halfTween = Tween<double>(begin: 0.0, end: 0.5);

   final ColorTween _borderColorTween = ColorTween();
   final ColorTween _headerColorTween = ColorTween();
   final ColorTween _iconColorTween = ColorTween();
   final ColorTween _backgroundColorTween = ColorTween();

   late AnimationController _controller;
   late Animation<double> _iconTurns;
   late Animation<double> _heightFactor;
   late Animation<Color?> _borderColor;
   late Animation<Color?> _headerColor;
   late Animation<Color?> _iconColor;
   late Animation<Color?> _backgroundColor;

   bool _isExpanded = false;

   @override
   void initState() {
     super.initState();
     _controller = AnimationController(duration: _kExpand, vsync: this);
     _heightFactor = _controller.drive(_easeInTween);
     _iconTurns = _controller.drive(_halfTween.chain(_easeInTween));
     _borderColor = _controller.drive(_borderColorTween.chain(_easeOutTween));
     _headerColor = _controller.drive(_headerColorTween.chain(_easeInTween));
     _iconColor = _controller.drive(_iconColorTween.chain(_easeInTween));
     _backgroundColor = _controller.drive(_backgroundColorTween.chain(_easeOutTween));

     _isExpanded = PageStorage.of(context)?.readState(context) as bool? ?? widget.initiallyExpanded;
     if (_isExpanded)
       _controller.value = 1.0;
   }

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

   void _handleTap() {
     setState(() {
       _isExpanded = !_isExpanded;
       if (_isExpanded) {
         _controller.forward();
       } else {
         _controller.reverse().then<void>((void value) {
           if (!mounted)
             return;
           setState(() {
             // Rebuild without widget.children.
           });
         });
       }
       PageStorage.of(context)?.writeState(context, _isExpanded);
     });
     if (widget.onExpansionChanged != null)
       widget.onExpansionChanged!(_isExpanded);
   }

   Widget _buildChildren(BuildContext context, Widget? child) {
     final Color borderSideColor = _borderColor.value ?? Colors.transparent;

     return Container(
       decoration: BoxDecoration(
         color: _backgroundColor.value ?? Colors.transparent,
         border: Border(
           top: BorderSide(color: borderSideColor),
           bottom: BorderSide(color: borderSideColor),
         ),
       ),
       child: Column(
         mainAxisSize: MainAxisSize.min,
         children: <Widget>[
           ListTileTheme.merge(
             iconColor: _iconColor.value,
             textColor: _headerColor.value,
             child: ListTile(
               onTap: _handleTap,
               contentPadding: widget.tilePadding,
               leading: widget.leading,
               title: widget.title,
               subtitle: widget.subtitle,
               trailing: widget.trailing ?? RotationTransition(
                 turns: _iconTurns,
                 child: const Icon(Icons.expand_more),
               ),
             ),
           ),
           ClipRect(
             child: Align(
               alignment: widget.expandedAlignment ?? Alignment.center,
               heightFactor: _heightFactor.value,
               child: child,
             ),
           ),
         ],
       ),
     );
   }

   @override
   void didChangeDependencies() {
     final ThemeData theme = Theme.of(context);
     _borderColorTween.end = theme.dividerColor;
     _headerColorTween
       ..begin = theme.textTheme.subtitle1!.color
       ..end = theme.accentColor;
     _iconColorTween
       ..begin = theme.unselectedWidgetColor
       ..end = theme.accentColor;
     _backgroundColorTween
       ..begin = widget.collapsedBackgroundColor
       ..end = widget.backgroundColor;
     super.didChangeDependencies();
   }

   @override
   Widget build(BuildContext context) {
     final bool closed = !_isExpanded && _controller.isDismissed;
     final bool shouldRemoveChildren = closed && !widget.maintainState;

     final Widget result = Offstage(
       child: TickerMode(
         child: Padding(
           padding: widget.childrenPadding ?? EdgeInsets.zero,
           child: Column(
             crossAxisAlignment: widget.expandedCrossAxisAlignment ?? CrossAxisAlignment.center,
             children: widget.children,
           ),
         ),
         enabled: !closed,
       ),
       offstage: closed
     );

     return AnimatedBuilder(
       animation: _controller.view,
       builder: _buildChildren,
       child: shouldRemoveChildren ? 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.

	import 'package:flutter/widgets.dart';

	import 'colors.dart';
	import 'icons.dart';
	import 'list_tile.dart';
	import 'theme.dart';
	import 'theme_data.dart';

	const Duration _kExpand = Duration(milliseconds: 200);

	/// A single-line [ListTile] with a trailing button that expands or collapses
	/// the tile to reveal or hide the [children].
	///
	/// 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 to enable the
	/// [ExpansionTile] to save and restore its expanded state when it is scrolled
	/// in and out of view.
	///
	/// See also:
	///
	/// * [ListTile], useful for creating expansion tile [children] when the
	/// expansion tile represents a sublist.
	/// * The "Expand/collapse" section of
	/// <https://material.io/guidelines/components/lists-controls.html>.
	class ExpansionTile extends StatefulWidget {
	/// Creates a single-line [ListTile] with a trailing button that expands or collapses
	/// the tile to reveal or hide the [children]. The [initiallyExpanded] property must
	/// be non-null.
	const ExpansionTile({
	Key? key,
	this.leading,
	required this.title,
	this.subtitle,
	this.backgroundColor,
	this.onExpansionChanged,
	this.children = const <Widget>[],
	this.trailing,
	this.initiallyExpanded = false,
	this.maintainState = false,
	this.tilePadding,
	this.expandedCrossAxisAlignment,
	this.expandedAlignment,
	this.childrenPadding,
	this.collapsedBackgroundColor,
	}) : assert(initiallyExpanded != null),
	assert(maintainState != null),
	assert(
	expandedCrossAxisAlignment != CrossAxisAlignment.baseline,
	'CrossAxisAlignment.baseline is not supported since the expanded children '
	'are aligned in a column, not a row. Try to use another constant.',
	),
	super(key: key);

	/// A widget to display before the title.
	///
	/// Typically a [CircleAvatar] widget.
	final Widget? leading;

	/// The primary content of the list item.
	///
	/// Typically a [Text] widget.
	final Widget title;

	/// Additional content displayed below the title.
	///
	/// Typically a [Text] widget.
	final Widget? subtitle;

	/// Called when the tile expands or collapses.
	///
	/// When the tile starts expanding, this function is called with the value
	/// true. When the tile starts collapsing, this function is called with
	/// the value false.
	final ValueChanged<bool>? onExpansionChanged;

	/// The widgets that are displayed when the tile expands.
	///
	/// Typically [ListTile] widgets.
	final List<Widget> children;

	/// The color to display behind the sublist when expanded.
	final Color? backgroundColor;

	/// When not null, defines the background color of tile when the sublist is collapsed.
	final Color? collapsedBackgroundColor;

	/// A widget to display instead of a rotating arrow icon.
	final Widget? trailing;

	/// Specifies if the list tile is initially expanded (true) or collapsed (false, the default).
	final bool initiallyExpanded;

	/// Specifies whether the state of the children is maintained when the tile expands and collapses.
	///
	/// When true, the children are kept in the tree while the tile is collapsed.
	/// When false (default), the children are removed from the tree when the tile is
	/// collapsed and recreated upon expansion.
	final bool maintainState;

	/// Specifies padding for the [ListTile].
	///
	/// Analogous to [ListTile.contentPadding], this property defines the insets for
	/// the [leading], [title], [subtitle] and [trailing] widgets. It does not inset
	/// the expanded [children] widgets.
	///
	/// When the value is null, the tile's padding is `EdgeInsets.symmetric(horizontal: 16.0)`.
	final EdgeInsetsGeometry? tilePadding;

	/// Specifies the alignment of [children], which are arranged in a column when
	/// the tile is expanded.
	///
	/// The internals of the expanded tile make use of a [Column] widget for
	/// [children], and [Align] widget to align the column. The `expandedAlignment`
	/// parameter is passed directly into the [Align].
	///
	/// Modifying this property controls the alignment of the column within the
	/// expanded tile, not the alignment of [children] widgets within the column.
	/// To align each child within [children], see [expandedCrossAxisAlignment].
	///
	/// The width of the column is the width of the widest child widget in [children].
	///
	/// When the value is null, the value of `expandedAlignment` is [Alignment.center].
	final Alignment? expandedAlignment;

	/// Specifies the alignment of each child within [children] when the tile is expanded.
	///
	/// The internals of the expanded tile make use of a [Column] widget for
	/// [children], and the `crossAxisAlignment` parameter is passed directly into the [Column].
	///
	/// Modifying this property controls the cross axis alignment of each child
	/// within its [Column]. Note that the width of the [Column] that houses
	/// [children] will be the same as the widest child widget in [children]. It is
	/// not necessarily the width of [Column] is equal to the width of expanded tile.
	///
	/// To align the [Column] along the expanded tile, use the [expandedAlignment] property
	/// instead.
	///
	/// When the value is null, the value of `expandedCrossAxisAlignment` is [CrossAxisAlignment.center].
	final CrossAxisAlignment? expandedCrossAxisAlignment;

	/// Specifies padding for [children].
	///
	/// When the value is null, the value of `childrenPadding` is [EdgeInsets.zero].
	final EdgeInsetsGeometry? childrenPadding;

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

	class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProviderStateMixin {
	static final Animatable<double> _easeOutTween = CurveTween(curve: Curves.easeOut);
	static final Animatable<double> _easeInTween = CurveTween(curve: Curves.easeIn);
	static final Animatable<double> _halfTween = Tween<double>(begin: 0.0, end: 0.5);

	final ColorTween _borderColorTween = ColorTween();
	final ColorTween _headerColorTween = ColorTween();
	final ColorTween _iconColorTween = ColorTween();
	final ColorTween _backgroundColorTween = ColorTween();

	late AnimationController _controller;
	late Animation<double> _iconTurns;
	late Animation<double> _heightFactor;
	late Animation<Color?> _borderColor;
	late Animation<Color?> _headerColor;
	late Animation<Color?> _iconColor;
	late Animation<Color?> _backgroundColor;

	bool _isExpanded = false;

	@override
	void initState() {
	super.initState();
	_controller = AnimationController(duration: _kExpand, vsync: this);
	_heightFactor = _controller.drive(_easeInTween);
	_iconTurns = _controller.drive(_halfTween.chain(_easeInTween));
	_borderColor = _controller.drive(_borderColorTween.chain(_easeOutTween));
	_headerColor = _controller.drive(_headerColorTween.chain(_easeInTween));
	_iconColor = _controller.drive(_iconColorTween.chain(_easeInTween));
	_backgroundColor = _controller.drive(_backgroundColorTween.chain(_easeOutTween));

	_isExpanded = PageStorage.of(context)?.readState(context) as bool? ?? widget.initiallyExpanded;
	if (_isExpanded)
	_controller.value = 1.0;
	}

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

	void _handleTap() {
	setState(() {
	_isExpanded = !_isExpanded;
	if (_isExpanded) {
	_controller.forward();
	} else {
	_controller.reverse().then<void>((void value) {
	if (!mounted)
	return;
	setState(() {
	// Rebuild without widget.children.
	});
	});
	}
	PageStorage.of(context)?.writeState(context, _isExpanded);
	});
	if (widget.onExpansionChanged != null)
	widget.onExpansionChanged!(_isExpanded);
	}

	Widget _buildChildren(BuildContext context, Widget? child) {
	final Color borderSideColor = _borderColor.value ?? Colors.transparent;

	return Container(
	decoration: BoxDecoration(
	color: _backgroundColor.value ?? Colors.transparent,
	border: Border(
	top: BorderSide(color: borderSideColor),
	bottom: BorderSide(color: borderSideColor),
	),
	),
	child: Column(
	mainAxisSize: MainAxisSize.min,
	children: <Widget>[
	ListTileTheme.merge(
	iconColor: _iconColor.value,
	textColor: _headerColor.value,
	child: ListTile(
	onTap: _handleTap,
	contentPadding: widget.tilePadding,
	leading: widget.leading,
	title: widget.title,
	subtitle: widget.subtitle,
	trailing: widget.trailing ?? RotationTransition(
	turns: _iconTurns,
	child: const Icon(Icons.expand_more),
	),
	),
	),
	ClipRect(
	child: Align(
	alignment: widget.expandedAlignment ?? Alignment.center,
	heightFactor: _heightFactor.value,
	child: child,
	),
	),
	],
	),
	);
	}

	@override
	void didChangeDependencies() {
	final ThemeData theme = Theme.of(context);
	_borderColorTween.end = theme.dividerColor;
	_headerColorTween
	..begin = theme.textTheme.subtitle1!.color
	..end = theme.accentColor;
	_iconColorTween
	..begin = theme.unselectedWidgetColor
	..end = theme.accentColor;
	_backgroundColorTween
	..begin = widget.collapsedBackgroundColor
	..end = widget.backgroundColor;
	super.didChangeDependencies();
	}

	@override
	Widget build(BuildContext context) {
	final bool closed = !_isExpanded && _controller.isDismissed;
	final bool shouldRemoveChildren = closed && !widget.maintainState;

	final Widget result = Offstage(
	child: TickerMode(
	child: Padding(
	padding: widget.childrenPadding ?? EdgeInsets.zero,
	child: Column(
	crossAxisAlignment: widget.expandedCrossAxisAlignment ?? CrossAxisAlignment.center,
	children: widget.children,
	),
	),
	enabled: !closed,
	),
	offstage: closed
	);

	return AnimatedBuilder(
	animation: _controller.view,
	builder: _buildChildren,
	child: shouldRemoveChildren ? null : result,
	);
	}
	}