ExpansionTileCard, based on Flutter's ExpansionTile, inspired by the Google Material Theme

expansion_tile_card.dart

// Originally based on ExpansionTile from Flutter.
//
// 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/material.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
/// [ExpansionTileCard] 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.
///  * [ExpansionTile], the original widget on which this widget is based.
///  * The "Expand/collapse" section of
///    <https://material.io/guidelines/components/lists-controls.html>.
class ExpansionTileCard 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 ExpansionTileCard({
    Key key,
    this.leading,
    @required this.title,
    this.subtitle,
    this.onExpansionChanged,
    this.children = const <Widget>[],
    this.trailing,
    this.borderRadius = const BorderRadius.all(Radius.circular(8.0)),
    this.elevation = 2.0,
    this.initiallyExpanded = false,
  })  : assert(initiallyExpanded != null),
        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;

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

  /// The radius used for the Material widget's border. Only visible once expanded.
  ///
  /// Defaults to a circular border with a radius of 8.0.
  final BorderRadiusGeometry borderRadius;

  /// The final elevation of the Material widget, once expanded.
  ///
  /// Defaults to 2.0.
  final double elevation;

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

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

class _ExpansionTileCardState extends State<ExpansionTileCard>
    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 _headerColorTween = ColorTween();
  final ColorTween _iconColorTween = ColorTween();
  final ColorTween _materialColorTween = ColorTween();

  AnimationController _controller;
  Animation<double> _iconTurns;
  Animation<double> _heightFactor;
  Animation<double> _elevationFactor;
  Animation<Color> _headerColor;
  Animation<Color> _iconColor;
  Animation<Color> _materialColor;

  bool _isExpanded = false;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(duration: _kExpand, vsync: this);
    _heightFactor = _controller.drive(_easeInTween);
    _iconTurns = _controller.drive(_halfTween.chain(_easeInTween));
    _headerColor = _controller.drive(_headerColorTween.chain(_easeInTween));
    _materialColor = _controller.drive(_materialColorTween.chain(_easeInTween));
    _iconColor = _controller.drive(_iconColorTween.chain(_easeInTween));
    _isExpanded = PageStorage.of(context)?.readState(context) as bool ??
        widget.initiallyExpanded;
    _elevationFactor = _controller.drive(_easeOutTween);
    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) {
    return Padding(
      padding: EdgeInsets.symmetric(
        vertical: 6.0 * _heightFactor.value,
      ),
      child: Material(
        type: MaterialType.card,
        color: _materialColor.value,
        borderRadius: widget.borderRadius,
        elevation: widget.elevation * _elevationFactor.value,
        child: Container(
          padding: EdgeInsets.all(2.0),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              ListTileTheme.merge(
                iconColor: _iconColor.value,
                textColor: _headerColor.value,
                child: ListTile(
                  onTap: _handleTap,
                  leading: widget.leading,
                  title: widget.title,
                  subtitle: widget.subtitle,
                  trailing: widget.trailing ??
                      RotationTransition(
                        turns: _iconTurns,
                        child: const Icon(Icons.expand_more),
                      ),
                ),
              ),
              ClipRect(
                child: Align(
                  heightFactor: _heightFactor.value,
                  child: child,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  @override
  void didChangeDependencies() {
    final ThemeData theme = Theme.of(context);
    _headerColorTween
      ..begin = theme.textTheme.subtitle1.color
      ..end = theme.accentColor;
    _iconColorTween
      ..begin = theme.unselectedWidgetColor
      ..end = theme.accentColor;
    _materialColorTween
      ..begin = theme.canvasColor
      ..end = theme.cardColor;
    super.didChangeDependencies();
  }

  @override
  Widget build(BuildContext context) {
    final bool closed = !_isExpanded && _controller.isDismissed;
    return AnimatedBuilder(
      animation: _controller.view,
      builder: _buildChildren,
      child: closed ? null : Column(children: widget.children),
    );
  }
}