// 修改了ExpansionPanelList的State的build方法
// 并删去了ExpansionPane(已经在原文件中定义),防止重定义
// 原版会在展开的项的上下添加及其难看的鸿沟

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';


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

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 => hashValues(runtimeType, salt, value);

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

typedef ExpansionPanelCallback = void Function(int panelIndex, bool isExpanded);
typedef ExpansionPanelHeaderBuilder = Widget Function(BuildContext context, bool isExpanded);

class ChznExpansionPanelList extends StatefulWidget {
  const ChznExpansionPanelList({
    Key? key,
    this.children = const <ExpansionPanel>[],
    this.expansionCallback,
    this.animationDuration = kThemeAnimationDuration,
    this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
    this.dividerColor,
    this.elevation = 2,
  }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = false,
        initialOpenPanelValue = null,
        super(key: key);
  const ChznExpansionPanelList.radio({
    Key? key,
    this.children = const <ExpansionPanelRadio>[],
    this.expansionCallback,
    this.animationDuration = kThemeAnimationDuration,
    this.initialOpenPanelValue,
    this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
    this.dividerColor,
    this.elevation = 2,
  }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = true,
        super(key: key);
  final List<ExpansionPanel> children;
  final ExpansionPanelCallback? expansionCallback;
  final Duration animationDuration;
  final bool _allowOnlyOnePanelOpen;
  final Object? initialOpenPanelValue;
  final EdgeInsets expandedHeaderPadding;
  final Color? dividerColor;
  final double elevation;

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

class _ExpansionPanelListState extends State<ChznExpansionPanelList> {
  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(ChznExpansionPanelList 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) {
    widget.expansionCallback?.call(index, isExpanded);

    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;
      });
    }
  }

  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)));

      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(
          isExpanded: _isChildExpanded(index),
          padding: const EdgeInsets.all(16.0),
          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)));
    }

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