A fix for flutters expansion panel list. Removed header icon and gap between selected panels

app_expansion_panel.dart

import 'package:flutter/material.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 => 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]';
  }
}

class AppExpansionPanelList 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 AppExpansionPanelList({
    Key? key,
    required this.children,
    this.expansionCallback,
    this.animationDuration = kThemeAnimationDuration,
    this.expandedHeaderPadding = EdgeInsets.zero,
    this.dividerColor,
    this.elevation = 2,
  })  : _allowOnlyOnePanelOpen = false,
        initialOpenPanelValue = null,
        super(key: key);

  /// 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 AppExpansionPanelList.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 AppExpansionPanelList, the callback needs to setState when it's notified
  /// about the closing/opening panel. On the other hand, the callback for
  /// AppExpansionPanelList.radio is simply meant 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 [AppExpansionPanelList.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;

  @override
  State<AppExpansionPanelList> createState() => _AppExpansionPanelListState();
}

class _AppExpansionPanelListState extends State<AppExpansionPanelList> {
  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(AppExpansionPanelList oldWidget) {
    super.didUpdateWidget(oldWidget);

    if (widget._allowOnlyOnePanelOpen) {
      assert(_allIdentifiersUnique(),
          'All ExpansionPanelRadio identifier values must be unique.');
      // If the previous widget was non-radio AppExpansionPanelList, 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) {
      //todo: Uncomment to add gap between selected panels
      /*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,
              ),
            ),
          ),
          //todo: Uncomment to remove header icon
          // 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,
      children: items,
    );
  }
}

I added some ToDos on the areas's that were edited

this is useful

thank you very much!