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Changing Icon Color Behind ListTile In An ExpansionPanelList.radio

- 1 answer

In a ListTile of ExpansionPanelList, the trialing is already given as an arrow down, and when the ListTile is expanded, the arrow changes to arrow up.

However, it's dark in color. I added trailing: Icon(blah blah). This merely added an icon between the title and the default trailing button(arrow down).

How do I change the color of the default trailing button while keeping its versatility that it turns into an arrow up when the box is expanded?

I tried wrapping the ListTile in a theme, which was what someone recommended, but the color of the arrow down doesn't change as seen below:

Widget _buildPanel() {
    return ExpansionPanelList.radio(
      initialOpenPanelValue: 0,
      children: _data.map<ExpansionPanelRadio>((Item item) {
        return ExpansionPanelRadio(
          backgroundColor: Colors.black,
          canTapOnHeader: true,
          value: item.id,
          headerBuilder: (BuildContext context, bool isExpanded) {
            return Theme(
              data: Theme.of(context)
                  .copyWith(unselectedWidgetColor: Colors.white),
              child: ListTile(
                title: Text(item.headerValue,
                  
...

Updated errors: ErrorError continued

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Answer

If you are using an ExpansionPanelList you can only change the color of it's ExpandIcon (The widget which has the arrow) via the theme brightness , here is an excerpt of the code inside the ExpandIcon:

    switch(Theme.of(context).brightness) {
      case Brightness.light:
        return Colors.black54;
      case Brightness.dark:
        return Colors.white60;
    }

This widget also has attributes to change its expanded and disabled color but unfortunately ExpansionPanelList does not passes these custom attributes to the ExpandIcon hence you will go to the fallback switch I pasted above.

One solution can be to extract ExpansionPanelList from the framework and add the properties color,disabledColor,expandedColor, to its constructor. I renamed the class ExpansionPanelList to ColoredExpansionPanelList and added the previous color attributes to pass them to its child ExpandIcon.

// The following code is the same that is found inside the Flutter framework 
//just adding the `color,disabledColor,expandedColor` for personalizing the `ExpandedIcon`

// 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';
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]';
  }
}

/// Signature for the callback that's called when an [ExpansionPanel] is
/// expanded or collapsed.
///
/// The position of the panel within an [ColoredExpansionPanelList] is given by
/// [panelIndex].
typedef ExpansionPanelCallback = void Function(int panelIndex, bool isExpanded);

/// Signature for the callback that's called when the header of the
/// [ExpansionPanel] needs to rebuild.
typedef ExpansionPanelHeaderBuilder = Widget Function(BuildContext context, bool isExpanded);

/// A material expansion panel. It has a header and a body and can be either
/// expanded or collapsed. The body of the panel is only visible when it is
/// expanded.
///
/// Expansion panels are only intended to be used as children for
/// [ColoredExpansionPanelList].
///
/// See [ColoredExpansionPanelList] for a sample implementation.
///
/// See also:
///
///  * [ColoredExpansionPanelList]
///  * <https://material.io/design/components/lists.html#types>
class ExpansionPanel {
  /// Creates an expansion panel to be used as a child for [ColoredExpansionPanelList].
  /// See [ColoredExpansionPanelList] for an example on how to use this widget.
  ///
  /// The [headerBuilder], [body], and [isExpanded] arguments must not be null.
  ExpansionPanel({
    required this.headerBuilder,
    required this.body,
    this.isExpanded = false,
    this.canTapOnHeader = false,
    this.backgroundColor,
  }) : assert(headerBuilder != null),
        assert(body != null),
        assert(isExpanded != null),
        assert(canTapOnHeader != null);

  /// The widget builder that builds the expansion panels' header.
  final ExpansionPanelHeaderBuilder headerBuilder;

  /// The body of the expansion panel that's displayed below the header.
  ///
  /// This widget is visible only when the panel is expanded.
  final Widget body;

  /// Whether the panel is expanded.
  ///
  /// Defaults to false.
  final bool isExpanded;

  /// Whether tapping on the panel's header will expand/collapse it.
  ///
  /// Defaults to false.
  final bool canTapOnHeader;

  /// Defines the background color of the panel.
  ///
  /// Defaults to [ThemeData.cardColor].
  final Color? backgroundColor;
}

/// An expansion panel that allows for radio-like functionality.
/// This means that at any given time, at most, one [ExpansionPanelRadio]
/// can remain expanded.
///
/// A unique identifier [value] must be assigned to each panel.
/// This identifier allows the [ColoredExpansionPanelList] to determine
/// which [ExpansionPanelRadio] instance should be expanded.
///
/// See [ExpansionPanelList.radio] for a sample implementation.
class ExpansionPanelRadio extends ExpansionPanel {

  /// An expansion panel that allows for radio functionality.
  ///
  /// A unique [value] must be passed into the constructor. The
  /// [headerBuilder], [body], [value] must not be null.
  ExpansionPanelRadio({
    required this.value,
    required ExpansionPanelHeaderBuilder headerBuilder,
    required Widget body,
    bool canTapOnHeader = false,
    Color? backgroundColor,
  }) : assert(value != null),
        super(
        body: body,
        headerBuilder: headerBuilder,
        canTapOnHeader: canTapOnHeader,
        backgroundColor: backgroundColor,
      );

  /// The value that uniquely identifies a radio panel so that the currently
  /// selected radio panel can be identified.
  final Object value;
}

/// A material expansion panel list that lays out its children and animates
/// expansions.
///
/// Note that [expansionCallback] behaves differently for [ColoredExpansionPanelList]
/// and [ExpansionPanelList.radio].
///
/// {@tool dartpad --template=stateful_widget_scaffold}
///
/// Here is a simple example of how to implement ExpansionPanelList.
///
/// ```dart preamble
/// // stores ExpansionPanel state information
/// class Item {
///   Item({
///     required this.expandedValue,
///     required this.headerValue,
///     this.isExpanded = false,
///   });
///
///   String expandedValue;
///   String headerValue;
///   bool isExpanded;
/// }
///
/// List<Item> generateItems(int numberOfItems) {
///   return List<Item>.generate(numberOfItems, (int index) {
///     return Item(
///       headerValue: 'Panel $index',
///       expandedValue: 'This is item number $index',
///     );
///   });
/// }
/// ```
///
/// ```dart
/// final List<Item> _data = generateItems(8);
///
/// @override
/// Widget build(BuildContext context) {
///   return SingleChildScrollView(
///     child: Container(
///       child: _buildPanel(),
///     ),
///   );
/// }
///
/// Widget _buildPanel() {
///   return ExpansionPanelList(
///     expansionCallback: (int index, bool isExpanded) {
///       setState(() {
///         _data[index].isExpanded = !isExpanded;
///       });
///     },
///     children: _data.map<ExpansionPanel>((Item item) {
///       return ExpansionPanel(
///         headerBuilder: (BuildContext context, bool isExpanded) {
///           return ListTile(
///             title: Text(item.headerValue),
///           );
///         },
///         body: ListTile(
///           title: Text(item.expandedValue),
///           subtitle: const Text('To delete this panel, tap the trash can icon'),
///           trailing: const Icon(Icons.delete),
///           onTap: () {
///             setState(() {
///               _data.removeWhere((Item currentItem) => item == currentItem);
///             });
///           }
///         ),
///         isExpanded: item.isExpanded,
///       );
///     }).toList(),
///   );
/// }
/// ```
/// {@end-tool}
///
/// See also:
///
///  * [ExpansionPanel]
///  * [ExpansionPanelList.radio]
///  * <https://material.io/design/components/lists.html#types>
class ColoredExpansionPanelList 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 ColoredExpansionPanelList({
    Key? key,
    this.children = const <ExpansionPanel>[],
    this.expansionCallback,
    this.animationDuration = kThemeAnimationDuration,
    this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
    this.dividerColor,
    this.color,
    this.disabledColor,
    this.expandedColor,
    this.elevation = 2,
  }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = false,
        initialOpenPanelValue = null,
        super(key: key);


  /// START OF CUSTOM CODE TO ADD COLOR TO EXPANDEDICON
  /// 
  /// 
  /// 
  /// The color of the icon.
  ///
  /// Defaults to [Colors.black54] when the theme's
  /// [ThemeData.brightness] is [Brightness.light] and to
  /// [Colors.white60] when it is [Brightness.dark]. This adheres to the
  /// Material Design specifications for [icons](https://material.io/design/iconography/system-icons.html#color)
  /// and for [dark theme](https://material.io/design/color/dark-theme.html#ui-application)
  final Color? color;

  /// The color of the icon when it is disabled,
  /// i.e. if [onPressed] is null.
  ///
  /// Defaults to [Colors.black38] when the theme's
  /// [ThemeData.brightness] is [Brightness.light] and to
  /// [Colors.white38] when it is [Brightness.dark]. This adheres to the
  /// Material Design specifications for [icons](https://material.io/design/iconography/system-icons.html#color)
  /// and for [dark theme](https://material.io/design/color/dark-theme.html#ui-application)
  final Color? disabledColor;

  /// The color of the icon when the icon is expanded.
  ///
  /// Defaults to [Colors.black54] when the theme's
  /// [ThemeData.brightness] is [Brightness.light] and to
  /// [Colors.white] when it is [Brightness.dark]. This adheres to the
  /// Material Design specifications for [icons](https://material.io/design/iconography/system-icons.html#color)
  /// and for [dark theme](https://material.io/design/color/dark-theme.html#ui-application)
  final Color? expandedColor;

  ///END OF CUSTOM CODE TO ADD COLOR TO EXPANDEDICON
  /// 
  /// 
  /// 
  /// 
  /// 


  /// Creates a radio expansion panel list widget.
  ///
  /// This widget allows for at most one panel in the list to be open.
  /// The expansion panel callback is triggered when an expansion panel
  /// expand/collapse button is pushed. The [children] and [animationDuration]
  /// arguments must not be null. The [children] objects must be instances
  /// of [ExpansionPanelRadio].
  ///
  /// {@tool dartpad --template=stateful_widget_scaffold}
  ///
  /// Here is a simple example of how to implement ExpansionPanelList.radio.
  ///
  /// ```dart preamble
  /// // stores ExpansionPanel state information
  /// class Item {
  ///   Item({
  ///     required this.id,
  ///     required this.expandedValue,
  ///     required this.headerValue,
  ///   });
  ///
  ///   int id;
  ///   String expandedValue;
  ///   String headerValue;
  /// }
  ///
  /// List<Item> generateItems(int numberOfItems) {
  ///   return List<Item>.generate(numberOfItems, (int index) {
  ///     return Item(
  ///       id: index,
  ///       headerValue: 'Panel $index',
  ///       expandedValue: 'This is item number $index',
  ///     );
  ///   });
  /// }
  /// ```
  ///
  /// ```dart
  /// final List<Item> _data = generateItems(8);
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return SingleChildScrollView(
  ///     child: Container(
  ///       child: _buildPanel(),
  ///     ),
  ///   );
  /// }
  ///
  /// Widget _buildPanel() {
  ///   return ExpansionPanelList.radio(
  ///     initialOpenPanelValue: 2,
  ///     children: _data.map<ExpansionPanelRadio>((Item item) {
  ///       return ExpansionPanelRadio(
  ///         value: item.id,
  ///         headerBuilder: (BuildContext context, bool isExpanded) {
  ///           return ListTile(
  ///             title: Text(item.headerValue),
  ///           );
  ///         },
  ///         body: ListTile(
  ///           title: Text(item.expandedValue),
  ///           subtitle: const Text('To delete this panel, tap the trash can icon'),
  ///           trailing: const Icon(Icons.delete),
  ///           onTap: () {
  ///             setState(() {
  ///               _data.removeWhere((Item currentItem) => item == currentItem);
  ///             });
  ///           }
  ///         )
  ///       );
  ///     }).toList(),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  const ColoredExpansionPanelList.radio({
    Key? key,
    this.children = const <ExpansionPanelRadio>[],
    this.expansionCallback,
    this.animationDuration = kThemeAnimationDuration,
    this.initialOpenPanelValue,
    this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
    this.dividerColor,
    this.color,
    this.disabledColor,
    this.expandedColor,
    this.elevation = 2,
  }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = true,
        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 ExpansionPanelList.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 ExpansionPanelList, the callback needs to setState when it's notified
  /// about the closing/opening panel. On the other hand, the callback for
  /// ExpansionPanelList.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 [ExpansionPanelList.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.
  ///
  /// This uses [kElevationToShadow] to simulate shadows, it does not use
  /// [Material]'s arbitrary elevation feature.
  ///
  /// The following values can be used to define the elevation: 0, 1, 2, 3, 4, 6,
  /// 8, 9, 12, 16, 24.
  ///
  /// By default, the value of elevation is 2.
  final int elevation;

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

class _ColoredExpansionPanelListState extends State<ColoredExpansionPanelList> {
  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(ColoredExpansionPanelList 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(
          color: widget.color,
          disabledColor: widget.disabledColor,
          expandedColor:  widget.expandedColor,
          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,
      children: items,
    );
  }
}

Original ExpansionPanelList code

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source: stackoverflow.com
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