Flutter有很多的基礎Widget，其中IconButton很常用，還有 PopupButton, 這裡擴充套件的這個 AppBarButton 是將兩者融合一起，用起來更方便了。

import 'package:flutter/material.dart';

class AppBarButton<T> extends StatelessWidget {
  final Widget child;
  final Color color, focusColor;
  final double iconSize;
  final String tooltip;
  final bool autofocus;
  
 
/// 選單列表
  ///
  /// [[{'title': '分享', 'icon': FIcons.share_2, 'type': SHARE}]]
  final List<Map<String, dynamic>> menus;
  final Offset menuOffset;
  final MenusWidgetBuilder<T> menuItemBuilder;
  final T menuInitialValue;
  final T menuSelected;
  final double minWidth, splashRadius;
  final EdgeInsetsGeometry padding;
  final VoidCallback onPressed;
  final ValueChanged
 
<T> onSelected;

  const AppBarButton({Key key, Widget child, Widget icon, this.color,
    this.focusColor, this.iconSize, this.tooltip,
    this.minWidth, this.splashRadius,
    this.autofocus,
    this.menus,
    this.menuOffset,
    this.menuItemBuilder,
    this.menuInitialValue,
    this.menuSelected,
    
 
this.onSelected,
    this.padding, VoidCallback onPressed, VoidCallback onTap}):
      child = child ?? icon,
      onPressed = onPressed ?? onTap,
      super(key: key);

  @override
  Widget build(BuildContext context) {
    if ((menus != null && menus.isNotEmpty) || menuItemBuilder != null)
      return _buildButton(context, () {
        showButtonMenu(context,
          menus: menus,
          menuItemBuilder: menuItemBuilder,
          menuInitialValue: menuInitialValue,
          menuOffset: menuOffset,
          menuSelected: menuSelected,
          onSelected: onSelected
        );
      });
    return _buildButton(context, onPressed);
  }

  Widget _buildButton(BuildContext context, VoidCallback onPressed) {
    final _btn = IconButton(
      iconSize: iconSize ?? Theme.of(context).appBarTheme.iconTheme.size,
      icon: child,
      autofocus: autofocus ?? false,
      focusColor: focusColor,
      tooltip: tooltip,
      color: color,
      splashRadius: splashRadius,
      padding: padding ?? const EdgeInsets.all(8.0),
      onPressed: onPressed,
    );
    return minWidth == null ? _btn : ButtonTheme(
      minWidth: minWidth,
      child: _btn,
    );
  }

  /// 顯示彈出選單
  static void showButtonMenu<T>(BuildContext context, {
    Offset menuOffset,
    Offset local,
    List<Map<String, dynamic>> menus,
    MenusWidgetBuilder<T> menuItemBuilder,
    T menuInitialValue,
    T menuSelected,
    ValueChanged<T> onSelected,
    VoidCallback onMenuCancel,
  }) {
    final PopupMenuThemeData popupMenuTheme = PopupMenuTheme.of(context);
    final RenderBox button = context.findRenderObject() as RenderBox;
    final RenderBox overlay = Overlay.of(context).context.findRenderObject() as RenderBox;
    final RelativeRect position = RelativeRect.fromRect(
      Rect.fromPoints(
        local != null ? local : button.localToGlobal(menuOffset ?? Offset(0, 40), ancestor: overlay),
        button.localToGlobal(button.size.bottomRight(Offset.zero), ancestor: overlay),
      ),
      Offset.zero & overlay.size,
    );
    final _primaryColor = Theme.of(context).primaryColor;
    final _textStyle = Theme.of(context).popupMenuTheme?.textStyle ??
        Theme.of(context).textTheme.subtitle1;
    final List<PopupMenuEntry<T>> items = menuItemBuilder != null 
        ? menuItemBuilder(context)
        : menus.map((element) {
          final _value = element['type'];
          return PopupMenuItem<T>(
            textStyle: _textStyle,
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: [
                (menuSelected == _value)
                  ? DefaultTextStyle(
                    style: _textStyle.copyWith(color: _primaryColor),
                    child: Text(element['title']))
                  : Text(element['title']),
                if (element['icon'] != null)
                  Icon(element['icon'], color: _primaryColor, size: 20),
              ],
            ),
            value: _value,
          );
    }).toList();
    // Only show the menu if there is something to show
    if (items.isNotEmpty) {
      showMenu<T>(
        context: context,
        elevation: popupMenuTheme.elevation,
        items: items,
        position: position,
        shape: popupMenuTheme.shape,
        color: popupMenuTheme.color,
        initialValue: menuInitialValue,
      ).then<void>((T newValue) {
        if (newValue == null) {
          if (onMenuCancel != null) onMenuCancel();
          return null;
        }
        if (onSelected != null)
          onSelected(newValue);
        return newValue;
      });
    }
  }
}

typedef MenusWidgetBuilder<T> = List<PopupMenuEntry<T>> Function(BuildContext context);

使用示例：

AppBarButton(
  icon: Icon(FIcons.save),
  iconSize: 21,
  tooltip: "儲存",
  onPressed: () => _saveRule(context),
),

AppBarButton(
  icon: Icon(FIcons.share_2),
  tooltip: "分享",
  menus:  [
    {'title': '分享', 'icon': FIcons.share_2, 'type': SHARE},
    {'title': '掃碼分享到遠端', 'icon': FIcons.bscan, 'type': SHARE_QRSCAN},
  ],
  onSelected: (value) => onPopupMenuClick(value, provider),
),

AppBarButton<SourceSortType>(
  icon: Icon(Icons.sort_by_alpha),
  tooltip: "排序",
  menus: [
    {'title': '按型別',
      'icon': _type == SourceSortType.type ? _axisIcon : null,
      'type': SourceSortType.type},
    {'title': '按名稱',
      'icon': _type == SourceSortType.name ? _axisIcon : null,
      'type': SourceSortType.name},
    {'title': '按作者',
      'icon': _type == SourceSortType.author ? _axisIcon : null,
      'type': SourceSortType.author},
    {'title': '按分組',
      'icon': _type == SourceSortType.group ? _axisIcon : null,
      'type': SourceSortType.group},
    {'title': '按修改時間',
      'icon': _type == SourceSortType.updateTime ? _axisIcon : null,
      'type': SourceSortType.updateTime},
    {'title': '按建立時間',
      'icon': _type == SourceSortType.createTime ? _axisIcon : null,
      'type': SourceSortType.createTime},
  ],
  menuSelected: _type,
  onSelected: (value) {
    provider.sort(value, _profile)
        .whenComplete(() => refreshData(provider));
  },
)