如何在Flutter中制作自定义底部导航栏

我有这样的底部导航栏设计：

我尽力使用以下代码创建自定义底部导航栏：

class FABBottomAppBarItem {
  FABBottomAppBarItem({this.iconData, this.text});
  IconData iconData;
  String text;
}

class FABBottomAppBar extends StatefulWidget {
  FABBottomAppBar({
    this.items,
    this.centerItemText,
    this.height: 60.0,
    this.iconSize: 24.0,
    this.backgroundColor,
    this.color,
    this.selectedColor,
    this.notchedShape,
    this.onTabSelected,
  }) {
    assert(this.items.length == 2 || this.items.length == 4);
  }
  final List<FABBottomAppBarItem> items;
  final String centerItemText;
  final double height;
  final double iconSize;
  final Color backgroundColor;
  final Color color;
  final Color selectedColor;
  final NotchedShape notchedShape;
  final ValueChanged<int> onTabSelected;

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

class FABBottomAppBarState extends State<FABBottomAppBar> {
  int _selectedIndex = 0;

  _updateIndex(int index) {
    widget.onTabSelected(index);
    setState(() {
      _selectedIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    List<Widget> items = List.generate(widget.items.length, (int index) {
      return _buildTabItem(
        item: widget.items[index],
        index: index,
        onPressed: _updateIndex,
      );
    });
    items.insert(items.length >> 1, _buildMiddleTabItem());

    return BottomAppBar(
      shape: widget.notchedShape,
      child: Row(
        mainAxisSize: MainAxisSize.max,
        mainAxisAlignment: MainAxisAlignment.spaceAround,
        children: items,
      ),
      color: widget.backgroundColor,
    );
  }

  Widget _buildMiddleTabItem() {
    return Expanded(
      child: SizedBox(
        height: widget.height,
        child: Column(
          mainAxisSize: MainAxisSize.min,
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            SizedBox(height: widget.iconSize),
            Text(
              widget.centerItemText ?? '',
              style: TextStyle(color: widget.color),
            ),
          ],
        ),
      ),
    );
  }

  Widget _buildTabItem({
    FABBottomAppBarItem item,
    int index,
    ValueChanged<int> onPressed,
  }) {
    Color color = _selectedIndex == index ? widget.selectedColor : widget.color;
    return Expanded(
      child: SizedBox(
        height: widget.height,
        child: Material(
          type: MaterialType.transparency,
          child: InkWell(
            onTap: () => onPressed(index),
            child: Column(
              mainAxisSize: MainAxisSize.min,
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Icon(item.iconData, color: color, size: widget.iconSize),
                Text(
                  item.text,
                  style: TextStyle(color: color),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

我实现了这样的自定义：

bottomNavigationBar: FABBottomAppBar(
          centerItemText: 'เสา',
          color: Colors.grey,
          backgroundColor: Colors.white,
          selectedColor: Colors.red,
          notchedShape: CircularNotchedRectangle(),
          onTabSelected: _onTapped,
          items: [
            FABBottomAppBarItem(iconData: Icons.home, text: 'หน้าแรก'),
            FABBottomAppBarItem(iconData: Icons.search, text: 'ค้นหา'),
            FABBottomAppBarItem(iconData: Icons.account_circle, text: 'โปรไฟล์'),
            FABBottomAppBarItem(iconData: Icons.more_horiz, text: 'อื่นๆ'),
          ],
        ),
        body: _list[_page],
        floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
        floatingActionButton: FloatingActionButton(
          onPressed: () {
          },
          child: Icon(Icons.add),
          elevation: 2.0,
        ),
      ),

该代码的结果是：

如何使我的Custom Nav Bar像设计一样？因为在设计中，FAB/Asset/Icon底部导航栏的内部和curved upwards我arrows在设计中标记的一样。