How to make custom Bottom Navigation Bar in Flutter

I have a design of Bottom Navigation Bar like this:

I try my best to create custom of Bottom Navigation Bar with this code:

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),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

And I implemented that custom like this:

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,
        ),
      ),

And the result of that code is:

How to make my Custom Nav Bar like the design? Because in the design, the FAB/Asset/Icon inside the Bottom Navigation Bar and have curved upwards as I marked with arrows in the design.

I edited CircularNotchedRectangle.

Use CircularOuterNotchedRectangle instead of CircularNotchedRectangle.

PS. I added extraOffset param for extra thick. but it's not working exactly correct. But I just wanted to show you how to approach.

class CircularOuterNotchedRectangle extends NotchedShape {
  /// Creates a [CircularOuterNotchedRectangle].
  ///
  /// The same object can be used to create multiple shapes.
  const CircularOuterNotchedRectangle({this.extraOffset = 10.0});

  final double extraOffset;

  /// Creates a [Path] that describes a rectangle with a smooth circular notch.
  ///
  /// `host` is the bounding box for the returned shape. Conceptually this is
  /// the rectangle to which the notch will be applied.
  ///
  /// `guest` is the bounding box of a circle that the notch accommodates. All
  /// points in the circle bounded by `guest` will be outside of the returned
  /// path.
  ///
  /// The notch is curve that smoothly connects the host's top edge and
  /// the guest circle.
  // TODO(amirh): add an example diagram here.
  @override
  Path getOuterPath(Rect host, Rect guest) {
    if (guest == null || !host.overlaps(guest)) return Path()..addRect(host);

    // The guest's shape is a circle bounded by the guest rectangle.
    // So the guest's radius is half the guest width.
    final double notchRadius = guest.width / 2.0;

    // We build a path for the notch from 3 segments:
    // Segment A - a Bezier curve from the host's top edge to segment B.
    // Segment B - an arc with radius notchRadius.
    // Segment C - a Bezier curve from segment B back to the host's top edge.
    //
    // A detailed explanation and the derivation of the formulas below is

    const double s1 = 15.0;
    const double s2 = 1.0;

    final double r = notchRadius + extraOffset/2;
    final double a = -1.0 * r - s2;
    final double b = host.top + guest.center.dy;

    final double n2 = math.sqrt(b * b * r * r * (a * a + b * b - r * r));
    final double p2xA = ((a * r * r) - n2) / (a * a + b * b);
    final double p2xB = ((a * r * r) + n2) / (a * a + b * b);
    final double p2yA = math.sqrt(r * r - p2xA * p2xA) - extraOffset/2;
    final double p2yB = math.sqrt(r * r - p2xB * p2xB) - extraOffset/2;

    final List<Offset> p = List<Offset>(6);

    // p0, p1, and p2 are the control points for segment A.
    p[0] = Offset(a - s1, b);
    p[1] = Offset(a, b);
    p[2] = p2yA > p2yB ? Offset(p2xA, -p2yA) : Offset(p2xB, p2yB);

    // p3, p4, and p5 are the control points for segment B, which is a mirror
    // of segment A around the y axis.
    p[3] = Offset(-1.0 * p[2].dx, -p[2].dy);
    p[4] = Offset(-1.0 * p[1].dx, p[1].dy);
    p[5] = Offset(-1.0 * p[0].dx, p[0].dy);

    // translate all points back to the absolute coordinate system.
    for (int i = 0; i < p.length; i += 1) p[i] += guest.center;

    return Path()
      ..moveTo(host.left, -host.top)
      ..lineTo(p[0].dx, p[0].dy)
      ..quadraticBezierTo(p[1].dx, p[1].dy, p[2].dx, -p[2].dy)
      ..arcToPoint(
        p[3],
        radius: Radius.circular(notchRadius),
        clockwise: true,
      )
      ..quadraticBezierTo(p[4].dx, p[4].dy, p[5].dx, p[5].dy)
      ..lineTo(host.right, host.top)
      ..lineTo(host.right, host.bottom)
      ..lineTo(host.left, host.bottom)
      ..close();
  }
}