How to set border radius to bottom app bar in a flutter app?

Question

I want to set the borderRadius to an Bottom Navigation App Bar as its shown in the image. I have tried to put Bottom Navigation App Bar to a ClipRRect borderRadius and in a Container decoration but it didn't worked. So how can I apply the topLeft, and topRight border radius to my bottom navigation bar. Kindly help to let me know how can I do it?

main.dart

    void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Food Ordering',
      theme: ThemeData(primarySwatch: Colors.blue, primaryColor: Colors.white),
      home: MyStatefulWidget(),
      routes: <String, WidgetBuilder>{
        '/detail-page': (BuildContext context) => MyDetailPage(),
      },
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  int _selectedIndex = 0;
  static const TextStyle optionStyle =
      TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
  static List<Widget> _widgetOptions = <Widget>[
    HomePage(),
    HomePage(),
    HomePage(),
    HomePage(),
  ];

  void _onItemTapped(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: _widgetOptions.elementAt(_selectedIndex),
      ),
      bottomNavigationBar: BottomNavigationBar(
          items: <BottomNavigationBarItem>[
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-home.png'),
              title: Text('Home'),
            ),
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-mentors.png'),
              title: Text('Mentors'),
            ),
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-messages.png'),
              title: Text('Messages'),
            ),
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-settings.png'),
              title: Text('Settings'),
            ),
          ],
          currentIndex: _selectedIndex,
          selectedItemColor: Colors.blue,
          onTap: _onItemTapped),
    );
  }
}

Answer 1

EDIT

Scaffold now has a property called extendBody which can be used to extend the body below a bottomBar. From the documentation,

If true, and bottomNavigationBar or persistentFooterButtons is specified, then the body extends to the bottom of the Scaffold, instead of only extending to the top of the bottomNavigationBar or the persistentFooterButtons.

This means that all you need to do is

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Some Text'),
      ),
      body: bodyContent,
      extendBody: true,
      bottomNavigationBar: bottomNavigationBar,
    );
  }

  Widget get bodyContent {
    return Container(color: Colors.red);
  }

  Widget get bottomNavigationBar {
    return ClipRRect(
      borderRadius: const BorderRadius.only(
        topRight: Radius.circular(40),
        topLeft: Radius.circular(40),
      ),
      child: BottomNavigationBar(
        items: const [
          BottomNavigationBarItem(icon: Icon(Icons.home), label: '1'),
          BottomNavigationBarItem(icon: Icon(Icons.usb), label: '2'),
          BottomNavigationBarItem(
              icon: Icon(Icons.assignment_ind), label: '3'),
          BottomNavigationBarItem(
              icon: Icon(Icons.multiline_chart), label: '4'),
        ],
        unselectedItemColor: Colors.grey,
        selectedItemColor: Colors.black,
        showUnselectedLabels: true,
      ),
    );
  }
}

OUTDATED

Put it inside a stack. Don't add the Bottom Navigation Bar to the scaffold directly.

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Some Text'),
      ),
      body: Stack(
        children: <Widget>[
          bodyContent,
          Positioned(
            left: 0,
            right: 0,
            bottom: 0,
            child: bottomNavigationBar,
          ),
        ],
      ),
    );
  }

  Widget get bodyContent {
    return Container(color: Colors.red);
  }

  Widget get bottomNavigationBar {
    return ClipRRect(
      borderRadius: BorderRadius.only(
        topRight: Radius.circular(40),
        topLeft: Radius.circular(40),
      ),
      child: BottomNavigationBar(
        items: [
          BottomNavigationBarItem(icon: Icon(Icons.home), title: Text('1')),
          BottomNavigationBarItem(icon: Icon(Icons.usb), title: Text('2')),
          BottomNavigationBarItem(
              icon: Icon(Icons.assignment_ind), title: Text('3')),
          BottomNavigationBarItem(
              icon: Icon(Icons.multiline_chart), title: Text('4')),
        ],
        unselectedItemColor: Colors.grey,
        selectedItemColor: Colors.black,
        showUnselectedLabels: true,
      ),
    );
  }
}