How do I pass the ontap function – Flutter

You can use callback method update widget based on tap,

You can follow this example code.

void main() => runApp(MaterialApp(home: MyApp()));

class MyApp extends StatefulWidget {
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  int currentIndex = 0;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: MyBottomNavBar(
          currentIndex: currentIndex,
          onTap: (index) {
            setState(() {
              currentIndex = index;
            });
          }),
      body: Center(
        child: [Text('1'), Text('2'), Text('3')][currentIndex],
      ),
    );
  }
}

class MyBottomNavBar extends StatelessWidget {
  const MyBottomNavBar({
    super.key,
    required this.onTap,
    required this.currentIndex,
  });

  final Function(int) onTap;
  final int currentIndex;

  @override
  Widget build(BuildContext context) {
    return BottomNavigationBar(
      currentIndex: currentIndex,
      onTap: onTap,
      items: const [
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
        ),
      ],
    );
  }
}

If you are using on same widget, Follow doc example