how to hide BottomNavigationView on android-navigation lib

The accepted answer works, and it's the one recommended in the official documentation, but as stated on comments, it does cause some flickering, as the callback is executed before the fragment is attached.

I find the below answer more flexible, and handles animations better:

supportFragmentManager.registerFragmentLifecycleCallbacks(object : FragmentManager.FragmentLifecycleCallbacks() {
        override fun onFragmentViewCreated(fm: FragmentManager, f: Fragment, v: View, savedInstanceState: Bundle?) {
            TransitionManager.beginDelayedTransition(binding.root, Slide(Gravity.BOTTOM).excludeTarget(R.id.nav_host_fragment, true))
            when (f) {
                is ModalFragment -> {
                    binding.bottomNavigation.visibility = View.GONE
                }
                else -> {
                    binding.bottomNavigation.visibility = View.VISIBLE
                }
            }
        }
    }, true)

You can customize it depending on the transitions between your fragments, by choosing different animation (on my example it's a Slide), or by making the call at another lifecycle callback.

You could do something like this in your activity's onCreate. When ever an item in the nav bar is selected it will show or hide the nav based on the fragment id's.

private fun setupNav() {
    val navController = findNavController(R.id.nav_host_fragment)
    findViewById<BottomNavigationView>(R.id.bottomNav)
        .setupWithNavController(navController)

    navController.addOnDestinationChangedListener { _, destination, _ ->
        when (destination.id) {
            R.id.mainFragment -> showBottomNav()
            R.id.mineFragment -> showBottomNav()
            else -> hideBottomNav()
        }
    }
}

private fun showBottomNav() {
    bottomNav.visibility = View.VISIBLE
    
}

private fun hideBottomNav() {
    bottomNav.visibility = View.GONE
   
}