I've got error:

navigation destination com.example.app.myapp/action_homeFragment_to_mainFragment is unknown to this NavController

mainFragmentViewModel.navigateToHome.observe(this, Observer {
            if (it) {
                this.findNavController().navigate(R.id.action_mainFragment_to_homeFragment)
            } else {
                this.findNavController().navigate(R.id.action_homeFragment_to_mainFragment)
            }
        })

You cannot access an action defined in HomeFragment tag of your Navigation graph in Main Fragment. You should handle going to MainFragment from HomeFragment in Home Fragment.

okay. get it now