UserFragment.kt

class UserFragment : Fragment() {

    private var usersViewModel: UserViewModel? = null

    override fun onCreateView(
        inflater: LayoutInflater, 
        container: ViewGroup?, 
        savedInstanceState: Bundle?
    ): View? = inflater.inflate(R.layout.fragment_users, container, false)

    override fun onViewCreated(
        view: View, 
        savedInstanceState: Bundle?
    ) {
        usersViewModel = ViewModelProviders
            .of(this)
            .get(UsersViewModel::class.java)

        usersViewModel.usersLiveData.observe(viewLifecycleOwner, Observer { resource ->
            resource.onSuccess { userList: HashMap<String, User> ->
                usersLoaded(user) 
            }.onLoading { partialUserList: HashMap<String, User> ->
                usersAreLoading(partialUserList)
                // show ProgressBar?
            }.onFailure { cause: Throwable ->
                userLoadFailed(cause)
            }
        })
    }

}