class MainActivity: AppCompatActivity() {
    //...Rest of MainActivity
    override fun onCreate(savedInstanceState: Bundle?) {
        //...Rest of onCreate()
        val model = ViewModelProviders.of(this).get(TeamViewModel::class.java)
        subscribeUi(model)
    }

    private fun subscribeUi(viewModel: TeamViewModel) {
        viewModel.getTeams().observe(this, Observer { teams ->
            // update UI
            teams?.let {
                (teams_list.adapter as TeamAdapter).setTeamList(it)
            }
        })

        viewModel.getIsLoading().observe(this, Observer { value ->
            value?.let { show ->
                loading_spinner.visibility = if (show) View.VISIBLE else View.GONE
            }
        })

        viewModel.shouldShowError().observe(this, Observer { value ->
            value?.let { show ->
                tv_error.visibility = if (show) View.VISIBLE else View.GONE
            }
        })
    }
}