class MainActivity : AppCompatActivity() {

    private lateinit var viewModel: MainViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        viewModel = ViewModelProviders.of(this).get(MainViewModel::class.java)
        viewModel.counter.observe(this, Observer {
            counter_button.text = it.toString()
        })

        counter_button.setOnClickListener {
            viewModel.incrementCounter()
        }
    }
}