MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
      
        val vm = ViewModelProviders.of(this)[CounterViewModel::class.java]
        vm.counter.observe(this, Observer({ textView.text = it.toString() }))
        button.setOnClickListener { vm.onButtonClick() }
    }
}