MainActivity.kt

class MainActivity : AppCompatActivity() {

    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)

        username.doAfterTextChanged { text -> viewModel.username = text?.toString() ?: "" }
        password.doAfterTextChanged { text -> viewModel.password = text?.toString() ?: "" }

        viewModel.isFormValid.observe(this, Observer { valid ->
            login.isEnabled = valid ?: false
        })
    }
}