CounterViewModel.kt

sealed class CounterIntent : Intent {
  object IncrementIntent : CounterIntent()
  object DecrementIntent : CounterIntent()
}

sealed class CounterAction : Action {
    data class UpdateCountAction(val count: Int) : CounterAction()
}

data class CounterState(val count: Int = 0) : State

class CounterViewModel : OwlViewModel<CounterIntent, CounterAction, CounterState>(initialState = CounterState()) {
    override fun intentToAction(intent: CounterIntent, state: CounterState): CounterAction = when (intent) {
        IncrementIntent -> UpdateCountAction(state.count + 1)
        DecrementIntent -> UpdateCountAction(state.count - 1)
    }

    override fun reducer(state: CounterState, action: CounterAction): CounterState = when (action) {
        is UpdateCountAction -> state.copy(count = action.count)
    }
}

class MainActivity : AppCompatActivity() {

    private val counterViewModel: CounterViewModel by lazy {
        ViewModelProviders.of(this).get(CounterViewModel::class.java)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        ...
        buttonIncrement.setOnClickListener {
            counterViewModel.dispatch(IncrementIntent)
        }
        buttonDecrement.setOnClickListener {
            counterViewModel.dispatch(DecrementIntent)
        }
        counterViewModel.state.observe(this, Observer { state ->
            textCount.text = "count: ${state.count}"
        })
        ...
    }
}