data class ApplicationState(
    val counter: Int = 0
)

sealed class CounterAction : Action {
    object Increment : CounterAction()
    object Reset : CounterAction()
}

object CounterReducer : Reducer<ApplicationState> {
    override fun reduce(action: Action, state: ApplicationState): ApplicationState =
        when (action) {
            is CounterAction.Increment ->
                state.copy(counter = state.counter.inc())
            is CounterAction.Reset ->
                state.copy(counter = 0)
            else ->
                state
        }
}

class ApplicationStore(
    initialState: ApplicationState,
    middlewares: List<Middleware<ApplicationState>>,
    reducers: List<Reducer<ApplicationState>>
) : AbstractStore<ApplicationState>(initialState, middlewares, reducers)

object AppComponent {
    val store = ApplicationStore(
        initialState = ApplicationState(),
        middlewares = emptyList(),
        reducers = listOf(CounterReducer)
    )
}

fun dispatch(action: Action) =
    AppComponent.store.dispatch(action)
fun subscribe(subscription: Subscription<ApplicationState>) =
    AppComponent.store.subscribe(subscription)
fun unsubscribe(subscription: Subscription<ApplicationState>) =
    AppComponent.store.unsubscribe(subscription)

class CounterView(
    context: Context
) : FrameLayout(context) {
    private val counterSubscription = SubStateSubscription<ApplicationState, Int>(
        transform = { it.counter },
        onStateChange = { state: Int, _: Boolean -> handleCounterStateChange(state) }
    )
    private lateinit var counterTextView: TextView
    private lateinit var floatingActionButton: FloatingActionButton
    init {
        inflate(context, R.layout.view_counter, this)
        findViewsById()
        setOnClickListeners()
    }
    private fun findViewsById() {
        counterTextView = findViewById(R.id.counterTextView)
        floatingActionButton = findViewById(R.id.floatingActionButton)
    }
    private fun setOnClickListeners() {
        floatingActionButton.setOnClickListener { dispatch(CounterAction.Increment) }
    }
    override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        subscribeToStateChanges()
    }
    private fun subscribeToStateChanges() {
        subscribe(counterSubscription)
    }
    override fun onDetachedFromWindow() {
        unsubscribeFromStateChanges()
        super.onDetachedFromWindow()
    }
    private fun unsubscribeFromStateChanges() {
        unsubscribe(counterSubscription)
    }
    private fun handleCounterStateChange(state: Int) {
        counterTextView.text = state.toString()
    }
}

...
counterTextView.text = state.toString()
...

class MainActivity : AppCompatActivity<ApplicationState>() {
    private lateinit var contentViewGroup: ViewGroup
    override fun getStore(): Store<ApplicationState> =
        AppComponent.store
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        findViewsById()
        addCounterView()
    }
    private fun findViewsById() {
        contentViewGroup = findViewById(R.id.contentViewGroup)
    }
    private fun addCounterView() {
        contentViewGroup.addView(CounterView(context = this))
    }
}

object ActivityLifecycleMiddleware : Middleware<ApplicationState> {
    override fun handleAction(
        action: Action,
        state: ApplicationState,
        next: Next<ApplicationState>
    ): Action {
        val newAction = when (action) {
            is ActivityLifecycleAction.OnDestroy ->
                handleActivityOnDestroy(action)
            else ->
                action
        }
        return next(newAction, state)
    }
    private fun handleActivityOnDestroy(action: ActivityLifecycleAction.OnDestroy): Action =
        if (action.isFinishing) CounterAction.Reset else action
}

store = ApplicationStore(
    initialState = ApplicationState(),
    middlewares = listOf(ActivityLifecycleMiddleware),
    reducers = listOf(CounterReducer)
)