Subscribe to substate?

[Dimillian]

I'm working on fairly complexe application, and I'm splitting my AppState into multiple substate, so I have multiple reducers and states. It works fine, but is there a way to subscribe to change only happening to a specific part of my state?

To get you an idea here is my AppState

class AppState {
    var userState = UserState()
}

and my UserState

class UserState {
    var currentUser: String? = null
    var users = mutableMapOf<String, User>()
}

and my AppReducer

class AppReducer: Reducer<AppState> {
    val userReducer = UserReducer()

    override fun reduce(state: AppState, action: Action?): AppState {
        userReducer.reduce(state.userState, action)
        return state
    }
}

So when I subscribe, I would want to do something like this:

store.subscribe { state.userState ->
  Log.d("State change", state.userState.currentUser)
}

is that something possible or planned?

Thanks!

[brianegan]

Hey there :) Sorry for the delay -- haven't been checking in too actively on this project as of late. We don't have built-in support, but I could see this being achieved pretty easily with a little helper class that looks something like:

class CurrentUserSubscriber(val store: Store<AppState>) {
    fun subscribe(subscriber: (User?) -> Unit): Subscription {
        val subscription = store.subscribe {
            // Use this part of the function to narrow down to the part of the state you care about
            subscriber(it.userState.currentUser)
        }

        return Subscription { subscription.unsubscribe() };
    }
}

This would be used like so:

@Test
    fun `store should be able to subscribe to substate`() {
        val reducer = Reducer<AppState> { state, action ->
            when (action) {
                is User -> state.copy(userState = state.userState.copy(currentUser = action))
                else -> state
            }
        }
        val store = BaseStore(AppState(), reducer);
        var currentUser: User? = null

        // Create an instance of our Substate-Subscriber     
        val currentUserSubscriber = CurrentUserSubscriber(store)

        // Subscribe to the substate using our class
        currentUserSubscriber.subscribe { currentUser = it }

        val user = User("Brian")

        // Dispatch a new User to the store
        store.dispatch(user)

        // Shows that our substate subscriber worked!
        assertThat(currentUser).isEqualTo(user)
    }