Less high level....

Thing.swift

public struct MessagesState: State, HasMessagesUnread, HasMessagesRead {
    public var unreadCount: UInt = 0
    public var messages = [String]()
    public var outbound = [String]()
    public var sentCount: UInt = 0
    public var readCount: UInt = 0
}

// supported state manipulation protocols.

public protocol HasMessagesUnread: State {
    var unreadCount: UInt { get set }
}

public protocol HasMessagesRead: State {
    var readCount: UInt { get set }
}

// .....

class MyController: Subscriber {

    let store = Store(state: MessagesState(), reducer: MessagesReducer())

    func doShit() {
        store.subscribe(self) { (state: HasMessagesUnread) in
            print("handling \(HasMessagesUnread.self)")
            print("unread, value = \(state.unreadCount)\n")
        }

        store.subscribe(self) { (state: HasMessagesRead) in
            print("handling \(HasMessagesRead.self)")
            print("read, value = \(state.readCount)\n")
        }

        store.subscribe(self) { (state: MessagesState) in
            print("handling \(MessagesState.self)")
            print("unread, value = \(state.unreadCount)")
            print("read, value = \(state.readCount)\n")
        }

        //let action1 = UpdateUnreadAction(value: 18)
        //store.dispatch(action1)

      /*
      This should hit the HasMessagesRead subscriber, as well as the
      MessagesState subscriber since the latter subscribes/implements the former.
      */
        let action2 = UpdateReadAction(value: 42)
        store.dispatch(action2)

        //let action3 = UpdateEverythingAction(unread: 1, read: 1)
        //store.dispatch(action3)
    }
    
}