RxSwift callback chaining

RxSwiftExample.swift

    func fetchUserId() -> Observable<String> {
        return create{ (observer) -> Disposable in
            Client.fetchUserId() { [unowned self]
                 (userId: String?, err: ErrorType?) -> Void in
                    if let _ = err{
                        observer.on(Event.Error(err!))
                    } else {
                        observer.on(Event.Next(userId))
                        observer.on(Event.Completed)
                    }
                }

            return NopDisposable.instance
        }
    }
    
    func fetchUserFollowersCount(userId: String) -> Observable<Int> {
        return create{ (observer) -> Disposable in
            Client.fetchUserFollowerCount(userId) { [unowned self]
                 (userFollowersCount: Int?, err: ErrorType?) -> Void in
                    if let _ = err{
                        observer.on(Event.Error(err!))
                    } else {
                        observer.on(Event.Next(userFollowersCount))
                        observer.on(Event.Completed)
                    }
                }

            return NopDisposable.instance
        }
    }
    
    // impl
    fetchUserId().flatMap() { userId in
      return fetchUserFollowersCount(userId)
    }
    .observeOn(MainScheduler.sharedInstance)
    .subscribeNext { userFollowersCount in
      ...
    }