FirebaseService3.swift

public class FirebaseService {
    private let rootRef: DatabaseReference
    public init(ref: DatabaseReference) {
        self.rootRef = ref.root
    }

    // MARK: Observing Paths
    func observeSingleEvent<T>(at path: Path<T>) -> Single<T>
        where T: Decodable {
            let ref = rootRef.child(path.rendered)
            return ref.rx.observeSingleEvent(of: .value)
    }

    func observe<T>(at path: Path<T>) -> Observable<DecodeResult<T>>
        where T: Decodable {
            let ref = rootRef.child(path.rendered)
            return ref.rx.observe(eventType: .value)
    }

    // MARK: Observing Collection Paths
    func observeSingleEvent<T>(of type: CollectionEventType,
                               at path: Path<T>) -> Single<T>
        where T: Decodable {
            let ref = rootRef.child(path.rendered)
            return ref.rx.observeSingleEvent(of: type.firebaseEventType)
    }

    func observe<T>(eventType type: CollectionEventType,
                    at path: Path<T>.Collection) -> Observable<DecodeResult<T>>
        where T: Decodable {
            let ref = rootRef.child(path.rendered)
            return ref.rx.observe(eventType: type.firebaseEventType)
    }
 
    // ...
}