Problems with creating Observable when using @escaping closures

Question

I am trying to code an observable for NSManagedObjectContext save() operation with no success. The function does not fire neither onNext nor onCompleted event and is being disposed immediately. When I debug with breakpoints it shows Disposables.create() and @escaping notification closure work on different threads. Please tell what am I doing wrong in the code below and help to fix the problem.

func save() -> Observable<Void> {
    guard base.hasChanges else { return Observable.just(()) }
    return Observable.create { observer -> Disposable in
        var token: NSObjectProtocol?
        token = NotificationCenter.default.addObserver(forName: .NSManagedObjectContextObjectsDidChange, object: self, queue: nil) { _ in
            observer.onNext(()) /* Is executed on the other thread and is never called */
            observer.onCompleted()
            NotificationCenter.default.removeObserver(token!)
        }
        
        base.perform {
            do {
                try base.save()
            } catch {
                observer.onError(Error.some)
            }
        }
        return Disposables.create() \* Immediately calls this func *\
    }
}

Answer 1

func save() -> Observable<Void> {
    guard base.hasChanges else { return Observable.just(()) }
    return Observable.create { observer -> Disposable in
        let queue = DispatchQueue(label: "Save")
        queue.sync {
            self.base.perform {
                do {
                    try self.base.save()
                    observer.onNext(())
                    observer.onCompleted()
                } catch {
                    observer.onError(NSManagedObjectContext.ReactiveError.save)
                }
            }                
        }
        return Disposables.create()
    }
}