let imageDataSubject = PublishSubject<Data>()

let scheduler = ConcurrentDispatchQueueScheduler(qos: .background)

imageDataSubject
    .observeOn(scheduler)
    .map { (imageData) -> UIImage in
        if let image = UIImage(data: imageData) {
            return image
        } else {
            return UIImage()
        }
    }
    .observeOn(MainScheduler.instance)
    .subscribe(onNext: { (image) in

        imageView.image = image
    })
    .addDisposableTo(disposeBag)

imageDataSubject.onNext(swiftImageData)