private func detectFaces(completion: @escaping ([VNFaceObservation]?) -> Void) {
guard let image = UIImage(named: photos[currentIndex]),
let cgImage = image.cgImage,
let orientation = CGImagePropertyOrientation(rawValue: UInt32(image.imageOrientation.rawValue)) else { return completion(nil) }
let request = VNDetectFaceRectanglesRequest()
let handler = VNImageRequestHandler(cgImage: cgImage, orientation: orientation, options: [:])
DispatchQueue.global().async {
try? handler.perform([request])
guard let observations = request.results as? [VNFaceObservation] else { return completion(nil) }
completion(observations)
}