ImageScanner+DetectRectangle.swift

extension ImageScanner {
    private func detectRectangle(on image: CIImage, orientation: CGImagePropertyOrientation) -> Result<VNRectangleObservation, Error> {
        var result: Result<VNRectangleObservation, Error> = .failure(ImageProviderError.internalError)
        let semaphore = DispatchSemaphore(value: 1) // ➊
        let rectanglesRequest = VNDetectRectanglesRequest { request, error in // ➋
            guard error == nil,
                  let observations = request.results as? [VNRectangleObservation],
                  let detectedRectangle = observations.first else {
                return
            }
            result = .success(detectedRectangle)
            semaphore.signal()
        }
        let handler = VNImageRequestHandler(ciImage: image, orientation: orientation) // ➌
        if (try? handler.perform([rectanglesRequest])) != nil { semaphore.wait() } // ➍
        return result
    }
}