call the handwriten recognition for an image

.swift

    public func retrieveTextOnImage(_ image: UIImage, completion: @escaping (String?, NSError?) -> ()) {
        
        assert(CognitiveServicesComputerVisionAPIKey.characters.count > 0, 
               "Please set the value of the API key variable (CognitiveServicesVisualFeaturesAPIKey) before attempting to use the application.")
        
        var urlString = CognitiveServicesConfiguration.HandwrittenOcrURL
        urlString += "?\(CognitiveServicesHTTPParameters.Handwriting)=true"
        
        let url = URL(string: urlString)
        print("calling the following URL: \(String(describing: url))")
        let request = NSMutableURLRequest(url: url!)
        
        request.addValue(CognitiveServicesComputerVisionAPIKey, 
                         forHTTPHeaderField: CognitiveServicesHTTPHeader.SubscriptionKey)
        request.addValue(CognitiveServicesHTTPContentType.OctetStream, 
                         forHTTPHeaderField: CognitiveServicesHTTPHeader.ContentType)
        
        let requestData = UIImageJPEGRepresentation(image, 
                                                    CognitiveServicesConfiguration.JPEGCompressionQuality)
        request.httpBody = requestData
        request.httpMethod = CognitiveServicesHTTPMethod.POST
        
        let session = URLSession.shared
        let task = session.dataTask(with: request as URLRequest) { (data, response, error) in
            print("executed task")
            if let error = error {
                completion(nil, error as NSError?)
                return
            }
            
            let headerFields = (response as! HTTPURLResponse).allHeaderFields
            
            let operationID = headerFields["Operation-Location"] as? String
            if let opID = operationID {
                completion(opID, nil)
            } else {
                completion(nil, nil)
            }
            
        }
        
        task.resume()
    }