didOutput sampleBuffer in ViewController.swift

// MARK: - AVCaptureVideoDataOutputSampleBufferDelegate

/* This delegate is fired periodically every time a new video frame is written. 
    It is called on the dispatch queue specified while setting up the capture session.
*/
func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        
        /* Initialise CVPixelBuffer from sample buffer 
           CVPixelBuffer is the input type we will feed our coremlmodel . 
        */
        guard let pixelBuffer: CVPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else { return }

        /* Initialise Core ML model 
           We create a model container to be used with VNCoreMLRequest based on our HandSigns Core ML model.
        */
        guard let handSignsModel = try? VNCoreMLModel(for: HandSigns().model) else { return }

        /* Create a Core ML Vision request 
           The completion block will execute when the request finishes execution and fetches a response.
         */
        let request =  VNCoreMLRequest(model: handSignsModel) { (finishedRequest, err) in

            /* Dealing with the result of the Core ML Vision request
              The request's result is an array of VNClassificationObservation object which holds 
              identifier - The prediction tag we had defined in our Custom Vision model - FiveHand, FistHand, VictoryHand, NoHand
              confidence - The confidence on the prediction made by the model on a scale of 0 to 1                                                 
            */                                                          
            guard let results = finishedRequest.results as? [VNClassificationObservation] else { return }

            /* Results array holds predictions iwth decreasing level of confidence. 
               Thus we choose the first one with highest confidence. */
            guard let firstResult = results.first else { return }
                                                               
            var predictionString = ""
            
            /* Depending on the identifier we set the UILabel text with it's confidence.
               We update UI on the main queue. */         
            DispatchQueue.main.async {
                switch firstResult.identifier {
                case HandSign.fistHand.rawValue:
                    predictionString = "Fist👊🏽"
                case HandSign.victoryHand.rawValue:
                    predictionString = "Victory✌🏽"
                case HandSign.fiveHand.rawValue:
                    predictionString = "High Five🖐🏽"
                case HandSign.noHand.rawValue:
                    predictionString = "No Hand ❎"
                default:
                    break
                }
                self.predictionLabel.text = predictionString + "(\(firstResult.confidence))"
            }
        }

        /* Perform the above request using Vision Image Request Handler
           We input our CVPixelbuffer to this handler along with the request declared above.
        */
        try? VNImageRequestHandler(cvPixelBuffer: pixelBuffer, options: [:]).perform([request])
    }

}