Update from swift 4 breaks app - ios

Question

My friends and I recently updated from swift 3.2 to swift 4. Unfortunately, our project no longer functions correctly for our QR reader function. The video output works fine, so when we load the page the screen displays when the phone is looking at. However, it will no longer place the sizing box on the screen or recognize the QR codes in front of it.
import UIKit
import AVFoundation
class ScanViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
#IBOutlet var messageLabel:UILabel!
#IBOutlet var topbar: UIView!
var captureSession: AVCaptureSession?
var videoPreviewLayer: AVCaptureVideoPreviewLayer?
var qrCodeFrameView: UIView?
override func viewDidLoad() {
super.viewDidLoad()
// Get an instance of the AVCaptureDevice class to initialize a device object and provide the video as the media type parameter.
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
do {
// Get an instance of the AVCaptureDeviceInput class using the previous device object.
let input = try AVCaptureDeviceInput(device: captureDevice)
// Initialize the captureSession object.
captureSession = AVCaptureSession()
// Set the input device on the capture session.
captureSession?.addInput(input)
// Initialize a AVCaptureMetadataOutput object and set it as the output device to the capture session.
let captureMetadataOutput = AVCaptureMetadataOutput()
captureSession?.addOutput(captureMetadataOutput)
// Set delegate and use the default dispatch queue to execute the call back
captureMetadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
captureMetadataOutput.metadataObjectTypes = [AVMetadataObjectTypeQRCode]
// Initialize the video preview layer and add it as a sublayer to the viewPreview view's layer.
videoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
videoPreviewLayer?.videoGravity = AVLayerVideoGravityResizeAspectFill
videoPreviewLayer?.frame = view.layer.bounds
view.layer.addSublayer(videoPreviewLayer!)
// Move the message label and top bar to the front
view.bringSubview(toFront: messageLabel)
view.bringSubview(toFront: topbar)
// Start video capture.
captureSession?.startRunning()
// Initialize QR Code Frame to highlight the QR code
qrCodeFrameView = UIView()
if let qrCodeFrameView = qrCodeFrameView {
qrCodeFrameView.layer.borderColor = UIColor.randomColor().cgColor
qrCodeFrameView.layer.borderWidth = 4
view.addSubview(qrCodeFrameView)
view.bringSubview(toFront: qrCodeFrameView)
}
} catch {
// If any error occurs, simply print it out and don't continue any more.
print(error)
return
}
}
func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputMetadataObjects metadataObjects: [Any]!, from connection: AVCaptureConnection!) {
// Check if the metadataObjects array is not nil and it contains at least one object.
if metadataObjects == nil || metadataObjects.count == 0 {
qrCodeFrameView?.frame = CGRect.zero
messageLabel.text = "No QR code is detected"
return
}
// Get the metadata object.
let metadataObj = metadataObjects[0] as! AVMetadataMachineReadableCodeObject
if metadataObj.type == AVMetadataObjectTypeQRCode {
// If the found metadata is equal to the QR code metadata then update the status label's text and set the bounds
let barCodeObject = videoPreviewLayer?.transformedMetadataObject(for: metadataObj)
qrCodeFrameView?.frame = barCodeObject!.bounds
if metadataObj.stringValue != nil { //Output
messageLabel.text = metadataObj.stringValue
performSegue(withIdentifier: "QRFound", sender: self)
}
}
}
#IBAction func cancel(_ sender: UIButton) {
if let owningNavController = navigationController {
owningNavController.popViewController(animated: true)
}
dismiss(animated: true, completion: nil)
}
}

Answer 1

Your delegate function is not correct, so it’s never called. The correct one is:
optional func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection)
See the documentation page.
That seems to have changed in Swift 4.
EDIT: Also see this answer.