title: Swift 4 QRCode Scanning
body: <p>That's the code I'm trying to scan QRCodes</p>
  
  <pre><code>import AVFoundation
  import UIKit
  
  class ScannerViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
      var captureSession: AVCaptureSession!
      var previewLayer: AVCaptureVideoPreviewLayer!
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          view.backgroundColor = UIColor.black
          captureSession = AVCaptureSession()
  
          guard let videoCaptureDevice = AVCaptureDevice.default(for: .video) else { return }
          let videoInput: AVCaptureDeviceInput
  
          do {
              videoInput = try AVCaptureDeviceInput(device: videoCaptureDevice)
          } catch {
              return
          }
  
          if (captureSession.canAddInput(videoInput)) {
              captureSession.addInput(videoInput)
          } else {
              failed()
              return
          }
  
          let metadataOutput = AVCaptureMetadataOutput()
  
          if (captureSession.canAddOutput(metadataOutput)) {
              captureSession.addOutput(metadataOutput)
  
              metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
              metadataOutput.metadataObjectTypes = [.qr]
          } else {
              failed()
              return
          }
  
          previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
          previewLayer.frame = view.layer.bounds
          previewLayer.videoGravity = .resizeAspectFill
          view.layer.addSublayer(previewLayer)
  
          captureSession.startRunning()
      }
  
      func failed() {
          let ac = UIAlertController(title: "Scanning not supported", message: "Your device does not support scanning a code from an item. Please use a device with a camera.", preferredStyle: .alert)
          ac.addAction(UIAlertAction(title: "OK", style: .default))
          present(ac, animated: true)
          captureSession = nil
      }
  
      override func viewWillAppear(_ animated: Bool) {
          super.viewWillAppear(animated)
  
          if (captureSession?.isRunning == false) {
              captureSession.startRunning()
          }
      }
  
      override func viewWillDisappear(_ animated: Bool) {
          super.viewWillDisappear(animated)
  
          if (captureSession?.isRunning == true) {
              captureSession.stopRunning()
          }
      }
  
      func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputMetadataObjects metadataObjects: [Any]!, from connection: AVCaptureConnection!) {
          captureSession.stopRunning()
  
          if let metadataObject = metadataObjects.first {
              guard let readableObject = metadataObject as? AVMetadataMachineReadableCodeObject else { return }
              guard let stringValue = readableObject.stringValue else { return }
              AudioServicesPlaySystemSound(SystemSoundID(kSystemSoundID_Vibrate))
              found(code: stringValue)
          }
  
          dismiss(animated: true)
      }
  
      func found(code: String) {
          print(code)
      }
  
      override var prefersStatusBarHidden: Bool {
          return true
      }
  
      override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
          return .portrait
      }
  }
  </code></pre>
  
  <p>Found it <a href="https://www.hackingwithswift.com/example-code/media/how-to-scan-a-qr-code" rel="nofollow noreferrer">here</a>. <br>
  But it doesn't work, everything is set correctly, I get the <code>ViewController</code> loaded and the <code>captureSession</code> running, but when I point to a QRcode nothing happens, <code>captureOutput()</code> is never called. <br></p>