Styjun

I am making a QR code scanner app where my camera finds the code and have to segue into another view. But i can segue into another view because the app randomly closes if i call the self.performSegue after i find the code.

If i do not call the segue the code prints but the camera is froze. The app does not close if i get rid of the segue line

import UIKit
import Firebase
import AVFoundation

class ScannerViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate 

 var intendedDest = String()
 var codex = String()

 var captureSession: AVCaptureSession!
 var previewLayer: AVCaptureVideoPreviewLayer!

 override func viewDidLoad() 
 super.viewDidLoad()
 // Do any additional setup after loading the view, typically from a nib.

 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.global(qos: .userInteractive))
 metadataOutput.metadataObjectTypes = [.ean8, .ean13, .pdf417, .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()
 
 

 override func didReceiveMemoryWarning() 
 super.didReceiveMemoryWarning()
 // Dispose of any resources that can be recreated.
 

 func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], 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)
 

 self.performSegue(withIdentifier: "toTimerView", sender: self)
 

 func found(code: String) 
 // qr code found!
 print(code)
 codex = code
 print(codex)
 

 override func prepare(for segue: UIStoryboardSegue, sender: Any?) 
 if let destination = segue.destination as? TimerViewController 
 destination.intendedDestxx = intendedDest
 
 

 override var prefersStatusBarHidden: Bool 
 return true
 

 override var supportedInterfaceOrientations: UIInterfaceOrientationMask 
 return .portrait
 

'''

According to your warning and crash message you accessing UI in background thread.

Make sure that your UI code is called in main thread.

In case of background threads you can fix it by:

DispatchQueue.main.async 
 /// your code here

In your code I see that you setting yourself as delegate of image stream on background queue. It mean your segue called on background as well. Fix it by:

func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], 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)
 

 DispatchQueue.main.async 
 self.performSegue(withIdentifier: "toTimerView", sender: self)