Implement scanning QRCode ViewController via AVCaptureSession, AVCaptureMetadataOutput and AVCaptureVideoPreviewLayer

My sample project source code in GitHub

Prerequisite

Initialization

Carry out all the initializations in viewDidLoad

Initialize AVCaptureSession

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captureSession = AVCaptureSession()

Add video input to the capture session

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guard let videoCaptureDevice = AVCaptureDevice.default(for: .video) else{
    return
}
guard let videoInput = try? AVCaptureDeviceInput(device: videoCaptureDevice) else{
    return
}

if (captureSession.canAddInput(videoInput)) {
    captureSession.addInput(videoInput)
} else {
    return
}

Add metadata output to the capture session

Use AVCaptureMetadataOutput for capturing QRCode or others
Metadata object type:
.qr -> QRCode
.code128 -> Bar code

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let metadataOutput = AVCaptureMetadataOutput()
if (captureSession.canAddOutput(metadataOutput)) {
    captureSession.addOutput(metadataOutput)

    metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
    metadataOutput.metadataObjectTypes = [.qr, .code128]
} else {
    return
}

Add preview layer to the ViewController

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previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
previewLayer.frame = view.layer.bounds
previewLayer.videoGravity = .resizeAspectFill
view.layer.addSublayer(previewLayer)

Start capture session

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DispatchQueue.global().async {
    self.captureSession.startRunning()
}

Start & Stop capture session

Start the capture session when view appears and stop the capture session when view disappears

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override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    
    if (captureSession?.isRunning == false) {
        DispatchQueue.global().async {
            self.captureSession.startRunning()
        }
    }
}

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)

    if (captureSession?.isRunning == true) {
        captureSession.stopRunning()
    }
}

Implement the delegate

Implement the AVCaptureMetadataOutputObjectsDelegate to get the scan result

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extension ScannerViewController: AVCaptureMetadataOutputObjectsDelegate{
    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 value = readableObject.stringValue else { return }
            AudioServicesPlaySystemSound(SystemSoundID(kSystemSoundID_Vibrate))
            
            // value here is the scanning result string
        }
    }
}