import AVFoundation
class QRScannerViewController: UIViewController,
     AVCaptureMetadataOutputObjectsDelegate {
   
    func viewDidLoad() {
        self.initCaptureSession()
    }
    
    private func initCaptureSession() {
        let captureDevice = AVCaptureDevice
            .defaultDevice(withMediaType: AVMediaTypeVideo)
        do {
            let input = try AVCaptureDeviceInput(device: captureDevice)
            let captureMetadataOutput = AVCaptureMetadataOutput()
            self.captureSession?.addOutput(captureMetadataOutput)
            captureMetadataOutput.setMetadataObjectsDelegate(self,
                 queue: DispatchQueue.main)
            captureMetadataOutput
                .metadataObjectTypes = [AVMetadataObjectTypeQRCode]
            
            self.videoPreviewLayer = 
                AVCaptureVideoPreviewLayer(session: self.captureSession)
            self.videoPreviewLayer?
                .videoGravity = AVLayerVideoGravityResizeAspectFill
            self.videoPreviewLayer?.frame =    
                self.view.layer.bounds

            self._viewController?.view.layer
                .addSublayer(videoPreviewLayer!)
            self.captureSession?.startRunning()
        } catch {
            //TODO: handle input open error
        }
    }
    private func dismissCaptureSession() {
        if let running = self.captureSession?.isRunning, running {
            self.captureSession?.stopRunning()
        }
        self.captureSession = nil
        self.videoPreviewLayer?.removeFromSuperLayer()
        self.videoPreviewLayer = nil
    }
    
    func captureOutput(_ captureOutput: AVCaptureOutput, 
        didOutputMetadataObjects metadataObjects: [Any]!, 
        from connection: AVCaptureConnection) {
        guard metadataObjects != nil && metadataObjects.count != 0 else {
            //Nothing captured
            return
        }

        if let metadataObj = 
            metadataObjects[0] as? AVMetadataMachineReadableCodeObject {
            guard metadataObj.type == AVMetadataObjectTypeQRCode else {
                return
            }

            let barCodeObject = videoPreviewLayer?
                .transformedMetadataObject(for: 
                    metadataObj as AVMetadataMachineReadableCodeObject)
                 as! AVMetadataMachineReadableCodeObject
        
            if let qrValue = metadataObj.stringValue {
                self.handleQRRead(value: qrValue)
            }
        }
    }

    private handleQRRead(value: String) {
        //TODO: Handle the read qr
    }
    private captureSession: AVCaptureSession?
    private videoPreviewLayer: AVCaptureVideo
}

handleQRRead - will be called on a successful scan initCaptureSession - initialize scanning for QR and camera input dismissCaptureSession - hide the camera input and stop scanning

Prior to iOS 7 when you want to scan a QR code, we might need to rely on third party frameworks or libraries like zBar or zXing. But Apple introduced AVCaptureMetaDataOutput from iOS 7 for reading barcodes.

To read QR code using AVFoundation we need to setup/create AVCaptureSession and use captureOutput:didOutputMetadataObjects:fromConnection: delegate method.

Step 1

Import AVFoundation framework and confirm to AVCaptureMetadataOutputObjectsDelegate protocol

import AVFoundation
class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate

Step 2

QR code reading is totally based on video capture. So to capture continuous video create an AVCaptureSession and set up device input and output. Add the below code in view controller viewDidLoad method

// Create an instance of the AVCaptureDevice and provide the video as the media type parameter.
let captureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
 
do {
    // Create an instance of the AVCaptureDeviceInput class using the device object and intialise capture session
    let input = try AVCaptureDeviceInput(device: captureDevice)
    captureSession = AVCaptureSession()
    captureSession?.addInput(input)
    
    // Create a instance of AVCaptureMetadataOutput object and set it as the output device the capture session.
    let captureMetadataOutput = AVCaptureMetadataOutput()
    captureSession?.addOutput(captureMetadataOutput)
    // Set delegate with a default dispatch queue
    captureMetadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
    //set meta data object type as QR code, here we can add more then one type as well 
    captureMetadataOutput.metadataObjectTypes = [AVMetadataObjectTypeQRCode]


    // Initialize the video preview layer and add it as a sublayer to the viewcontroller view's layer.
    videoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
    videoPreviewLayer?.videoGravity = AVLayerVideoGravityResizeAspectFill
    videoPreviewLayer?.frame = view.layer.bounds
    view.layer.addSublayer(videoPreviewLayer!)

    // Start capture session.
    captureSession?.startRunning()
} catch {
    // If any error occurs, let the user know. For the example purpose just print out the error
    print(error)
    return
}

Step 3

Implement AVCaptureMetadataOutputObjectsDelegate delegate method to read the QR code

func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputMetadataObjects metadataObjects: [Any]!, from connection: AVCaptureConnection!) {
    
    // Check if the metadataObjects array contains at least one object. If not no QR code is in our video capture
    if metadataObjects == nil || metadataObjects.count == 0 {
        // NO QR code is being detected.
        return
    }
    
    // Get the metadata object and cast it to `AVMetadataMachineReadableCodeObject`
    let metadataObj = metadataObjects[0] as! AVMetadataMachineReadableCodeObject
    
    if metadataObj.type == AVMetadataObjectTypeQRCode {
        // If the found metadata is equal to the QR code metadata then get the string value from meta data
        let barCodeObject = videoPreviewLayer?.transformedMetadataObject(for: metadataObj)
        
        if metadataObj.stringValue != nil {
           // metadataObj.stringValue is our QR code
        }
    }
} 

here metadata object can give you the bounds of the QR code read on the camera feed as well. To get the bounds simply pass the metadata object to videoPreviewLayer' s transformedMetadataObject method like below.

let barCodeObject = videoPreviewLayer?.transformedMetadataObject(for: metadataObj)
        qrCodeFrameView?.frame = barCodeObject!.bounds