条形码扫描器不调用metadataOutput

Question

我正在关注本教程：https://www.appcoda.com/barcode-reader-swift/

我有这段代码：

import Foundation
import UIKit
import AVFoundation

class ScanBPViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {

    var captureSession = AVCaptureSession()
    var videoPreviewLayer:AVCaptureVideoPreviewLayer?
    var qrCodeFrameView:UIView?

    override func viewDidLoad() {
        super.viewDidLoad()
        // Get the back-facing camera for capturing videos
        let deviceDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInWideAngleCamera], mediaType: AVMediaType.video, position: .back)

        guard let captureDevice = deviceDiscoverySession.devices.first else {
            print("Failed to get the camera device")
            return
        }

        do {
            // Get an instance of the AVCaptureDeviceInput class using the previous device object.
            let input = try AVCaptureDeviceInput(device: captureDevice)

            // 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 = [AVMetadataObject.ObjectType.pdf417]

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

            // Start video capture.
            captureSession.startRunning()

            // Initialize QR Code Frame to highlight the QR code
            qrCodeFrameView = UIView()

            if let qrCodeFrameView = qrCodeFrameView {
                qrCodeFrameView.layer.borderColor = UIColor.green.cgColor
                qrCodeFrameView.layer.borderWidth = 2
                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 metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
        // Check if the metadataObjects array is not nil and it contains at least one object.
        if metadataObjects.count == 0 {
            qrCodeFrameView?.frame = CGRect.zero
            print("No barcode is detected")
            return
        }

        // Get the metadata object.
        let metadataObj = metadataObjects[0] as! AVMetadataMachineReadableCodeObject

        if metadataObj.type == AVMetadataObject.ObjectType.qr {
            // 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 {
                print(metadataObj.stringValue)
            }
        }
    }

}

但是当我构建并运行应用程序时，它会调出摄像机视图，但不会识别我放在它前面的pdf417条形码，也不会调用metadataOutput函数，我猜测是因为它没有识别条形码。有谁看到我可能会出错？谢谢！