点击OK按钮（使用AVCaptureVideoPreviewLayer）时如何在屏幕中央显示活动指示？

Question

点击OK按钮时如何在屏幕中央显示活动指示？我正在使用AVCaptureVideoPreviewLayer对象并显示相机，当我们扫描QR码时会出现警告框，按下OK按钮后它会调用API，此时我想出现UIActivityIndi​​cator。

import UIKit
import AVFoundation

class QRCodeScanner: UIViewController, AVCaptureMetadataOutputObjectsDelegate {

    @IBOutlet weak var imgClose: UIImageView!

    var video = AVCaptureVideoPreviewLayer()

    var strQR = String()


    override func viewDidLoad() {
        super.viewDidLoad()

        //Creating session
        let session = AVCaptureSession()

        //Capture device
        //var captureDevice = AVCaptureDevice.default(for: AVMediaType.video)
        let captureDevice = AVCaptureDevice.default(AVCaptureDevice.DeviceType.builtInWideAngleCamera, for: AVMediaType.video, position: .front)

        do {
            let input = try AVCaptureDeviceInput(device: captureDevice!)
            session.addInput(input)
        }
        catch {
            print("Error")
        }

        let output = AVCaptureMetadataOutput()
        session.addOutput(output)

        output.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
        output.metadataObjectTypes = [AVMetadataObject.ObjectType.qr]

        video = AVCaptureVideoPreviewLayer(session: session)
        video.frame = view.layer.bounds
        view.layer.addSublayer(video)

        //Show Image in Camera Mode
        let myLayer = CALayer()
        let myImage = UIImage(named: "icn-close")?.cgImage
        myLayer.frame = CGRect(x: self.view.frame.size.width - CGFloat((myImage?.width)!) - 10, y: 16, width: 32, height: 35)
        myLayer.contents = myImage
        video.addSublayer(myLayer)


        self.view.bringSubview(toFront: self.view)

        session.startRunning()
    }


    @IBAction func btnClose(_ sender: Any)
    {
        self.navigationController?.popViewController(animated: true)
    }



    func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection)
    {
        if metadataObjects.count == 0 {
            //qrCodeFrameView?.frame = CGRect.zero
            //messageLabel.text = "No QR code is detected"
            return
        }

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

        let supportedCodeTypes = AVMetadataObject.ObjectType.qr

        if supportedCodeTypes == .qr {
            // If the found metadata is equal to the QR code metadata (or barcode) then update the status label's text and set the bounds
            //let barCodeObject = videoPwreviewLayer?.transformedMetadataObject(for: metadataObj)
            //qrCodeFrameView?.frame = barCodeObject!.bounds

            let alert = UIAlertController(title: "QR Code", message: object?.stringValue, preferredStyle: .alert)

            alert.addAction(UIAlertAction(title: "Retake", style: .default, handler: nil))
            alert.addAction(UIAlertAction(title: "OK", style: .default, handler: { (nil) in

                self.strQR = (object?.stringValue)!
                //print("strQR")

                activityIndicator.startAnimating()
                self.api(strURL: urlQRCode)
            }))

            present(alert, animated: true, completion: nil)


            if object?.stringValue != nil {
                //launchApp(decodedURL: metadataObj.stringValue!)
                //messageLabel.text = metadataObj.stringValue
            }
        }

    }

    func api(strURL: String)
    {
        //URL
        let myURL = URL(string: strURL)

        //URL Request
        let request = NSMutableURLRequest(url: myURL!)
        request.httpMethod = "POST"


        //Passing the strOTP text in the dictionary variable
        let postString = ["qr_code": strQR]


        request.addValue("application/json", forHTTPHeaderField: "Content-Type")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        let token = "Bearer " + strToken
        request.addValue(token, forHTTPHeaderField: "Authorization")

        do {
            // pass dictionary to nsdata object and set it as request body
            request.httpBody = try JSONSerialization.data(withJSONObject: postString, options: .prettyPrinted)

            //print("Successfully passed data to server")
        } catch let error {
            print(error.localizedDescription)
        }

        let postTask = URLSession.shared.dataTask(with: request as URLRequest) { (data, response, error) in

            //print(response!)

            guard error == nil else {
                return
            }

            guard let data = data else {
                return
            }

            do {
                //create json object from data
                if let json = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as? [String: Any] {
                    print("POST Method :\(json)")

                    let dict = json as? [String: String]
                    let status = dict!["status"]
                    print("Status : \(String(describing: status))")

                    //Match the status code (received from response through the server
                    if dict!["status"] == "1"
                    {
                        DispatchQueue.main.async {
                            //Stop Activity Indicator
                            activityIndicator.stopAnimating()

                            print("The status received from server is 1. Entered Successfully to OTP Screen")
                            let thankVC = self.storyboard?.instantiateViewController(withIdentifier: "ThankYouVC") as! ThankYouVC
                            thankVC.strEmpName = dict!["whom_to_meet"]!
                            self.navigationController?.pushViewController(thankVC, animated: true)

                        }
                    }
                    else
                    {
                        print("The status received from server is 0.")

                        let whoopsPopupVC = self.storyboard?.instantiateViewController(withIdentifier: "Popup_Whoops") as! Popup_Whoops
                        whoopsPopupVC.strDetail = "You are not a registered user."
                        self.navigationController?.present(whoopsPopupVC, animated: true, completion: nil)
                    }

                    // handle json...
                }
            } catch let error {
                print(error.localizedDescription)
            }
        }
        postTask.resume()
    }


}