Argument of '#selector' refers to instance method 'startDownloadTask().......Objecのエラー

前提・実現したいこと

URL先のサイトより動画ファイルを取得。
URLSessionによるファイルダウンロードを行いたいです。
参考としたサイト
https://joyplot.com/documents/2016/09/30/swift-urlsession-progressbar/

http://galakutaapp.blogspot.com/2017/09/swift-4.html

発生している問題・エラーメッセージ

エラーメッセージ

Argument of '#selector' refers to instance method 'startDownloadTask()' that is not exposed to Objective-C

該当のソースコード

ソースコード
import UIKit

class ViewController: UIViewController, URLSessionDownloadDelegate {
    
    
    
    var progressBar: UIProgressView!
    
    @objc override func viewDidLoad() {
        
        super.viewDidLoad()
        
        
        
        
        
        let button = UIButton(type: .system)
        
        button.setTitle("ダウンロード開始", for: .normal)
        
        button.titleLabel?.font = UIFont(name: "Arial", size: 24)
        
        button.addTarget(self, action: #selector(self.startDownloadTask), for: .touchUpInside)
        
        button.sizeToFit()
        
        button.center = self.view.center
        
        self.view.addSubview(button)
        
        //button.addTarget(self, action: #selector(self.startDownloadTask), for: .touchUpInside)
        
        
        
        progressBar = UIProgressView(progressViewStyle: .default)
        
        progressBar.layer.position = CGPoint(x: self.view.center.x, y: self.view.frame.height / 4)
        
        self.view.addSubview(progressBar)
        
    }
    
    
    
    func startDownloadTask() {
        
        
        
        let sessionConfig = URLSessionConfiguration.background(withIdentifier: "myapp-background")
        
        let session = URLSession(configuration: sessionConfig, delegate: self, delegateQueue: nil)
        
        let url = URL(string: "任意の動画URL")!
        
        let downloadTask = session.downloadTask(with: url)
        
        downloadTask.resume()
        
    }
    
    
    
    
    
    func getIdFromDateTime() -> String {
        
        let dateFormatter = DateFormatter()
        
        dateFormatter.dateFormat = "yyyy-MM-dd-HH-mm-ss"
        
        return dateFormatter.string(from: Date())
        
    }
    
    
    
    
    
    func getSaveDirectory() -> String {
        
        
        
        let fileManager = Foundation.FileManager.default
        
        
        
        
        
        let path = NSSearchPathForDirectoriesInDomains(Foundation.FileManager.SearchPathDirectory.libraryDirectory, Foundation.FileManager.SearchPathDomainMask.userDomainMask, true)[0] + "/DownloadFiles/"
        
        
        
        
        
        if !fileManager.fileExists(atPath: path) {
            
            createDir(path: path)
            
        }
        
        
        
        return path
        
    }
    
    
    
    
    
    func createDir(path: String) {
        
        do {
            
            let fileManager = Foundation.FileManager.default
            
            try fileManager.createDirectory(atPath: path, withIntermediateDirectories: true, attributes: nil)
            
        } catch let error as NSError {
            
            print("createDir: (error)")
            
        }
        
    }
    
    
    
    
    
    
    
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        
        
        
        
        
        print("didFinishDownloading")
        
        
        
        do {
            
            if let data = NSData(contentsOf: location) {
                
                
                
                let fileExtension = location.pathExtension
                
                let filePath = getSaveDirectory() + getIdFromDateTime() + "." + fileExtension
                
                
                
                print(filePath)
                
                
                
                try data.write(toFile: filePath, options: .atomic)
                
            }
            
        } catch let error as NSError {
            
            print("download error: (error)")
            
        }
        
    }
    
    
    
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        
        
        
        
        
        let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
        
        
        
        
        
        print(String(format: "%.2f", progress * 100) + "%")
        
        
        
        
        
        DispatchQueue.main.async(execute: {
            
            self.progressBar.setProgress(progress, animated: true)
            
        })
        
    }
    
    
    
    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        
        
        
        if error != nil {
            
            print("download error")
            
        }
        
    }
    
    override func didReceiveMemoryWarning() {
        
        super.didReceiveMemoryWarning()
        
    }
    
}