如何在Swift for iOS中实现代码，使MapKit中的批注可拖动？

import MapKit

class CustomAnnotation: NSObject, MKAnnotation {
    var coordinate: CLLocationCoordinate2D
    var title: String?
    var subtitle: String?
    
    init(coordinate: CLLocationCoordinate2D) {
        self.coordinate = coordinate
        super.init()
    }
}

import UIKit
import MapKit

class ViewController: UIViewController, MKMapViewDelegate {
    @IBOutlet weak var mapView: MKMapView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        mapView.delegate = self
        
        // 添加一个自定义的批注到地图上
        let annotation = CustomAnnotation(coordinate: CLLocationCoordinate2D(latitude: 37.331705, longitude: -122.030237))
        annotation.title = "Apple Park"
        annotation.subtitle = "Cupertino, CA"
        mapView.addAnnotation(annotation)
    }
    
    // 实现MKMapViewDelegate中的方法，返回自定义的批注视图
    func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
        guard let annotation = annotation as? CustomAnnotation else {
            return nil
        }
        
        let identifier = "CustomAnnotation"
        var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: identifier)
        
        if annotationView == nil {
            annotationView = MKPinAnnotationView(annotation: annotation, reuseIdentifier: identifier)
            annotationView?.canShowCallout = true
        } else {
            annotationView?.annotation = annotation
        }
        
        // 允许批注视图拖动
        annotationView?.isDraggable = true
        
        return annotationView
    }
}