我正在尝试在Swift中更改MKMapView上的大头针图像，但不幸的是它不起作用。知道我在做什么错吗？我在这里看到了一些示例，但是没有用。

import UIKit
import MapKit

class AlarmMapViewController: UIViewController {
    @IBOutlet weak var map: MKMapView!

    override func viewDidLoad() {
        super.viewDidLoad()
        showAlarms()
        map.showsUserLocation = true
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

    func showAlarms(){

        map.region.center.latitude = 49
        map.region.center.longitude = 12
        map.region.span.latitudeDelta = 1
        map.region.span.longitudeDelta = 1

        for alarm in Alarms.sharedInstance.alarms {

            let location = CLLocationCoordinate2D(
                latitude: Double(alarm.latitude),
                longitude: Double(alarm.longtitude)
            )

            let annotation = MKPointAnnotation()
            annotation.setCoordinate(location)
            annotation.title = alarm.name
            annotation.subtitle = alarm.description
            mapView(map, viewForAnnotation: annotation).annotation = annotation
            map.addAnnotation(annotation)
        }
    }

    @IBAction func zoomIn(sender: AnyObject) {
    }

    @IBAction func changeMapType(sender: AnyObject) {
    }

    func mapView(mapView: MKMapView!,
        viewForAnnotation annotation: MKAnnotation!) -> MKAnnotationView! {

        if annotation is MKUserLocation {
            //return nil so map view draws "blue dot" for standard user location
            return nil
        }

        let reuseId = "pin"
        var pinView = mapView.dequeueReusableAnnotationViewWithIdentifier(reuseId) as? MKPinAnnotationView
        if pinView == nil {
            pinView = MKPinAnnotationView(annotation: annotation, reuseIdentifier: reuseId)
            pinView!.canShowCallout = true
            pinView!.animatesDrop = true
            pinView!.image = UIImage(named:"GreenDot")!

        }
        else {
            pinView!.annotation = annotation
        }

        return pinView
    }
}

GreenDot图片可在其他地方使用。