MKAnnotationView不显示自定义图像（Swift）

Question

我正在尝试为地图图钉显示自定义注释图像。但是它始终显示默认的红色引脚。正确调用以下委托方法。

func mapView(mapView: MKMapView!, viewForAnnotation annotation: MKAnnotation!) -> MKAnnotationView! {
    if annotation is MKUserLocation {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:"marker")
        pinView!.sizeToFit()
    }
    else {
        pinView!.annotation = annotation
    }


    return pinView
}

自定义引脚的初始化

var startPin:MapMarker = MapMarker(coordinate: self.mapView.userLocation.coordinate, title: "", subtitle: "")
        self.mapView.addAnnotation(startPin)

自定义类

class MapMarker: NSObject, MKAnnotation {

var coordinate: CLLocationCoordinate2D
var title: String
var subtitle: String
var image: UIImage?

init(coordinate: CLLocationCoordinate2D, title: String, subtitle: String) {
    self.coordinate = coordinate
    self.title = title
    self.subtitle = subtitle
}

}