如何使用MKAnnotationView创建自定义"pin-drop"动画？

import MapKit

class CustomAnnotationView: MKAnnotationView {
    var pinImageView: UIImageView!
    
    override init(annotation: MKAnnotation?, reuseIdentifier: String?) {
        super.init(annotation: annotation, reuseIdentifier: reuseIdentifier)
        
        pinImageView = UIImageView(image: UIImage(named: "pin-drop.png"))
        addSubview(pinImageView)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        pinImageView.frame = bounds
    }
    
    func animateDrop() {
        let scaleUpTransform = CGAffineTransformMakeScale(1.2, 1.2)
        let scaleDownTransform = CGAffineTransformMakeScale(1.0, 1.0)
        
        pinImageView.transform = scaleUpTransform
        
        UIView.animateWithDuration(0.3, delay: 0.0, options: .CurveEaseInOut, animations: {
            self.pinImageView.transform = scaleDownTransform
        }, completion: nil)
    }
}

func mapView(mapView: MKMapView, viewForAnnotation annotation: MKAnnotation) -> MKAnnotationView? {
    if annotation is MKUserLocation {
        return nil
    }
    
    let reuseIdentifier = "customAnnotationView"
    var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: reuseIdentifier) as? CustomAnnotationView
    
    if annotationView == nil {
        annotationView = CustomAnnotationView(annotation: annotation, reuseIdentifier: reuseIdentifier)
    }
    
    annotationView?.animateDrop()
    
    return annotationView
}