Xamarin.Forms.Maps如何像谷歌地图一样在图钉旁边/上方放置标签？

public class CustomPin : Pin
{
    public string Label { get; set; }
}

public class CustomMap : Map
{
    protected override MKAnnotationView GetViewForAnnotation(MKMapView mapView, IMKAnnotation annotation)
    {
        if (annotation is MKUserLocation)
            return null;

        var customPin = annotation as CustomPin;
        if (customPin == null)
            return null;

        var pinView = new MKAnnotationView(annotation, customPin.Label);
        pinView.Image = UIImage.FromFile("pin.png");
        pinView.CalloutOffset = new CGPoint(0, 0);
        pinView.LeftCalloutAccessoryView = new UIImageView(UIImage.FromFile("label.png"));

        return pinView;
    }
}

var customMap = new CustomMap
{
    MapType = MapType.Street,
    IsShowingUser = true,
};

var pin = new CustomPin
{
    Position = new Position(37.79752, -122.40183),
    Label = "Custom Pin",
};

customMap.Pins.Add(pin);