MKMapView在后台线程中添加注释

Question

移动地图时，MKMapView会调用下面的委托功能。由于我们的应用程序的性质，我们在一个单独的线程中添加新的车辆到地图。我们注意到，执行此代码时应用程序可能会崩溃。错误发生在for循环的第5行。任何人都可以解释为什么循环返回零注释？

func regionDidChange(mapView: MKMapView, animated: Bool) {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        var vehiclesToAdd = [VehicleAnnotation]()

        for (var i = 0; i < self.mapView.annotations.count; i++) {
            if (self.mapView.annotations[i].isKindOfClass(VehicleAnnotation)) {
                let vehicleAnnotation: VehicleAnnotation! = self.mapView.annotations[i] as VehicleAnnotation

                if (!MKMapRectContainsPoint(mapView.visibleMapRect, MKMapPointForCoordinate(vehicleAnnotation.coordinate))) {
                    vehiclesToAdd.append(vehicleAnnotation)
                }
            }
        }

        dispatch_async(dispatch_get_main_queue()) {
            self.mapView.addAnnotations(vehiclesToAdd)
        }
    })
}

为什么会发生这种情况？