func mapView(mapView: MKMapView!, regionDidChangeAnimated animated: Bool) {
        dispatch_async(self._queue, {
            if (self.busStops.count > 0) {
                if (mapView.camera!.altitude <= 1000) {
                    for (var i = 0; i < self.busStops.count; i++) {
                        if (MKMapRectContainsPoint(mapView.visibleMapRect, MKMapPointForCoordinate(self.busStops[i].position))) {
                            let stop = BusAnno()
                            stop.setCoordinate(self.busStops[i].position)
                            stop.type = "stop"
                            stop.title = self.busStops[i].name
                            stop.subtitle = self.busStops[i].street
                            self.activeStops.append(stop)
                        }
                    }
                    dispatch_async(self._queue, {
                        self.mapView.addAnnotations(self.activeStops)
                    })
                } else if (self.activeStops.count > 0) {
                    mapView.removeAnnotations(self.activeStops)
                    self.activeStops = []
                }
            }
        })
    }