如何通过移动对象来扩展多边形？

Question

需要实时扩展多边形移动对象（白色正方形）。 如何实现呢？以下是与此对应的代码 photo。我会很高兴有帮助

    func polygonCircleWhatHere(coordinate: CLLocationCoordinate2D, withMeterRadius: Double) {
        let degreesBetweenPoints = 8.0
        //45 sides
        let numberOfPoints = floor(360.0 / degreesBetweenPoints)
        let distRadians: Double = withMeterRadius / 6371000.0
        // earth radius in meters
        let centerLatRadians: Double = coordinate.latitude * Double.pi / 180
        let centerLonRadians: Double = coordinate.longitude * Double.pi / 180
        var coordinates = [CLLocationCoordinate2D]()
        //array to hold all the points
        for index in 0 ..< Int(numberOfPoints) {
            let degrees: Double = Double(index) * Double(degreesBetweenPoints)
            let degreeRadians: Double = degrees * Double.pi / 180
            let pointLatRadians: Double = asin(sin(centerLatRadians) * cos(distRadians) + cos(centerLatRadians) * sin(distRadians) * cos(degreeRadians))
            let pointLonRadians: Double = centerLonRadians + atan2(sin(degreeRadians) * sin(distRadians) * cos(centerLatRadians), cos(distRadians) - sin(centerLatRadians) * sin(pointLatRadians))
            let pointLat: Double = pointLatRadians * 180 / Double.pi
            let pointLon: Double = pointLonRadians * 180 / Double.pi
            let point: CLLocationCoordinate2D = CLLocationCoordinate2DMake(pointLat, pointLon)
            coordinates.append(point)
        }
        let polygon = MGLPolygon(coordinates: &coordinates, count: UInt(coordinates.count))

        draggableAnnotationPoint.coordinate = CLLocationCoordinate2D(latitude: coordinates[0].latitude, longitude: coordinates[0].longitude)

        let centerAnnotationPoint = MGLPointAnnotation()
        centerAnnotationPoint.coordinate = CLLocationCoordinate2D(latitude: coordinate.latitude, longitude: coordinate.longitude)
        self.mapView.addAnnotations([polygon,centerAnnotationPoint,draggableAnnotationPoint])
    }