在UIImageView IOS上绘制圆的最佳方式

I have a UIImageView that shows a picture the user has just taken with the camera. On that image i need to draw a transparent movable circle thats a set size. So when the user drags their finger over the image the circle moves with it. Then if the user stops moving their finger, the circle stays in the same place.I have a UIImageView that shows a picture the u