How to fill outer overlay circle on a map using Swift

I am trying to create an overlay on a map exactly as shown in the picture in this post: How to fill an outer overlay circle in iOS 7 on a map . I have read these instructions to achieve this goal in Objective C and tried my best to convert this to swift as shown below:

     override func drawMapRect(mapRect: MKMapRect, zoomScale: MKZoomScale, inContext context: CGContext) {
        print("overlay")
        // Fill full map rect with some color.
        let rect: CGRect = rectForMapRect(mapRect)
        CGContextSaveGState(context)
        CGContextAddRect(context, rect)
        CGContextSetFillColorWithColor(context, UIColor.whiteColor().colorWithAlphaComponent(0.4).CGColor)
        CGContextFillRect(context, rect)
        CGContextRestoreGState(context)

        // Clip rounded hole.
        CGContextSaveGState(context)
        CGContextSetFillColorWithColor(context, UIColor.whiteColor().CGColor)
        CGContextSetBlendMode(context, CGBlendMode.Clear)
        CGContextFillEllipseInRect(context, rect)
        CGContextRestoreGState(context)
        // Draw circle
        super.drawMapRect(mapRect, zoomScale:zoomScale, inContext:context)
    }

      

        
        
        
      

    

I know I am missing how to make this code work as an overlay. As you can probably tell, I'm a bit new to this, but this is the first time I've gotten so stuck I can't handle it from the other answers here!