Layering images in Swift - ios

Question

My goal is to overlay two images (the first a photo from camera roll, the second a PNG of a cartoon ghost).
I've gotten far enough that I'm passing in a camera roll image and a selected ghost image to a view controller. But where I'm stuck is how to layer these images in a useable way.
I can flow in the original photo a couple of ways (either by starting with an image view or by programmatically creating one in a view), and I can add the ghost on top of it (I've mostly been doing this programmatically).
But I can only control the ghost's size and placement manually. I'd like it to come in centered on the original image and match either its height or width, depending on which is smaller as it can be horizontal or vertical.
After an evening of searching, I've come up blank. But surely there's got to be a way to grab the image view's coordinates and make that calculation, right?
Here's what I've got:
import UIKit
class TwoLayerViewController: UIViewController {
#IBOutlet weak var bottomLayerImage: UIImageView!
var originalPhoto: UIImage?
var chosenGhostPhoto: UIImage?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
bottomLayerImage.image = originalPhoto
bottomLayerImage.contentMode = UIViewContentMode.ScaleAspectFit
var ghostView = UIImageView(frame:CGRectMake(bottomLayerImage.frame.origin.x, bottomLayerImage.frame.origin.y, 100, 100))
ghostView.image = chosenGhostPhoto
ghostView.backgroundColor = UIColor.grayColor()
bottomLayerImage.addSubview(ghostView)
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}

Answer 1

Try putting the top image not in an ImageView, but in a sublayer over the other one. Like this:
class YourImageView: UIImageView{
let containerLayer = CALayer()
func drawImageOnTop(img: UIImage){
let piclayer = CALayer()
let sz = originalPhoto?.size ?? CGSize(width: 0, height: 0)
piclayer.frame = CGRect(origin: self.layer.contentsRect.origin, size: sz)
piclayer.position = CGPoint(x: sz.width/2, y: sz.height/2)
piclayer.contentsGravity = .resizeAspect
piclayer.contents = img.cgImage
containerLayer.addSublayer(piclayer)
}