I had the need to create a UIImage on the fly but it needed to have transparent shape INSIDE
I hassled with Floodfill to draw around the shapes… that kinda worked, but was slow
I tried creating masked images, like you would in Xojo, that was a royal PITA
Then I ran across a SUPER SIMPLE way
UIGraphicsBeginImageContext(layerImageSize)
let context : Context = UIGraphicsGetCurrentContext()!
context.setFillColor(UIColor.blue.cgColor) // fill area with BLUE
context.fill(rect)
//
context.setFillColor(UIColor.clear.cgColor)
context.setBlendMode(.clear) // this is the "magic"
//
....draw whatever shapes... they will all leave transparent voids in the "blue"
//
context.setBlendMode(.normal)