Transparency when drawing in CGContext does not work Swift - swift

Question

Hello i'm developing a drawing application. I try to draw multiple lines with transparency in a context (one line over another) but the transparencies do not work as I thought they work when drawing one image over another. I've made this test with 2 squares:
var imageView:UIImageView = UIImageView()
imageView.frame = CGRect(x: 0, y: 0, width: 400, height: 200)
self.addSubview(imageView)
/* CREATE A SQUARE UIImage */
let color:UIColor = UIColor(red:1.00, green:0.40, blue:0.21, alpha:1.0)
let bounds:CGRect = CGRect(x: 0, y: 0, width: 200, height: 200)
UIGraphicsBeginImageContextWithOptions(bounds.size, false, 0.0)
var ctx = UIGraphicsGetCurrentContext()
ctx?.setFillColor(color.cgColor)
ctx?.fill(bounds)
let rectangleUIImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
UIGraphicsEndImageContext()
/* DRAW SCUARE WHIT ALPHA 1 */
UIGraphicsBeginImageContextWithOptions(imageView.frame.size, false, 0.0)
ctx = UIGraphicsGetCurrentContext()
ctx?.setAlpha(1.0)
ctx?.draw(rectangleUIImage.cgImage!, in: CGRect(x: 0, y: 0, width: 200, height: 200))
/* DRAW 10 SQUARES WHIT ALPHA 0.1 IN THE SAME PLACE */
ctx?.setAlpha(0.1)
for i in 0..<10 {
ctx?.draw(rectangleUIImage.cgImage!, in: CGRect(x: 200, y: 0, width: 200, height: 200))
}
imageView.image = UIGraphicsGetImageFromCurrentImageContext()!
UIGraphicsEndImageContext()
I would expect the above code to result in this:Two boxes visually with the same transparency
But no matter how many times I draw a transparent square on another I always find this :( :
Two boxes, one without transparency and one with transparency