Add annotation to resizable image within scrollview - swift

Question

I have a Scroll View in which I load an Image that can be zoomed and where I can "navigate".
I'm trying to add buttons programmatically to the view such as MKAnnotation that are used in MapKit.
Is that possible?
The methods I use are scrollViewDidZoom
func scrollViewDidZoom(scrollView: UIScrollView) {
centerScrollViewContents()
}
where the function centerScrollViewContents is
func centerScrollViewContents() {
let boundsSize = scrollView.bounds.size
var contentFrame = imageView.frame
if contentFrame.size.width < boundsSize.width {
contentFrame.origin.x = (boundsSize.width - contentFrame.size.width) / 2
} else { contentFrame.origin.x = 0 }
if contentFrame.size.height < boundsSize.height {
contentFrame.origin.y = (boundsSize.height - contentFrame.size.height) / 2
} else { contentFrame.origin.y = 0 }
imageView.frame = contentFrame
}
I add the button/annotation with a UITapGestureRecognizer implemented as
let posTocco = sender.locationInView(scrollView)
println("\(posTocco), \(scrollView.zoomScale)")
var button = UIButton()
let image = UIImage(named: "arrow.png") as UIImage?
button.frame = CGRectMake(posTocco.x, posTocco.y, 30, 30)
button.setImage(image, forState: .Normal)
scrollView.addSubview(button)
For sure I need to define the button/annotation outside the tapGesture function but I cannot figure out how to move the button within the image zoom.
Any help?
EDIT:
I partially figured out how to solve this, but the button slightly move at the first zoom in/out.
I create a new variable which stores the latest zoomValue fattoreScalaCorrente and in the scrollViewDidZoom(scrollView:) I do the magic
var newLoc = CGPoint(x: posAssolute.x * scrollView.zoomScale, y: posAssolute.y * scrollView.zoomScale)
button.frame = CGRectMake(newLoc.x, newLoc.y, button.frame.width, button.frame.height)
Do you have any idea why my button is moving slightly at the first zoom?
EDIT2:
After the suggestion given using
button.frame = CGRectMake(newLoc.x - button.frame.width/2, newLoc.y - button.frame.height/2, button.frame.width, button.frame.height)
The button seems to move a little and I cannot understand why.

Answer 1

Try to replace
button.frame = CGRectMake(newLoc.x, newLoc.y, button.frame.width, button.frame.height)
with
button.frame = CGRectMake(newLoc.x - button.frame.width/2, newLoc.y - button.frame.height/2, button.frame.width, button.frame.height)