UIScrollView Zooming & contentInset - swift

Question

Simliar to iOS Photos App where the user is zooming in and out of an image by pinching:
UIView > UIScrollView > UIImageView > UIImage
Initially, I had the issue of zooming below scale 1: image being off centered. I got it fixed by doing this:
func scrollViewDidZoom(scrollView: UIScrollView) {
let offsetX = max((scrollView.bounds.width - scrollView.contentSize.width) * 0.5, 0)
let offsetY = max((scrollView.bounds.height - scrollView.contentSize.height) * 0.5, 0)
scrollView.contentInset = UIEdgeInsetsMake(offsetY, offsetX, 0, 0)
}
This works well when zooming out.
UIImage content mode is aspectFit
Issue
When I ZOOM IN, when zoomScale is above 1, scroll view insets need to hug the surroundings of the UIImage that the scroll view contains. This takes away the dead-space that was surrounding the UIImage. IE, Photos app when zooming-in by pinching or double tapping.
Tried
func scrollViewDidZoom(scrollView: UIScrollView) {
if scrollView.zoomScale > 1 {
let imageScale = (self.imageView.bounds.width/self.imageView.image!.size.width)
let imageWidth = self.imageView.image!.size.width * imageScale
let imageHeight = self.imageView.image!.size.height * imageScale
scrollView.contentInset = UIEdgeInsetsMake(((scrollView.frame.height - imageHeight) * 0.5), (scrollView.frame.width - imageWidth) * 0.5 , 0, 0)
print (scrollView.contentInset.top)
}
else {
let offsetX = max((scrollView.bounds.width - scrollView.contentSize.width) * 0.5, 0)
let offsetY = max((scrollView.bounds.height - scrollView.contentSize.height) * 0.5, 0)
scrollView.contentInset = UIEdgeInsetsMake(offsetY, offsetX, 0, 0)
}
}
Above addition seems to vary the inset amount still.
Update (images added)
First image shows the default layout. Rest shows when zoomed in.....

Answer 1

Your approach looks correct. You need to update your code as below.
func scrollViewDidZoom(scrollView: UIScrollView) {
if scrollView.zoomScale > 1 {
if let image = imageView.image {
let ratioW = imageView.frame.width / image.size.width
let ratioH = imageView.frame.height / image.size.height
let ratio = ratioW < ratioH ? ratioW:ratioH
let newWidth = image.size.width*ratio
let newHeight = image.size.height*ratio
let left = 0.5 * (newWidth * scrollView.zoomScale > imageView.frame.width ? (newWidth - imageView.frame.width) : (scrollView.frame.width - scrollView.contentSize.width))
let top = 0.5 * (newHeight * scrollView.zoomScale > imageView.frame.height ? (newHeight - imageView.frame.height) : (scrollView.frame.height - scrollView.contentSize.height))
scrollView.contentInset = UIEdgeInsetsMake(top, left, top, left)
}
} else {
scrollView.contentInset = UIEdgeInsetsZero
}
}
Swift 5
func scrollViewDidZoom(scrollView: UIScrollView) {
if scrollView.zoomScale > 1 {
if let image = imageView.image {
let ratioW = imageView.frame.width / image.size.width
let ratioH = imageView.frame.height / image.size.height
let ratio = ratioW < ratioH ? ratioW:ratioH
let newWidth = image.size.width*ratio
let newHeight = image.size.height*ratio
let left = 0.5 * (newWidth * scrollView.zoomScale > imageView.frame.width ? (newWidth - imageView.frame.width) : (scrollView.frame.width - scrollView.contentSize.width))
let top = 0.5 * (newHeight * scrollView.zoomScale > imageView.frame.height ? (newHeight - imageView.frame.height) : (scrollView.frame.height - scrollView.contentSize.height))
scrollView.contentInset = UIEdgeInsets(top: top, left: left, bottom: top, right: left)
}
} else {
scrollView.contentInset = .zero
}
}

Answer 2

Swift 5
func scrollViewDidZoom(_ scrollView: UIScrollView) {
if scrollView.zoomScale > 1 {
if let image = imageView.image {
let ratioW = imageView.frame.width / image.size.width
let ratioH = imageView.frame.height / image.size.height
let ratio = ratioW < ratioH ? ratioW : ratioH
let newWidth = image.size.width * ratio
let newHeight = image.size.height * ratio
let left = 0.5 * (newWidth * scrollView.zoomScale > imageView.frame.width ? (newWidth - imageView.frame.width) : (scrollView.frame.width - scrollView.contentSize.width))
let top = 0.5 * (newHeight * scrollView.zoomScale > imageView.frame.height ? (newHeight - imageView.frame.height) : (scrollView.frame.height - scrollView.contentSize.height))
scrollView.contentInset = UIEdgeInsets(top: top, left: left, bottom: top, right: left)
}
} else {
scrollView.contentInset = UIEdgeInsets.zero
}
}

Answer 3

for swift 4
func scrollViewDidZoom(_ scrollView: UIScrollView) {
if scrollView.zoomScale > 1 {
if let image = self.imageViewZoom.image {
let ratioW = self.imageViewZoom.frame.width / image.size.width
let ratioH = self.imageViewZoom.frame.height / image.size.height
let ratio = ratioW < ratioH ? ratioW:ratioH
let newWidth = image.size.width*ratio
let newHeight = image.size.height*ratio
let left = 0.5 * (newWidth * scrollView.zoomScale > self.imageViewZoom.frame.width ? (newWidth - self.imageViewZoom.frame.width) : (scrollView.frame.width - scrollView.contentSize.width))
let top = 0.5 * (newHeight * scrollView.zoomScale > self.imageViewZoom.frame.height ? (newHeight - self.imageViewZoom.frame.height) : (scrollView.frame.height - scrollView.contentSize.height))
scrollView.contentInset = UIEdgeInsetsMake(top, left, top, left)
}
} else {
scrollView.contentInset = UIEdgeInsets.zero
}
}