Zoom UIImageView in UIScrollView – not centered - ios

Question

I have an UIScrollView with an UIImageView in it. Just a simple test case: The UIImageView is full width of the scroll view and centered to Y with auto layout. However, in the final result on my device it is not really centered by Y.
But i have the problem that when zooming the UIImageView with the scroll view it "drifts" to the bottom area – not just scaled in the center as it should be. It also "drifts" to the left when zoom out (at the very end of the video) and bounces back to the center.
I've made a small preview video of this behavior: https://www.youtube.com/watch?v=ivRNkzNrcEA
Here is my simple test code:
class TestController: UIViewController, UIScrollViewDelegate {
#IBOutlet weak var SCROLL: UIScrollView!
#IBOutlet weak var IMAGE: UIImageView!
override func viewDidLoad() {
super.viewDidLoad()
SCROLL.minimumZoomScale = 1;
SCROLL.maximumZoomScale = 6.0;
SCROLL.zoomScale = 1.0;
SCROLL.contentSize = IMAGE.frame.size;
SCROLL.delegate = self;
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
}
override func viewDidAppear(animated: Bool) {
super.viewDidAppear(animated)
}
func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
return IMAGE
}
func scrollViewDidZoom(scrollView: UIScrollView) {
}
func scrollViewDidEndZooming(scrollView: UIScrollView, withView view: UIView?, atScale scale: CGFloat) {
}
}
And here is my test auto layout:

Answer 1

I am not sure if it is possible to keep centering with just auto-layout. One approach that works for me is to to sub-class UIScrollView and modify layoutSubviews as illustrated in Apple WWDC example. Code below
-(void) layoutSubviews
{
[super layoutSubviews];
UIImageView *v = (UIImageView *)[self.delegate viewForZoomingInScrollView:self];
//Centering code
CGFloat svw = self.bounds.size.width;
CGFloat svh = self.bounds.size.height;
CGFloat vw = v.frame.size.width;
CGFloat vh = v.frame.size.height;
CGRect f = v.frame;
if (svw > vw)
{
f.origin.x = (svw - vw)/2.0;
}
else
{
f.origin.x = 0;
}
if (svh > vh)
{
f.origin.y = (svh - vh)/2.0;
}
else
{
f.origin.y = 0;
}
v.frame = f;
}