Center an UIImageView on the screen when zoom out - iphone

Question

I have an UIImageView inside an UIScrollView. I want that the user can zoom and navigate the image.
This is my work code:
//img is the UIImageView
//scroller is the UIScrollView
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView {
return img;
}
- (void)viewDidLoad {
[super viewDidLoad];
UIImage *image = [UIImage imageNamed:#"map_screen.png"];
img = [[UIImageView alloc] initWithImage:image];
scroller.delegate = self;
scroller.autoresizesSubviews = YES;
scroller.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
scroller.contentSize = img.frame.size;
scroller.scrollEnabled = YES;
scroller.directionalLockEnabled = NO;
scroller.userInteractionEnabled = YES;
CGSize ivsize = img.frame.size;
CGSize ssize = scroller.frame.size;
float scalex = ssize.width / ivsize.width;
float scaley = ssize.height / ivsize.height;
scroller.minimumZoomScale = fmin(1.0, fmax(scaley, scalex));
scroller.zoomScale = fmin(1.0, fmax(scalex, scaley));
[scroller addSubview:img];
img.userInteractionEnabled = YES;
}
all works, but this happened: the minimum zoom is the height of the screen.
My image has the width bigger than the height, so i want that the minumum zoom is the width.
If i write
scroller.minimumZoomScale = fmin(1.0, scalex);
works, but when the user zooms out, the image is not at the center of the screen, but at the top.
i've tried something like this
CGPoint scrollCenter = [scroller center];
[img setCenter:CGPointMake(scrollCenter.x, scrollCenter.y)];
or
img.center = scroller.center;
but with this solution, the image is not completly scrollable, and if i zoom out, it stay again at the top of the screen, but is not completly visible!
What can i do for fix it?

Answer 1

You have to do it manually while the zooming is in progress using the scrollViewDidZoom delegate function... something like
- (void)scrollViewDidZoom:(UIScrollView *)scrollView
{
// center the image as it becomes smaller than the size of the screen
CGSize boundsSize = scrollView.bounds.size;
CGRect frameToCenter = imageView.frame;
// center horizontally
if (frameToCenter.size.width < boundsSize.width)
{
frameToCenter.origin.x = (boundsSize.width - frameToCenter.size.width) / 2;
} else {
frameToCenter.origin.x = 0;
}
// center vertically
if (frameToCenter.size.height < boundsSize.height)
{
frameToCenter.origin.y = (boundsSize.height - frameToCenter.size.height) / 2;
} else {
frameToCenter.origin.y = 0;
}
imageView.frame = frameToCenter;
}

Answer 2

A Swift 3.0 version of Bastian 's answer, a little more succinct:
func scrollViewDidZoom(_ scrollView: UIScrollView) {
// center the image as it becomes smaller than the size of the screen
let boundsSize = scrollView.bounds.size
var frameToCenter = imageView.frame
// center horizontally and vertically
let widthDiff = boundsSize.width - frameToCenter.size.width
let heightDiff = boundsSize.height - frameToCenter.size.height
frameToCenter.origin.x = (widthDiff > 0) ? widthDiff / 2 : 0;
frameToCenter.origin.y = (heightDiff > 0) ? heightDiff / 2 : 0;
imageView.frame = frameToCenter;
}