In my application, I would like to present the user with a full-screen photo viewer much like the one used in the Photos app. This is just for a single photo and as such should be quite simple. I just want the user to be able to view this one photo with the ability to zoom and pan.

I have most of it working. And, if I do not center my UIImageView, everything behaves perfectly. However, I really want the UIImageView to be centered on the screen when the image is sufficiently zoomed out. I do not want it stuck to the top-left corner of the scroll view.

Once I attempt to center this view, my vertical scrollable area appears to be greater than it should be. As such, once I zoom in a little, I am able to scroll about 100 pixels past the top of the image. What am I doing wrong?

@interface MyPhotoViewController : UIViewController <UIScrollViewDelegate>
{
    UIImage* photo;
    UIImageView *imageView;
}
- (id)initWithPhoto:(UIImage *)aPhoto;
@end

@implementation MyPhotoViewController

- (id)initWithPhoto:(UIImage *)aPhoto
{
    if (self = [super init])
    {
        photo = [aPhoto retain];

        // Some 3.0 SDK code here to ensure this view has a full-screen
        // layout.
    }

    return self;
}

- (void)dealloc
{
    [photo release];
    [imageView release];
    [super dealloc];
}

- (void)loadView
{
    // Set the main view of this UIViewController to be a UIScrollView.
    UIScrollView *scrollView = [[UIScrollView alloc] init];
    [self setView:scrollView];
    [scrollView release];
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Initialize the scroll view.
    CGSize photoSize = [photo size];
    UIScrollView *scrollView = (UIScrollView *)[self view];
    [scrollView setDelegate:self];
    [scrollView setBackgroundColor:[UIColor blackColor]];

    // Create the image view. We push the origin to (0, -44) to ensure
    // that this view displays behind the navigation bar.
    imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0.0, -44.0,
        photoSize.width, photoSize.height)];
    [imageView setImage:photo];
    [scrollView addSubview:imageView];

    // Configure zooming.
    CGSize screenSize = [[UIScreen mainScreen] bounds].size;
    CGFloat widthRatio = screenSize.width / photoSize.width;
    CGFloat heightRatio = screenSize.height / photoSize.height;
    CGFloat initialZoom = (widthRatio > heightRatio) ? heightRatio : widthRatio;
    [scrollView setMaximumZoomScale:3.0];
    [scrollView setMinimumZoomScale:initialZoom];
    [scrollView setZoomScale:initialZoom];
    [scrollView setBouncesZoom:YES];
    [scrollView setContentSize:CGSizeMake(photoSize.width * initialZoom,
        photoSize.height * initialZoom)];

    // Center the photo. Again we push the center point up by 44 pixels
    // to account for the translucent navigation bar.
    CGPoint scrollCenter = [scrollView center];
    [imageView setCenter:CGPointMake(scrollCenter.x,
        scrollCenter.y - 44.0)];
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    [[[self navigationController] navigationBar] setBarStyle:UIBarStyleBlackTranslucent];
    [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleBlackTranslucent animated:YES];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [[[self navigationController] navigationBar] setBarStyle:UIBarStyleDefault];
    [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleDefault animated:YES];
}

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    return imageView;
}

@end