How do I center a UIImageView within a full-screen UIScrollView?
- by Sebastian Celis
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
