Figuring out how to juggle UIImageViews as a UIScrollview scrolls.

JugglingShelvesViewController.m

- (void)viewDidLoad
{
    [super viewDidLoad];
	// We'll want the controller to become the scrollView's delegate
    // in order to respond to changes in the content offset position.
    shelfScrollView.delegate = self;
    
    // We'll generate an array of image views to serve as the shelf
    // background. These shelves will reposition as they scroll out
    // of the viewport.
    shelves = [[NSMutableArray alloc] init];
    for (int shelvesToCreate = 7; shelvesToCreate > 0; shelvesToCreate--) {
        UIImageView *shelfImage = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"SteelShelfLandscape.png"]];
        [shelves addObject:shelfImage];
    }
    
    topShelfIndex = 0;
    bottomShelfIndex = 6;
}

- (void)viewWillAppear:(BOOL)animated {
    // We'll set the position of the shelves without using an animation 
    // whenever the view is about to appear. This ensures the proper
    // orientation is presented at any given time.
    shelfScrollView.contentSize = CGSizeMake(shelfScrollView.frame.size.width, shelfScrollView.frame.size.height*2);
    
    // The shelves are actually image views rather than a background
    // pattern. This is required for performance reasons. Using a large
    // image as a background pattern causes the device framerate to drop
    // to nearly 6fps. -JJ
    UIImage *image;
    
    UIInterfaceOrientation interfaceOrientation = [[UIDevice currentDevice] orientation];
    if (UIDeviceOrientationIsValidInterfaceOrientation(interfaceOrientation) && UIInterfaceOrientationIsLandscape(interfaceOrientation)) {
        image = [UIImage imageNamed:@"SteelShelfLandscape.png"];
    } else {
        image = [UIImage imageNamed:@"SteelShelf.png"];
    }
    
    NSEnumerator *shelfEnumerator = [shelves objectEnumerator];
    UIImageView *shelfImage;
    float currentShelf = 0.0;
    while (shelfImage = [shelfEnumerator nextObject]) {
        shelfImage.image = image;
        [shelfImage setFrame:CGRectMake(0, currentShelf*shelfImage.frame.size.height, shelfScrollView.bounds.size.width, shelfImage.frame.size.height)];
        [shelfScrollView addSubview:shelfImage];
        currentShelf++;
    }
}

#pragma mark - Scrollview Delegate

-(void)scrollViewDidScroll:(UIScrollView *)scrollView {
    UIImageView *shelf = [shelves objectAtIndex:0];
    if (scrollView.contentOffset.y < topShelfIndex*shelf.bounds.size.height) {
        topShelfIndex--;
        bottomShelfIndex--;
        
        // We'll assign the first shelf in the array as the lowest and continue
        // to test and reassign the lowest shelf until we've exhausted all
        // possible options.
        UIImageView *lowestImageView = shelf;
        UIImageView *currentShelfImage;
        NSEnumerator *shelfEnumerator = [shelves objectEnumerator];
        while (currentShelfImage = [shelfEnumerator nextObject]) {
            if (currentShelfImage.frame.origin.y > lowestImageView.frame.origin.y) {
                lowestImageView = currentShelfImage;
            }
        }
        
        [lowestImageView setFrame:CGRectMake(0, lowestImageView.bounds.size.height*topShelfIndex, scrollView.bounds.size.width, lowestImageView.bounds.size.height)];
    } else if (scrollView.contentOffset.y + scrollView.bounds.size.height > bottomShelfIndex*shelf.bounds.size.height) {
        topShelfIndex++;
        bottomShelfIndex++;
        
        // We'll assign the first shelf in the array as the topmost and continue
        // to test and reassign the topmost shelf until we've exhausted all
        // possible options.
        UIImageView *highestImageView = shelf;
        UIImageView *currentShelfImage;
        NSEnumerator *shelfEnumerator = [shelves objectEnumerator];
        while (currentShelfImage = [shelfEnumerator nextObject]) {
            if (currentShelfImage.frame.origin.y < highestImageView.frame.origin.y) {
                highestImageView = currentShelfImage;
            }
        }
        [highestImageView setFrame:CGRectMake(0, highestImageView.bounds.size.height*bottomShelfIndex, scrollView.bounds.size.width, highestImageView.bounds.size.height)];
    }
}