Image scrollview crashes - ios

Question

I've implemented a scrollview with paging to scroll between some images (graphs) at full page (like the Photo app installed in the iPhone).
I found the code below that use the classical 3 pages solution (I made some small modification for my application) but, even if it "works", the scrolling seems to be slow and often after I've scrolled some images the application crashes.
I'm using Xcode 4.2 with ARC option enabled and testing both on an iPad device.
Images (10 jpg) are 2048x1539 with a mean dimension of 200/250Kb each.
Is there anyone that can help me in finding the cause of the problem ?
Thanks,
Corrado
const int numImages = 10;
const float kPageWidth = 1024.0f;
const float kPageHeight = 768.0f;
- (void)viewDidLoad {
[super viewDidLoad];
scroll.contentSize = CGSizeMake(kPageWidth * numImages, kPageHeight);
imageview1 = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, kPageWidth, kPageHeight)];
imageview2 = [[UIImageView alloc] initWithFrame:CGRectMake(kPageWidth, 0, kPageWidth, kPageHeight)];
imageview3 = [[UIImageView alloc] initWithFrame:CGRectMake(kPageWidth * 2, 0, kPageWidth, kPageHeight)];
scroll.contentOffset = CGPointMake(0, 0);
[imageview1 setImage:[UIImage imageNamed:#"grafico_0.jpg"]];
imageview1.contentMode = UIViewContentModeScaleAspectFit;
[imageview1 setTag:1];
imageview2.contentMode = UIViewContentModeScaleAspectFit;
[imageview2 setTag:2];
imageview3.contentMode = UIViewContentModeScaleAspectFit;
[imageview3 setTag:3];
[scroll addSubview:imageview1];
[scroll addSubview:imageview2];
[scroll addSubview:imageview3];
}
- (void)scrollViewDidScroll:(UIScrollView*)scrollView {
const CGFloat currPos = scrollView.contentOffset.x;
const NSInteger selectedPage = lroundf(currPos * (1.0f / kPageWidth));
const NSInteger zone = 1 + (selectedPage % 3);
const NSInteger nextPage = selectedPage + 1;
const NSInteger prevPage = selectedPage - 1;
/// Next page
if (nextPage < numImages)
{
NSInteger nextViewTag = zone + 1;
if (nextViewTag == 4)
nextViewTag = 1;
UIImageView* nextView = (UIImageView*)[scrollView viewWithTag:nextViewTag];
nextView.frame = (CGRect){.origin.x = nextPage * kPageHeight, .origin.y = 0.0f, kPageHeight, kPageWidth};
NSString *str = [NSString stringWithFormat:#"grafico_%d.jpg", nextPage];
UIImage* img = [UIImage imageNamed:str];
nextView.image = img;
}
/// Prev page
if (prevPage >= 0)
{
NSInteger prevViewTag = zone - 1;
if (!prevViewTag)
prevViewTag = 3;
UIImageView* prevView = (UIImageView*)[scrollView viewWithTag:prevViewTag];
prevView.frame = (CGRect){.origin.x = prevPage * kPageHeight, .origin.y = 0.0f, kPageHeight, kPageWidth};
NSString *str = [NSString stringWithFormat:#"grafico_%d.jpg", prevPage];
UIImage* img = [UIImage imageNamed:str];
prevView.image = img;
}
}

Answer 1

You should not use imageNamed: for the loading of your large images, because that method caches the images and should only be used for small images that you use multiple times in your App (like images for buttons etc.). That method is notorious for causing memory problems when used with many large images.
Switch to imageWithContentsOfFile: instead. Loading your images with that methods secures that the images are not cached and the memory is freed after you do not use that images any more.
If the scrolling seems to be sluggish you can move the loading of the image to a background thread using performSelectorInBackground:
[self performSelectorInBackground:#selector(retrieveImageData:) withObject:imagePath];
the loading of the UIImage happens in this method:
- (void)retrieveImageData:(NSString *)imagePath {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
[self performSelectorOnMainThread:#selector(imageDataRetrieved:) withObject:image waitUntilDone:NO];
[pool release];
}
and the attachment of the image to the UIImageView on the main thread (UI manipulations must not happen on a background Thread):
- (void)imageDataRetrieved:(UIImage)*image {
yourImageView.image = image;
}