memory crash when storing NSOperationQueues in a NSCache - ios

Question

I have a UITableView that loads thumbnails into cells aynchronously as follows:
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:
^{
ThumbnailButtonView *thumbnailButtonView = [tableViewCell.contentView.subviews objectAtIndex:i];
UIImage *image = [self imageAtIndex:startingThumbnailIndex + i];
[self.thumbnailsCache setObject: image forKey:[NSNumber numberWithInt:startingThumbnailIndex + i]];
[[NSOperationQueue mainQueue] addOperationWithBlock:
^{
UITableViewCell *tableViewCell = [self cellForRowAtIndexPath:indexPath];
if (tableViewCell)
{
[activityIndicatorView stopAnimating];
[self setThumbnailButtonView:thumbnailButtonView withImage:image];
}
}];
}];
[self.operationQueue addOperation:operation];
[self.operationQueues setObject:operation forKey:[NSNumber numberWithInt:startingThumbnailIndex + i]];
As per a technique I learned in a WWDC presentation, I store all of my operation queues in a NSCache called operationQueues so that later on I can cancel them if the cell scrolls off the screen (there are 3 thumbnails in a cell):
- (void) tableView:(UITableView *)tableView didEndDisplayingCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath
{
NSInteger startingThumbnailIndex = [indexPath row] * self.thumbnailsPerCell;
for (int i = 0; i < 3; i++)
{
NSNumber *key = [[NSNumber alloc] initWithInt:i + startingThumbnailIndex];
NSOperation *operation = [self.operationQueues objectForKey:key];
if (operation)
{
[operation cancel];
[self.operationQueues removeObjectForKey:key];
}
}
}
However, I notice if I repeatedly launch, load, then close my UITableView, I start recieving memory warnings, and then eventually the app crashes. When I remove this line:
[self.operationQueues setObject:operation forKey:[NSNumber numberWithInt:startingThumbnailIndex + i]];
The memory issues go away. Does anyone have any clue on why storing the operation queues in a cache or an array causes the app to crash?

Answer 1

Note: I learnt about NSCache and NSOperationQueue a couple of days ago so I might be wrong.
I don't think that this is a problem with NSOperationQueue, you are adding images to your thumbnailsCache but when the view scrolls off-screen they are still in memory. I am guessing that when the cells scroll back in, you re-create your images. This is probably clogs your memory.
Also, shouldn't you be caching your images instead of your operations?
EDIT
I did some detailed testing with NSCache by adding images and strings until my app crashed. It doesn't seem to be evicting any items so I wrote my custom cache, which seems to work:
#implementation MemoryManagedCache : NSCache
- (id)init
{
self = [super init];
if (self) {
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(reduceMemoryFootprint) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
}
return self;
}
- (void)reduceMemoryFootprint
{
[self setCountLimit:self.countLimit/2];
}
#end