NSCache objectForKey: always return nil after memory warning on iOS 8

Question

I came across a problem that only happened on iOS 8. I used the NSCache to store my images. After receiving memory warning, I would fetch images and store into cache again. However the cache can't store my images anymore after warning. It always returns nil to me by using objectForKey:.
Here's part of my code:

 @interface ViewController ()
{
    NSCache *imageCache;
}

@implementation ViewController
- (instancetype)init
{
    self = [super init];
    if (self) {
        imageCache = [[NSCache alloc] init];
        [imageCache setTotalCostLimit:1024 * 1024 * 1];
    }
    return self;
}
- (void)imageDownloadManager:(ImageDownloadManager *)manager didReceiveImage:(UIImage *)image forObjectID:(NSString *)objecID
{   
    NSUInteger cost = [UIImageJPEGRepresentation(image, 0) length];
    image = [image smallImageWithCGSize:kImageThumbSize];
    [self.imageCache setObject:image forKey:objectID cost:cost];
    NSLog("image: %@",[self.imageCache objectForKey:objectID]);  //return nil
}
@end

Thanks :)

SOLUTIONS

You have to set countLimit and the value must greater than 0. Then you could use totalCostLimit as well.

“Yet, when memory is low, it will automatically discard some of its elements in order to free up memory for other application”… from here https://developer.apple.com/library/ios/documentation/Performance/Conceptual/ManagingMemory/Articles/CachingandPurgeableMemory.html — TonyMkenu, Dec 04 '14 at 09:09
yeah! but it could init after memory warning and supposed to be able to store/retrieve. It seems that it can't `setObject:` again after warning, and i'm sure `imageCache` is NOT nil as well. — yuhua, Dec 04 '14 at 09:15

score 1 · Accepted Answer · answered Mar 10 '15 at 10:26

I experienced the same problem (and only under iOS 8.1) and got it working by assigning a countLimit instead of a totalCostLimit.

// getter
- (NSCache *)cache
{
   if (!_cache) {
      _cache = [[NSCache alloc] init];
      _cache.countLimit = aLimit;
   }
   return _cache;
}

NSCache objectForKey: always return nil after memory warning on iOS 8

1 Answers1