Question

I’m trying to cache images I load from Flickr. If I load the same image, I’m hoping to use the cached version instead. For some reason, when I download an image from the internet, it works, but if I load it from my cache, it displays a blank image. I checked cacheData, and it has the same amount of bits as the image I put in, so it appears loading the file is working.

Here is how I cache images:

+ (void)cachePhoto:(NSData *)photo withKey:(NSString *)key {
    if (photo) {
        NSArray * urlArray = [fileManager URLsForDirectory:NSCachesDirectory inDomains:NSUserDomainMask];
        NSURL * targetDirectory = (NSURL *)[urlArray objectAtIndex:0];
        targetDirectory = [targetDirectory URLByAppendingPathComponent:key];
        [photo writeToURL:targetDirectory atomically:YES];
        [cachedPhotos addObject:key];
        NSLog(@"target url %@", targetDirectory);
    }
}

+ (NSData *)photoInCache:(NSString *)key {
    if ([cachedPhotos containsObject:key]) {
        NSString * path = [[cacheDirectory URLByAppendingPathComponent:key] path];
        NSLog(@"path: %@", path);
        return [fileManager contentsAtPath:path];
    } else {
        return nil;
    }
}

And my code to get it back:

NSData * cacheData = [PhotoCache photoInCache:key];

if (cacheData) {
    self.imageView.image = [UIImage imageWithData:cacheData];
    [spinner stopAnimating];
    NSLog(@"used cached image");
} else {
    dispatch_queue_t downloadQueue = dispatch_queue_create("get photo from flickr", NULL);
    dispatch_async(downloadQueue, ^{

        NSData * imageData = [[NSData alloc] initWithContentsOfURL:url];
        dispatch_async(dispatch_get_main_queue(), ^{
            [spinner stopAnimating];
            self.imageView.image = [UIImage imageWithData:imageData];
            [PhotoCache cachePhoto:imageData 
                           withKey:key];
        });
    });
}

Answer 1

I figured it out – I was loading the image into the imageView in my prepareForSegue, where the ImageView was not yet loaded. I loaded the image in viewDidLoad and it worked. I also had to use UIImagePNGRepresentation, like suggested in the comments.