Cacheing UIImage as NSData and getting it back - objective-c

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.