NSURLCache is emptied when app is closed - ios

Question

i'm trying to implement a NSURLCache with AFHTTPRequestOperation, this is what i have done:
AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:0 diskCapacity:50 * 1024 * 1024 diskPath:nil];
[NSURLCache setSharedURLCache:URLCache];
....
}
Then to use it i do this:
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:URLString] cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:30.0f];
UIImage *image = [[UIImageView sharedImageCache] cachedImageForRequest:urlRequest];
if (image != nil) {
//Cached image
} else {
AFHTTPRequestOperation *postOperation = [[AFHTTPRequestOperation alloc] initWithRequest:urlRequest];
postOperation.responseSerializer = [AFImageResponseSerializer serializer];
[postOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
UIImage *image = responseObject;
[[UIImageView sharedImageCache] cacheImage:image forRequest:urlRequest];
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"Image error: %#", error);
}];
[postOperation start];
}
the first time i open the view when there is the code above download the image and store it in the cache, then i close the view and i open it again and read it from the cache, instead if i close the app, and i try again the image is downloaded again, so how i can create a NSURLCache persistent?
EDIT:
I have tried what Duncan Bubbage suggest, and i have add a cache path instead of the nil, and i have do it in this way:
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
NSString *cachePath = [paths objectAtIndex:0];
cachePath = [cachePath stringByAppendingPathComponent:#"temp_img"];
NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:0 diskCapacity:50 * 1024 * 1024 diskPath:cachePath];
[NSURLCache setSharedURLCache:URLCache];
but i still cannot cache perstitently across session, what i have wrong? i have also check in the Finder the Cache Folder, and there is the temp_img folder, i have tried to create the folder manually with NSFileManager and then pass the path, but the folder temp_img remain empty...

Answer 1

Are you sure it is valid to set diskPath:nil when you are wanting to cache to disk persistently across sessions? The parameter is optional but that may only be for the use case of caching in memory only, in which case you'd expect to see what you're seeing here?
Alternatively, the Apple documentation for this method states:
NOTE
In iOS, the on-disk cache may be purged when the system runs low on disk space, but only when your app is not running.
This could just be expected behaviour?