NSURLCache issue - images being released from cache? - ios

Question

I have an app where I'm downloading quite a few images for display later on.
The app needs to function even when there's no internet connection, so the images are loaded at one point and persisted using NSURLCache.
This is a neat solution since I can use normal networking libraries and easily take advantage of custom cache settings.
I realize that this type of caching doesn't guarantee that the files are persisted until they expire since it's up to the system to release cache whenever it deems necessary. That's not a huge issue since the images should be able to be re-downloaded and hence re-persisted.
However, I've noticed that it decides to randomly release images from cache, and it doesn't seem to persist them when I download them again. I'm not sure where I'm going wrong.
First of all, I define the cache capacity like so:
NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:50 * 1024 * 1024
diskCapacity:200 * 1024 * 1024
diskPath:#"netcache"];
[NSURLCache setSharedURLCache:URLCache];
(50MB in memory and 200MB on disk)
When downloading the images (using AFNetworking) I modify the response headers to set Cache-Control to max-age=31536000 which means it should cache the response for one year.
NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:60.0];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
// Download completed
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
// Download failed
}];
[operation setCacheResponseBlock:^NSCachedURLResponse *(NSURLConnection *connection, NSCachedURLResponse *cachedResponse) {
NSURLResponse *response = cachedResponse.response;
NSHTTPURLResponse *HTTPResponse = (NSHTTPURLResponse*)response;
NSDictionary *headers = HTTPResponse.allHeaderFields;
NSMutableDictionary *modifiedHeaders = headers.mutableCopy;
modifiedHeaders[#"Cache-Control"] = #"max-age=31536000"; // 1 year in seconds
NSHTTPURLResponse *modifiedHTTPResponse = [[NSHTTPURLResponse alloc]
initWithURL:HTTPResponse.URL
statusCode:HTTPResponse.statusCode
HTTPVersion:#"HTTP/1.1"
headerFields:modifiedHeaders];
return [[NSCachedURLResponse alloc] initWithResponse:modifiedHTTPResponse data:cachedResponse.data userInfo:cachedResponse.userInfo storagePolicy:NSURLCacheStorageAllowed];
}];
[self.operationQueue addOperation:operation];
...and yet it seems like images are released and won't even get "re-cached" when downloaded again. (I believe they are reported as cached, even though they won't load when trying to display them when the device doesn't have any connection.)
The images are later displayed using AFNetworking's UIImageView+AFNetworking.h category, like so:
[self.imageView setImageWithURL:url];
Any ideas?

Answer 1

I have faced the same problem. First of all you can use AFURLSessionManager's method
- (void)setDataTaskWillCacheResponseBlock:(NSCachedURLResponse * (^)(NSURLSession *session, NSURLSessionDataTask *dataTask, NSCachedURLResponse *proposedResponse))block;
to set cache block for all requests at the same time.
Second - check that 'Cache-Control' field in response headers is 'public'. For example:
[someAFHTTPSessionManager setDataTaskWillCacheResponseBlock:^NSCachedURLResponse *(NSURLSession *session, NSURLSessionDataTask *dataTask, NSCachedURLResponse *proposedResponse)
{
NSHTTPURLResponse *resp = (NSHTTPURLResponse*)proposedResponse.response;
NSMutableDictionary *newHeaders = [[resp allHeaderFields] mutableCopy];
if (newHeaders[#"Cache-Control"] == nil) {
newHeaders[#"Cache-Control"] = #"public";
}
NSHTTPURLResponse *response2 = [[NSHTTPURLResponse alloc] initWithURL:resp.URL statusCode:resp.statusCode HTTPVersion:#"1.1" headerFields:newHeaders];
NSCachedURLResponse *cachedResponse2 = [[NSCachedURLResponse alloc] initWithResponse:response2
data:[proposedResponse data]
userInfo:[proposedResponse userInfo]
storagePolicy:NSURLCacheStorageAllowed];
return cachedResponse2;
}];
Third: If image's size is bigger than 5% of total cache space it will not be cached. Also it seems that there are some additional hidden rules.