More responsive UIImageView+AFNetworking using NSURLCache

UIImageView-Cache-Then-Network.m

// It's debatable if it's actually a good idea, because most of the time the image will be
// loaded from the cache without revalidation anyway, but it still has some utility in cases
// when max-age is short, or the image is big
//
// Also, it is more useful if we assume that max-age is smaller than the average user session,
// and that the iOS device never has to evict the images in the UIImageView+AFNetworking cache,
// because then *every* request for the image that actually goes through the NSURLCache will
// trigger the double request codepath. That being said, it's still probably excessive, because
// the amortised delay before the image is shown is still not affected much
- (void)setImageWithURLRequest:(NSURLRequest *)originalURLRequest
          placeholderImage:(UIImage *)placeholderImage
                   success:(void (^)(NSURLRequest *, NSHTTPURLResponse *, UIImage *))success
                   failure:(void (^)(NSURLRequest *, NSHTTPURLResponse *, NSError *))failure
{
    __weak __typeof(self) weakSelf = self;
    
    // first, try to find the image in the cache (both NSURLCache and UIImageView+AFNetworking caches)
    NSMutableURLRequest *cacheOnlyURLRequest = [originalURLRequest mutableCopy];
    cacheOnlyURLRequest.cachePolicy = NSURLRequestReturnCacheDataDontLoad;
    [self
     superSetImageWithURLRequest:cacheOnlyURLRequest
                placeholderImage:placeholderImage
                         // found the image in the cache
                         success:^void(NSURLRequest *urlRequest, NSHTTPURLResponse *urlResponse, UIImage *image) {
                             __typeof(self) strongSelf = weakSelf;
                             if (!strongSelf) {
                                 return;
                             }
                             
                             if (!urlResponse) {
                                 // the image was in the UIImageView+AFNetworking cache,
                                 // so we should just use it to avoid excessive network traffic
                                 return;
                             }
                             
                             // kill the image in the UIImageView+AFNetworking cache so that the next HTTP request
                             // goes through NSURLConnection
                             // WARNING: assumes that +sharedImageCache is actually an NSCache instance!
                             [(NSCache *)[[super class] sharedImageCache] removeObjectForKey:[[urlRequest URL] absoluteString]];
                             
                             // repeat the request, but now allow NSURLCache to revalidate it if needed
                             // use the previously received image as a placeholder
                             [strongSelf superSetImageWithURLRequest:originalURLRequest placeholderImage:image success:success failure:failure];
                         }
                         // image not found in the cache
                         failure:^void(NSURLRequest *urlRequest, NSHTTPURLResponse *urlResponse, NSError *error) {
                             // default behavior
                             [weakSelf superSetImageWithURLRequest:originalURLRequest placeholderImage:placeholderImage success:success failure:failure];
                         }];
}

// a trick to avoid implicitly retaining self when using super
- (void)superSetImageWithURLRequest:(NSURLRequest *)urlRequest
                   placeholderImage:(UIImage *)placeholderImage
                            success:(void (^)(NSURLRequest *, NSHTTPURLResponse *, UIImage *))success
                            failure:(void (^)(NSURLRequest *, NSHTTPURLResponse *, NSError *))failure
{
    [super setImageWithURLRequest:urlRequest placeholderImage:placeholderImage success:success failure:failure];
}