NSURLCache trick to control cache on client side

gistfile1.m

typedef NSCachedURLResponse * (^WGNCachedResponseBlock)(NSURLConnection *connection, NSCachedURLResponse *cachedResponse);

- (WGNCachedResponseBlock)cacheResponseBlockWithTime:(NSTimeInterval)time {
	return ^NSCachedURLResponse *(NSURLConnection *connection, NSCachedURLResponse *cachedResponse) {
		NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)[cachedResponse response];
		
		NSDictionary *headers = [httpResponse allHeaderFields];
		NSString *cacheControl = [headers valueForKey:@"Cache-Control"];
		NSString *expires = [headers valueForKey:@"Expires"];
		if((cacheControl == nil) && (expires == nil)) {
			NSMutableDictionary *modifiedHeaders = [httpResponse.allHeaderFields mutableCopy];
			[modifiedHeaders addEntriesFromDictionary:@{@"Expires": [NSDate RFC822StringFromDate:[[NSDate date] dateByAddingTimeInterval:time]], @"Cache-Control": @"public"}];
			
			NSHTTPURLResponse *response = [[NSHTTPURLResponse alloc] initWithURL:httpResponse.URL
																	  statusCode:httpResponse.statusCode
																	 HTTPVersion:@"HTTP/1.1" headerFields:modifiedHeaders];
			NSCachedURLResponse *modifiedCachedResponse = [[NSCachedURLResponse alloc] initWithResponse:response
																								   data:cachedResponse.data
																							   userInfo:cachedResponse.userInfo
																						  storagePolicy:NSURLCacheStorageAllowed];
			return modifiedCachedResponse;
			
		}
		return cachedResponse;
	};
}