How to set NSURLRequest cache expiration?

Question

I'm using AFNetworking and need to cache data in one response for a several minutes. So I set NSUrlCache in app delegate and then in my request setting up it:

NSMutableURLRequest *request = //obtain request; 
request.cachePolicy = NSURLRequestReturnCacheDataElseLoad;

How then set expiration date: if the data was loaded more than n minutes ago, ask response from server and not from disk?

EDIT:

Assume that server doesn't support caching, I need to manage it in code.

Answer 1

The expiration of responses in the NSURLCache is controlled via the Cache-Control header in the HTTP response.

EDIT I see you've updated your question. If the server doesn't provide the Cache-Control header in the response, it won't be cached. Every request to that endpoint will load the endpoint rather than return a cached response.

Answer 2

Swift equivalent of @HotJard's solution using URLSession

extension CachedURLResponse {
    func response(withExpirationDuration duration: Int) -> CachedURLResponse {
        var cachedResponse = self
        if let httpResponse = cachedResponse.response as? HTTPURLResponse, var headers = httpResponse.allHeaderFields as? [String : String], let url = httpResponse.url{

            headers["Cache-Control"] = "max-age=\(duration)"
            headers.removeValue(forKey: "Expires")
            headers.removeValue(forKey: "s-maxage")

            if let newResponse = HTTPURLResponse(url: url, statusCode: httpResponse.statusCode, httpVersion: "HTTP/1.1", headerFields: headers) {
            cachedResponse = CachedURLResponse(response: newResponse, data: cachedResponse.data, userInfo: headers, storagePolicy: cachedResponse.storagePolicy)
            }
        }
        return cachedResponse
    }
}

Then implement URLSessionDataDelegate protocol in your custom class

func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, willCacheResponse proposedResponse: CachedURLResponse, completionHandler: @escaping (CachedURLResponse?) -> Void) {

    if dataTask.currentRequest?.cachePolicy == .useProtocolCachePolicy {
        let newResponse = proposedResponse.response(withExpirationDuration: 60)
        completionHandler(newResponse)
    }else {
        completionHandler(proposedResponse)
    }
}

Don't forget to create your configuration and session, passing in the your custom class as the delegate reference e.g.

let session = URLSession(
        configuration: URLSession.shared.configuration,
        delegate: *delegateReference*,
        delegateQueue: URLSession.shared.delegateQueue
    )
let task = session.dataTask(with: request)
task.resume()

Answer 3

So, I found the solution.

The idea is to use connection:willCacheResponse: method. Before cache the response it will be executed and there we can change response and return new, or return nil and the response will not be cached. As I use AFNetworking, there is a nice method in operation:

- (void)setCacheResponseBlock:(NSCachedURLResponse * (^)(NSURLConnection *connection, NSCachedURLResponse *cachedResponse))block;

Add code:

  [operation setCacheResponseBlock:^NSCachedURLResponse *(NSURLConnection *connection, NSCachedURLResponse *cachedResponse) {
    if([connection currentRequest].cachePolicy == NSURLRequestUseProtocolCachePolicy) {
      cachedResponse = [cachedResponse responseWithExpirationDuration:60];
    }
    return cachedResponse;
  }];

Where responseWithExpirationDuration from category:

@interface NSCachedURLResponse (Expiration)
-(NSCachedURLResponse*)responseWithExpirationDuration:(int)duration;
@end

@implementation NSCachedURLResponse (Expiration)

-(NSCachedURLResponse*)responseWithExpirationDuration:(int)duration {
  NSCachedURLResponse* cachedResponse = self;
  NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)[cachedResponse response];
  NSDictionary *headers = [httpResponse allHeaderFields];
  NSMutableDictionary* newHeaders = [headers mutableCopy];

  newHeaders[@"Cache-Control"] = [NSString stringWithFormat:@"max-age=%i", duration];
  [newHeaders removeObjectForKey:@"Expires"];
  [newHeaders removeObjectForKey:@"s-maxage"];

  NSHTTPURLResponse* newResponse = [[NSHTTPURLResponse alloc] initWithURL:httpResponse.URL
                                                               statusCode:httpResponse.statusCode
                                                              HTTPVersion:@"HTTP/1.1"
                                                             headerFields:newHeaders];

  cachedResponse = [[NSCachedURLResponse alloc] initWithResponse:newResponse
                                                            data:[cachedResponse.data mutableCopy]
                                                        userInfo:newHeaders
                                                   storagePolicy:cachedResponse.storagePolicy];
  return cachedResponse;
}

@end

So, we set expiration in seconds in http header according to http/1.1 For that we need one of headers to be set up: Expires, Cache-Control: s-maxage or max-age Then create new cache response, because the properties is read only, and return new object.