NSURLCache using cache for requests older than max-age

Question

I've been banging my head against the while now trying to figure out what's going on with NSURLCache.

Basically, the server I am connecting to doesn't set any cache control headers... So following various guides and apple docs (ie https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/URLLoadingSystem/Concepts/CachePolicies.html) I set my own cache control headers in the willCacheResponse delegate and then return that modified response in the completion handler. The apple docs and the resource I've read seem to indicate this should work. But what I'm seeing is that the cached data is returned after it should be expired based on max-age. It seems like max-age is being ignored and the NSURLCache is using another heuristic to determine if it should pull data from the cache or not.

I set the max-age=60 cache-control header and I've verified using Charles that the data is pulled from cache and no network requests are made long after 60 seconds. Eventually (seems non-deterministic) a new request will be made that actually goes to the server (usually after a few hours have passed and I try the request again).

Here is the code, for testing purposes I'm just hardcoding the max-age to 60 seconds:

func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, willCacheResponse proposedResponse: CachedURLResponse, completionHandler: @escaping (CachedURLResponse?) -> Void) {
    var modifiedReponse: URLResponse? = nil
    if let HTTPResponse = proposedResponse.response as? HTTPURLResponse {

        if var newHeaders = HTTPResponse.allHeaderFields as? [String : String] {

            if newHeaders["Cache-Control"] == nil {
                newHeaders["Cache-Control"] = "max-age=60"
            }

            modifiedReponse = HTTPURLResponse(url: HTTPResponse.url!, statusCode: HTTPResponse.statusCode, httpVersion: "HTTP/1.1", headerFields: newHeaders)
        }
    }
    let response = modifiedReponse ?? proposedResponse.response
    var newCachedResponse: CachedURLResponse? = nil
    newCachedResponse = CachedURLResponse(response: response, data: proposedResponse.data, storagePolicy: proposedResponse.storagePolicy)
}

There are a few optional checks in there but I have confirmed that the response on newCachedResponse I am returning has the cache control header set to max-age=60. Am I doing something obviously wrong here? Or is NSURLCache just F'd?

DanielGaffey DanielGaffey · Accepted Answer · 2018-02-01T00:55:31

I know this is super late, but I think you just need to pass that newCachedResponse back into the completion handler closure provided to you.

Such as the last line being:

completionHandler(newCachedResponse)

NSURLCache using cache for requests older than max-age

1 Answers