How to integrate Etag in Swift

etag

let API_HEADER_FIELD_NONE_MATCH             : String    = "If-None-Match"
let API_HEADER_FIELD_ETAG                   : String    = "Etag"
let API_REQUEST_SUCCESS                     : Int       = 200


func ETagForURL(urlString: String) -> String? {
    // return the saved ETag value for the given URL
    return NSUserDefaults.standardUserDefaults().objectForKey(urlString) as String?
}
    
func updateETag(urlResponse: NSURLResponse!) {
    if let httpResponse = urlResponse as? NSHTTPURLResponse {
        if let urlString = httpResponse.URL?.absoluteString {
            if let etag = httpResponse.allHeaderFields[API_HEADER_FIELD_ETAG] as? String {
                // save the etag header value with the url as a key.
                NSUserDefaults.standardUserDefaults().setObject(etag, forKey: urlString)
                NSUserDefaults.standardUserDefaults().synchronize()
            }
        }
    }
}

func readFromBackend() {

    let urlString = "https://herderbeten.s3.eu-central-1.amazonaws.com/impressum/imprint.html"

    // create a mutable request
    var request = NSMutableURLRequest(URL: NSURL(string: urlString)!, cachePolicy: NSURLRequestCachePolicy.ReloadIgnoringLocalCacheData, timeoutInterval: 60);

    // add etag header value
    if let etag = self.ETagForURL(urlString) {
        request.addValue(etag, forHTTPHeaderField: API_HEADER_FIELD_NONE_MATCH)
    }

    NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue()) { (response: NSURLResponse!, data: NSData!, error: NSError!) -> Void in

        if (error != nil) {
            UIAlertView(title: L("API_ERROR"), message: error.localizedDescription, delegate: nil, cancelButtonTitle: nil, otherButtonTitles:"ok").show()
        }
        if (response != nil) {
            self.updateETag(response)

            if let httpResponse = response as? NSHTTPURLResponse {
                // Only update the entities if the status code is K_API_REQUEST_SUCCESS (200) and if there is no ERROR
                // The status code could also be K_API_REQUEST_CACHED (304) depending on the etag status
                if (httpResponse.statusCode == API_REQUEST_SUCCESS && error == nil && data != nil) {
                    // New data fetched
                    let htmlText = NSString(data: data, encoding: NSUTF8StringEncoding)
                    return
                }
            }
        }
        // An error occured OR nothing changed. The application should use cached data.
    }
}

// MARK: - Write files to cache folder

func writeDataToFile(data: NSData, filename: String) {
    if let path = self.getCacheDirectoryFilepath(filename) {
        data.writeToFile(path, atomically: false)
        self.addSkipBackupAttributeToItemAtURL(NSURL.fileURLWithPath(path as NSString))
    }
}

func getCacheDirectoryFilepath(filename: String) -> String? {
    let directories : [String]? = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.CachesDirectory, NSSearchPathDomainMask.AllDomainsMask, true) as? [String]
    if let cacheFolderPath = directories?.first {
        return cacheFolderPath.stringByAppendingPathComponent(filename)
    }
    return nil
}

func addSkipBackupAttributeToItemAtURL(URL: NSURL!) -> Bool {
    assert(NSFileManager.defaultManager().fileExistsAtPath(URL.path!))
    var error : NSError? = nil
    var success : Bool = URL.setResourceValue(true, forKey: NSURLIsExcludedFromBackupKey, error: &error)
    if (success == false || error != nil) {
        println("Error excluding \(URL.lastPathComponent) from backup \(error) ")
    }
    return success
}

@kevindelord why do you need to do this all stuff when NSURLRequest.CachePolicy.useProtocolCachePolicy provides all sorts of implementation by default based on RFC 2616.

https://developer.apple.com/documentation/foundation/nsurlrequest/cachepolicy/useprotocolcachepolicy
https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13

Probably because cache could be lost but you could have a copy of some file locally that you don't need to update so you don't get too much traffic on your hosting. That's my case at least.