Swift NSURLSession Check File Last Modified Date With HEAD Request

Implemenation.swift

func checkShouldDownloadFileAtLocation(urlString:String, completion:((shouldDownload:Bool) -> ())?) {
    var request = NSMutableURLRequest(URL: NSURL(string: urlString)!)
    request.HTTPMethod = "HEAD"
    var session = NSURLSession.sharedSession()
    
    var err: NSError?
    
    var task = session.dataTaskWithRequest(request, completionHandler: { [weak self] data, response, error -> Void in
        if let strongSelf = self {
            var isModified = false
            
            var err: NSError?
            if let httpResp: NSHTTPURLResponse = response as? NSHTTPURLResponse {
                let lastModifiedDate = httpResp.allHeaderFields["Last-Modified"] as? String
                if lastModifiedDate != nil {
                    let newLastModifiedDate = strongSelf.dateFormatter.dateFromString(lastModifiedDate!)
                    
                    if newLastModifiedDate != nil {
                        let currentLastModifiedDate = NSUserDefaults.standardUserDefaults().objectForKey("LastModifiedDate") as? NSDate
                        if currentLastModifiedDate == nil {
                            isModified = true
                        } else {
                            isModified = !newLastModifiedDate!.isEqual(currentLastModifiedDate!)
                        }
                        
                        NSUserDefaults.standardUserDefaults().setObject(newLastModifiedDate!, forKey: "LastModifiedDate")
                        NSUserDefaults.standardUserDefaults().synchronize()
                    }
                }
                
            }
            
            if completion != nil {
                dispatch_async(dispatch_get_main_queue(), { () -> Void in
                    completion!(shouldDownload: isModified)
                })
            }
        }
        
        })
    
    task.resume()
}

Usage.swift

self.checkShouldDownloadFileAtLocation("http://urltolocation.com/of/file.jpg", completion: { (shouldDownload) -> () in
    if shouldDownload {
        // Go download the file now
        
    }
})

Try this

import Foundation
extension URLResponse {
    var lastModifiedHeader: String? {
        get {
            if let httpResp = self as? HTTPURLResponse, let lastMod = httpResp.allHeaderFields["Last-Modified"] as? String {
                return lastMod
            }
            return nil
        }
    }
    var lastModified: Date? {
        get {
            if let lastModifiedHeader = self.lastModifiedHeader {
                let gmtDateFormatter = DateFormatter()
                gmtDateFormatter.dateFormat = "EEEE, dd LLL yyyy HH:mm:ss zzz"
                if let serverDate = gmtDateFormatter.date(from: lastModifiedHeader) {
                    return serverDate
                }
            }
            return nil
        }
    }
}