NSURLSession到底如何工作？

Question

The following lines of code are written inside a function and session_id and session_name are global variables.

let task = NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
        guard let _:NSData = data, let _:NSURLResponse = response  where error == nil else {
            print("error")
            return
        }
        let dataString = NSString(data: data!, encoding: NSUTF8StringEncoding)
        print("dataString1 = \(dataString)")
        var json = try! NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.AllowFragments) as? NSDictionary
        if(error != nil) {
            let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
            print("Error could not parse JSON: '\(jsonStr)'")
            print(error)
        }
        else
            if let parseJSON = json {
                print("jsonstr = \(json)")
                if let success = parseJSON["success"] as? String {
                    if success == "true" {
                        let session_valid = parseJSON["session_valid"] as! String
                        if session_valid == "true"{
                            let response = parseJSON.objectForKey("response") as! NSDictionary
                            print("response = \(response)")
                            session_id = response.objectForKey("session_id") as! String
                            session_name = response.objectForKey("session_name") as! String
                            dispatch_async(dispatch_get_main_queue(), { () -> Void in
                                if let resultController = self.storyboard!.instantiateViewControllerWithIdentifier("splitViewController") as? UISplitViewController {
                                    self.presentViewController(resultController, animated: true, completion: nil)
                                }
                            })
                        }
                    }
                }
                print("values1 are \(session_id) and \(session_name)") //print stmt 1
                return
            }//if
            else {
                print("json not parsed")
                let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
                print("Error could not parse JSON: \(jsonStr)")
                print(error)
        }//else
    }
    print("values are \(session_id) and \(session_name)") //print stmt 2
    task.resume()

The output relevant to the query is: (session_id and session_name are initialised to random values 1 and a)

values are 1 and a

and then it prints: (values of the two variables in the response from the php script)

values1 are ckj0uuj2q18m97m78m1uvje7f5 and d72d1363f44031cac4148b0e6fa295d6

My query is that how is 'print stmt 2' printed before 'print stmt 1'? I am new to swift. Am I missing any concept here? Also, why does 'print stmt 2' print the initial values of the variables and not the new values? I know the two questions are related but I am not getting how

Answer 1

The code inside that big block (Swift calls it a closure) gets run after the request completes. There's not a way to get a value over the Internet immediately, because it can take up to 90 seconds, and you don't want your entire app blocked for that long (particularly because iOS will kill your app outright after 30).

The way you solve that is by changing the code that needs the values so that it provides code to run after the values have been retrieved, to do something with those values.

See https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html for more info.