NSURLSession sometimes returns data but sometimes don't iOS 9 - Swift 2.0

Question

I'm developing an application what's based on JSON objects requested from specified URL but sometimes (not every time) I get no result
Gonna show some code snippets that can help determining this simple (?) problem
I have an array called "Data" (var Data:[String] = []) in my class MyTestClass and the following function:

func loadData(onComplete: ([String]) -> ()) {
        // Declaring a local array
        var Local:[String] = []

        // Preparing JSON request
        let address = "<URL goes here>"
        let url = NSURL(string: address)
        let request = NSURLRequest(URL: url!)

        let session = NSURLSession.sharedSession()
        let task = session.dataTaskWithRequest(request) { (data, response, error) in
            if (data?.length > 0 && error == nil) {
                let responseData = data as NSData!

                if responseData == nil {
                    // Presenting some alert
                } else {
                    let jsonObject: AnyObject? = try? NSJSONSerialization.JSONObjectWithData(responseData!, options: NSJSONReadingOptions.MutableContainers)

                    if jsonObject == nil {
                        // Presenting some alert
                    } else {
                        let json: NSArray = jsonObject as! NSArray

                        for (var i = 0; i < json.count; i++) {
                            if let obj = json[i] as? NSDictionary {
                                if let m = obj["m"] as? String {
                                    if m != "Unknown" {
                                        Local.append(m)
                                    }
                                }
                            }
                        }

                        onComplete(Local)
                    }

                }
            } else if error != nil {
                NSLog("Error: %@", error!)
            }
        }

        // Starting task
        task.resume()
    }

In viewDidLoad() I'm doing the following:

// Loading data
loadData { (d) in
    for dat in d {
        self.Data.append(dat)
    }
}

I'm using this solution (the functions are 100% the same [CTRL+C / CTRL+V]) in every of my ViewControllers, my hierarchy is the following: Table1 -> Table2 -> Table3

Table1 works fine, has no problem, displays the results every time fine, Table2 works most of the time (sometimes very rarely displays no result) but Table3 goes crazy ~80% of the time.

I'm getting mad because of this thing happening to me, before iOS 9 I used synchronous calls and had no problems because it's a small application, but now I don't even know how to make it work with this session.dataTaskWithRequest... thing.

As an aside, I'd suggest following Cocoa naming conventions and start all of your variables with lowercase letters. Also "data" is misleading, because, by convention, that sort of name tends to be used with `NSData` objects. I might suggest `strings` (or, even better, something that suggests what sort of strings those are) instead of `Data`. — Rob, Oct 08 '15 at 22:04
I was just using that name as an example for my problem, in my real application it's different but thanks for suggestion. — Szilárd Dóró, Oct 08 '15 at 22:05

score 2 · Accepted Answer · edited May 23 '17 at 12:31

2

You need to use dispatch async and reload the data of the table view on completion of your loadData block

use dispatch async with dataTaskWithRequest

edited May 23 '17 at 12:31

Community

1
1

answered Oct 08 '15 at 21:35

matanwrites

812
1
10
18

Thank you, it's working now, but I have an other similar question. What if I'm working with structures? How to load them properly? `struct Information { var x = String() var y = String() var z = String() }` I'm using the same method as above to get data from web and pass it through onComplete, but the data seems lost. – Szilárd Dóró Oct 08 '15 at 22:01
1

The process is largely the same. Just make sure you call `reloadData` (and do all of that on the main queue) and it should be fine. By the way, I might suggest `struct Information { var x : String var y : String var z : String }` and then using `Information(x: a, y: b, z: c)` (where `a`, `b`, and `c` would be your strings). If that's not working, create new question with [example](http://stackoverflow.com/help/mcve). – Rob Oct 08 '15 at 22:12
And you would need to change the type of the `Local` variable to return your structure – matanwrites Oct 08 '15 at 22:19

Rob · Answer 2 · 2015-10-08T21:58:54.777

You should call tableView.reloadData() (from the main queue) the request is done, after you're done appending all of those strings to your array.

loadData { (d) in
    dispatch_async(dispatch_get_main_queue()) {
        for dat in d {
            self.Data.append(dat)
        }
        self.tableView.reloadData()
    }
}

For the record, it's excellent that you have retired the synchronous network calls. The synchronous calls may have been easy, but they can cause problems.

NSURLSession sometimes returns data but sometimes don't iOS 9 - Swift 2.0

2 Answers2