Why do my async calls still block me from interacting with UI elements in Swift?

Question

My scenerio:

I asynchronously call a downloader object function to try to download a file.

Within the downloader's function, it asynchronously initiates a connection for downloading.

Even so, I cannot interact with my UI elements.

For example, I cannot edit my text field while download is in progress.

I want to be able to interact with my UI elements while downloading.

Where did I miss? What should I do? I truly appreciate your help!

---

ViewController's snippet for async download

//relevant UI elements I am referring to
@IBOutlet var renderButton: UIButton!
@IBOutlet var urlField: UITextField!

func handleOpenURL(url: NSURL){
    var downloader: aDownloader = aDownloader(url: url)

    //I try to put download to background thread
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {() -> Void in

        //This function initiates a connection call in an attempt to download the file
        downloader.executeConnection({ 
            (dataURL:NSURL!,error:NSError!) -> Void in  

                 dispatch_async(dispatch_get_main_queue(), { () -> Void in
                    self.imageView.image = UIImage(data:NSData(contentsOfURL: url)!)
                 })

                 //rest are nonsense
            }, progress: { (percentage:Double) -> Void in
                 //nonsense
            }
        )

     })
}

aDownloader.swift 's snippet for relevant code

class aDownloader: NSObject, allOtherProtocols {
    unowned var url: NSURL

    //this block reports the progress as a % number 
    var progressBlock : ((Double)->Void)?

    //this block will return either the destinationFileURL after download finishes,
    //or return error of any sort
    var dataBlock: ((NSURL!,NSError!)->Void)?

    init(url: NSURL) {
        self.url = url
    }

    func executeConnection(received:(NSURL!,NSError!)->Void, progress:(Double)->Void){
        var request : NSURLRequest = NSURLRequest(URL: self.url, 
                                                  cachePolicy: .ReloadIgnoringLocalCacheData,
                                                  timeoutInterval: 60.0)

        //I attempt to async-ly download the file here
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), { () -> Void in
            var connectionManager : NSURLConnection? = NSURLConnection(request:request, 
                                                                       delegate: self, 
                                                                       startImmediately: false)
            connectionManager?.scheduleInRunLoop(NSRunLoop.mainRunLoop(), 
                                                 forMode: NSRunLoopCommonModes)
            connectionManager?.start()
        })

        self.dataBlock = received
        self.progressBlock = progress
    }

    //nonsense
}

Answer 1

Two things.

1. Most importantly, you are still configuring your configuration manager to run on the main loop:

   connectionManager?.scheduleInRunLoop(NSRunLoop.mainRunLoop(), 
                                            forMode: NSRunLoopCommonModes)
   

   You will probably find it easier to just use NSURLConnection's class method sendAsynchronousRequest:queue:completionHandler: instead of manually starting the request in a background thread.

2. Also, it is a bad idea to update UI elements from a background thread:

   dispatch_async(dispatch_get_main_queue(), { () -> Void in
       self.imageView.image = UIImage(data:NSData(contentsOfURL: url)!)
   })
   

   Instead, you should load the image into a temporary variable in the background and then perform the actual assignment back on the main thread.