No Data in Array after appending using Swift and xcode

I am trying to download images from a url and then save them in an array of NSData.

I have a Class called Data Manager in which all my data is stored as well as functions for downloading images and getting data from URL.

In the same class I declare a variable called imageData of type [NSData] and let it equal an empty array as follows:

   var imageData: [NSData] = []

here is what my other 2 functions look like:

func getDataFromUrl(url:NSURL, completion: ((data: NSData?, response: NSURLResponse?, error: NSError? ) -> Void)) {
NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) in
completion(data: data, response: response, error: error)
}.resume()
}

func downloadImage(url: NSURL){
    print("Download Started")
    print("lastPathComponent: " + (url.lastPathComponent ?? ""))
    getDataFromUrl(url) { (data, response, error)  in
        dispatch_async(dispatch_get_main_queue()) { () -> Void in
            guard let data = data where error == nil else { return }
            print(response?.suggestedFilename ?? "")
            print("Download Finished")
            self.imageData.append(data)
            print("you have \(self.imageData.count)")
        }
        print("you still do have \(self.imageData.count)")
    }
}

I call these functions in my app Delegate class under the function didFinishLaunchingWithOptions as so

let dataManager = DataManager()
func application(application: UIApplication, didFinishLaunchingWithOptions      launchOptions: [NSObject: AnyObject]?) -> Bool {
    // Override point for customization after application launch.

    dataManager.URLStringArray.removeAll()

  for url in dataManager.objects.imageURLS {
dataManager.URLStringArray.append(url)
  }
    for url in dataManager.URLStringArray {   
    dataManager.downloadImage(NSURL(string: url)!)
    print(url)
    }

return true
}

In my view controller I go to get the data in the image array via following function:

func returnImageData() -> [NSData] {
    print("your image count is \(imageData.count))")
    return imageData
}

but the array is empty! Even though through the whole process I noticed that the array was becoming larger and larger because the print to the logs were showing the array increasing!

Thanks!