Why isn't my method getting called? - swift

Question

I have a viewController communicating with DownloaderHandler using DownloaderDelegate protocol.
My protocol is defined as:
protocol DownloaderDelegate : class {
func didReceive(data:Data)
}
I have a viewController
class ViewController: UIViewController {
weak var downloadHandler : DownloaderHandler?
override func viewDidLoad() {
super.viewDidLoad()
downloadHandler = DownloaderHandler()
downloadHandler?.delegate = self
changeBackground()
}
func changeBackground (){
let googleURL = URL(fileURLWithPath: "https://www.google.com/doodle4google/images/splashes/featured.png")
print(googleURL)
downloadHandler?.downloadData(url:googleURL) // Line BB
}
}
extension ViewController : DownloaderDelegate{
func didReceive(data: Data) {
let image = UIImage(data: data)
let imageView = UIImageView(image: image!)
view.insertSubview(imageView, at: 0)
}
}
And I have a Delegating class as :
class DownloaderHandler : NSObject, URLSessionDelegate{
weak var delegate :DownloaderDelegate?
var downloadsSession: URLSession = {
let configuration = URLSessionConfiguration.default
let session = URLSession(configuration: configuration)
return session // Line AA
}()
func downloadData(url: URL){ // Line CC
downloadsSession.dataTask(with: url){ data, response, error in
print("error is \(error), data is \(data) and response is \(response)") // Line DD
if let badError = error {
print(" the url didn't succeeed error is \(badError.localizedDescription)")
}
else if let someResponse = response as? HTTPURLResponse {
if someResponse.statusCode == 200{
self.delegate?.didReceive(data: data!)
}
}
}
}
}
Using breakpoints: Line AA, gets loaded. Line BB calls. Line CC never gets called. Why? What am I doing wrong?!

Answer 1

You have declared:
weak var downloadHandler : DownloaderHandler?
Then you say:
downloadHandler = DownloaderHandler()
downloadHandler is a weak reference, and nothing else retains this DownloaderHandler instance, so it vanishes in a puff of smoke after it is created. Your logging shows it being created, but if you were to log on its deinit you would also see it vanish immediately afterward. By the time you say downloadHandler?.downloadData(url:googleURL), your downloadHandler reference is nil and so nothing happens; you are talking to nobody at that point.
[You are probably slavishly following a mental rule that delegate references should be weak. But that rule is predicated on the assumption that the delegate has an independent existence, and thus should not be "owned" by the referrer. This object, however, has no independent existence; it is more a decorator object (what I would call a Helper). Thus, the reference needs to be strong. The back-reference is still weak, so you won't get a circular retain cycle.]

Answer 2

Remove the "weak" qualifier from the downloadHandler property on your view controller.
As it is the only reference to the downloadHandler object, it will be removed from memory as soon as the viewDidLoad method finishes executing.
You can make a small test; add a breakpoint to line BB and check if downloadHandler has a value. I suspect it will be "nil", because it is a weak property.