Swift: Stored Properties in Extensions unrecognized selector exception

Question

I have an extension of NSURLSessionDownloadTask, inside of this Extension, I created a stored property called 'id' using Objective C Associated Objects. When I try to set the property, I get 'an unrecognized selector sent to instance'. Here's my code below:

extension NSURLSessionDownloadTask {

  private struct AssociatedKeys {
    static var id: String?
  }

  var id: String? {
    get {
      return objc_getAssociatedObject(self, &AssociatedKeys.id) as? String
    }
    set {
      if let newValue = newValue {
        objc_setAssociatedObject(self, &AssociatedKeys.id, newValue as String?, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
      }
    }
  }
}

Later on;

let task = session.downloadTaskWithURL(url)
task.id = identifier

Answer 1

I can not fully explain it, but the problem seems to be that session.downloadTaskWithURL(url) returns an instance of some internal subclass __NSCFLocalDownloadTask.

If you define the extension as an extension of NSURLSessionTask instead of NSURLSessionDownloadTask

extension NSURLSessionTask { ... }

then it worked in my test.