Why URLSession doesn't go in URLSessionDownloadDelegate? - swift

Question

i try to print progress of downloading using URLSessionDownloadDelegate, but delegate's methods don't work
Although image is download, progress don't print
I have button
#IBAction func downloadTapped(_ sender: UIButton) {
let image = "https://neilpatel-qvjnwj7eutn3.netdna-ssl.com/wp-content/uploads/2016/02/applelogo.jpg"
guard let url = URL(string: image) else {return}
let operationQueue = OperationQueue()
let session = URLSession(configuration: .default, delegate: self, delegateQueue: operationQueue)
session.downloadTask(with: url) { (data, response, error) in
guard let url = data else {return}
do {
let data = try Data(contentsOf: url)
OperationQueue.main.addOperation {
self.imageView.image = UIImage(data: data)
}
} catch {
}
}.resume()
}
And extension
extension DownloadingViewController: URLSessionDownloadDelegate {
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
print("=====FINISH=====")
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
let progress = Float(bytesWritten) / Float(totalBytesWritten)
print(progress)
}
}
nothing at all

Answer 1

You are calling
session.downloadTask(with: url) { (data, response, error) in
This means that the URLSession's delegate is ignored, because the download task has a completion handler which is used instead. So what you are seeing is the expected behavior.
If you wish to use a delegate, call
session.downloadTask(with: url)
and do everything in the delegate, including receiving the downloaded file.
On the other hand, if your goal is merely to display progress, there is no need for the delegate. The download task vends a progress object for this purpose. Example:
let task = session.downloadTask(with:url) { fileURL, resp, err in
// whatever
}
// self.prog is a UIProgressView
self.prog.observedProgress = task.progress
task.resume()