URLSessionDelegate methods not called

Question

Firstly, I already checked similar existing questions, and none of the answers apply.
NSURLSession delegates not called
URLSessionDelegate Function Not Being Called

I am trying to download a file using URLSessionDownloadTask, like so

class MyNetworkManager : NSObject
{
    static let instance = MyNetworkManager()

    var downloadSession : URLSession?

    init()
    {
        super.init()

        let downloadConfiguration = URLSessionConfiguration.default
        downloadSession = URLSession(configuration: downloadConfiguration, delegate: self, delegateQueue: nil)
    }

    func download(_ url : URL)
    {
        var urlRequest = URLRequest(url: url)
        urlRequest.httpMethod = "GET"
        let downloadTask = downloadSession?.downloadTask(with: urlRequest)

        downloadTask?.resume()
    }
}

extension MyNetworkManager : URLSessionDelegate
{
    func urlSession(_ session: URLSession,
                    downloadTask: URLSessionDownloadTask,
                    didWriteData bytesWritten: Int64,
                    totalBytesWritten: Int64,
                    totalBytesExpectedToWrite: Int64)
    {
        // 
    }

    func urlSession(_ session: URLSession,
                    downloadTask: URLSessionDownloadTask,
                    didFinishDownloadingTo location: URL)
    {
        //
    }

    func urlSession(_ session: URLSession,
                    task: URLSessionTask,
                    didCompleteWithError error: Error?)
    {
        //
    }
}

However, no URLSessionDelegate methods are called.

Normally, delegate methods are not called if you create a task with completion handler - that is not the case, I'm only using URLRequest as parameter when creating a task.

Session's delegate is properly set, and after calling downloadTask?.resume() its state property is running

MyNetworkManager is a singleton, I'm using it like so

MyNetworkManager.instance.download(someURL)

so an instance is definitely retained.

Am I missing something here?

Between the bad signature delegate methods, the MyNetworkManager instance that might be released too soon, a missing delegate method (imagine you got error and not success), etc. It's laking some infos. — Larme
– Larme, Commented Oct 16, 2019 at 10:50
@Larme That's not the case, MyNetworkManager instance is retained — mag_zbc
– mag_zbc, Commented Oct 16, 2019 at 10:52
@matt Good point. Edited the question. MyNetworkManager is a singleton, trying to download using MyNetworkManager.instance.download(someURL) — mag_zbc
– mag_zbc, Commented Oct 16, 2019 at 10:57

Rob · Accepted Answer · 2019-10-16 11:08:02Z

You must conform to the relevant protocols, e.g.:

extension MyNetworkManager: URLSessionDelegate {
    // this is intentionally blank

    // obviously, if you implement any delegate methods for this protocol, put them here
}

extension MyNetworkManager: URLSessionDownloadDelegate {
    func urlSession(_ session: URLSession,
                    downloadTask: URLSessionDownloadTask,
                    didWriteData bytesWritten: Int64,
                    totalBytesWritten: Int64,
                    totalBytesExpectedToWrite: Int64) {
        print(#function)
    }

    func urlSession(_ session: URLSession,
                    downloadTask: URLSessionDownloadTask,
                    didFinishDownloadingTo location: URL) {
        print(#function)
    }
}

extension MyNetworkManager: URLSessionTaskDelegate {
    func urlSession(_ session: URLSession,
                    task: URLSessionTask,
                    didCompleteWithError error: Error?) {
        print(#function, error ?? "No error")
    }
}

If you don’t conform to URLSessionDownloadDelegate, it won’t call URLSessionDownloadDelegate methods.

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