Wait for Download Task to Finish in Nsurlsession

Wait for download task to finish in NSURLSession

As I said before in the comments of the main post the download NSURLSession.sharedSession().dataTaskWithRequest(request) run synchronously to the main thread which means it downloads the data synchronously to the displaying. So in short the print("test0") will be executed when the dispatched thread is finished (aka downloaded the data). To fix this I had to use semaphores (I also combined the first code and the second code from the original question):

private func downloadData(completion: (String) -> ()) {
   let semaphore = dispatch_semaphore_create(0);
   let request = NSMutableURLRequest(URL: url)

   var theJSONData = NSData()
   do {
       theJSONData = try NSJSONSerialization.dataWithJSONObject(json, options: NSJSONWritingOptions())
   } catch {
       completion(String())
   }

   request.HTTPMethod = "POST"
   request.setValue("application/json", forHTTPHeaderField: "Content-Type")
   request.HTTPBody = theJSONData

   let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
       data, response, error in
       data == nil ? completion(String()) : completion(String(data: data!, encoding: NSUTF8StringEncoding)!)
       dispatch_semaphore_signal(semaphore);
   }
   task.resume()
   dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}

This may not be the most efficient nor the most elegant solution but in my case I really had to wait for the downloading to be finished else it shouldn't do anything.

How to wait for all tasks of an NSURLSession to complete?

I found a solution that avoids invalidating the session, using the suggested DispatchGroup.

(answer is in Swift, while question is in ObjC ... but it's the same logic)

Note that when using, uploadTaskWithStreamedRequest:, we need to implement an URLSessionTaskDelegate and func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?). So, to simplify the answer, I will demonstrate the use of DispatchGroup with uploadTaskWithRequest:from:completionHandler:.

// strong reference to the dispatch group
let dispatchGroup = DispatchGroup()

func performManyThings() {
    for _ in 1...3 {
        let request = URLRequest(url: URL(string: "http://example.com")!)
        dispatchGroup.enter()
        let uploadTask = self.session.uploadTask(with: request, from: nil) { [weak self] _, _, _ in
            self?.dispatchGroup.leave()
        }
        uploadTask.resume()
    }
    dispatchGroup.notify(queue: .main) {
        // here, all the tasks are completed
    }
}

Function does not wait until the data is downloaded

The other answer is not a good replacement for the code you already had. A better way would be to continue using NSURLSession's data tasks to keep the download operation asynchronous and adding your own callback block to the method. You need to understand that the contents of the download task's block are not executed before you return from your method. Just look at where the call to resume() is for further evidence.

Instead, I recommend something like this:

func getImageFromServerById(imageId: String, completion: ((image: UIImage?) -> Void)) {
    let url:String = "https://dummyUrl.com/\(imageId).jpg"

    let task = NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: url)!) {(data, response, error) in
        completion(image: UIImage(data: data))
    }

    task.resume()
}

Which can be called like this

getImageFromServerById("some string") { image in
    dispatch_async(dispatch_get_main_queue()) {
        // go to something on the main thread with the image like setting to UIImageView
    }
}

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