Does NSURLSession Take place in a separate thread?

Question

I was designing an app that uses NSURLSession and thinking about putting it in a different thread with Grand Central Dispatch, but if NSURLSession automatically does that in the background, I wouldn't have to use GCD then, correct?

So in other words, does NSURLSession automatically use Grand Central Dispatch in the background, so we don't have to worry about it?

Answer 1

Yes,

NSURLSession (URLSession in Swift) does its work in a background thread. The download ALWAYS takes place asynchronously on a background thread.

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EDIT:

There is no reason to wrap your code that invokes NSURLSession (or URLSession in Swift 3 or later) in a GCD call.

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You can control whether its completion methods are executed on a background thread or not by the queue you pass in in the delegateQueue parameter to the init method. If you pass in nil, it creates a (background thread) serial operation queue where your completion methods are called. If you pass in NSOperationQueue.mainQueue() (OperationQueue.mainQueue() in recent versions of Swift) then your completion delegate methods/closures will be invoked on the main thread and you won't have to wrap UI calls in dispatch_async() calls to the main thread.

Answer 2

Here's an example of an NSURLSession request:

[[session dataTaskWithURL:[NSURL URLWithString:someURL]
      completionHandler:^(NSData *data,
                          NSURLResponse *response,
                          NSError *error) {
        // handle response

  }] resume];

From the Documentation: "This method is intended as an alternative to the sendAsynchronousRequest:queue:completionHandler: method of NSURLConnection, with the added ability to support custom authentication and cancellation." Short answer is: yes, NSURLSession will do background operations. You don't have to worry about this blocking your UI.