How do you know when the NSURLSession object has been invalidated by iOS?

ios, nsurlsession

Solution

When you create your `NSURLSession` object use the `sessionWithConfiguration:(NSURLSessionConfiguration *)configuration delegate:(id <NSURLSessionDelegate>)delegate delegateQueue:(NSOperationQueue *)queue;` method and specify yourself as the delegate.

You will then get a callback in: `- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error;` when the session is invalidated.

You cannot trust that `invalidateAndCancel` will instantly stop all tasks since it is up to the tasks to handle the cancel-call.

From the header of `NSURLSession`:

/* -invalidateAndCancel acts as -finishTasksAndInvalidate, but issues
* -cancel to all outstanding tasks for this session.  Note task 
* cancellation is subject to the state of the task, and some tasks may
* have already have completed at the time they are sent -cancel. 
*/

Problem

I am seeing an error in my tests where occasionally I get the following iOS error: A background URLSession with identifier {GUID} already exists! Even though I call invalidateAndCancel on the NSURLSession in the cleanup call after every test. I am looking for a way to wait until I am sure the NSURLSession object has been invalidated, before proceeding to the next test.

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