iOS: What happens if I execute a background job and then leave the view controller?

Question

Let's say I have view controller A and view controller B.

In VC A, I push VC B. Then in VC B, I execute some background tasks using NSOperation. In the background tasks, I modify VC B's variables.

What happens if the background tasks are not finished and I quit VC B? Will the operations be cancelled or will they still be executing? When debugging, it seems like they are still executing. In that case, wouldn't they be accessing already released variables (since I quitted VC B).

I'm a bit confused by this, anyone can clear me up? :)

Thanks,

Answer 1

You are correct, the operation does not magically disappear just because the object that spawned it did.

You will cause the OS to throw an exception as it tries to access the now deallocated view controller object. This is the danger of doing background threaded operations.

You need to plan accordingly, in this case, be able to cancel your operation when VC B gets deallocated. This means subclassing the NSOperation, implementing main() and checking for isCancelled.

See Apple's documentation regarding NSOperation, NSOperationQueues, and Concurrency Programming.