The operation queue provided when this object was created.
- iOS 7.0+
- macOS 10.9+
- tvOS 9.0+
- watchOS 2.0+
All delegate method calls and completion handlers related to the session are performed on this queue. The session object keeps a strong reference to this queue until your app exits or the session object is deallocated. If you do not invalidate the session, your app leaks memory until it exits.