mikeash/MAZeroingWeakRef — pull request #20 by asarazan (2013-01-04 19:07:21+00:00)
Title: Return removeable handle from addWeakObserver.

`addWeakObserver:selector:name:object:` now returns an opaque handle that can be removed with NSNotificationCenter's `removeObserver:` method.

This allows for temporary and cancelable observation.