KVO does not notify when a weakly observed property goes to nil

Number:rdar://12219416 Date Originated:31-Aug-2012 04:34 PM
Status:Open Resolved:
Product:Mac OS X Product Version:12B19
Classification:Serious Bug Reproducible:Always
31-Aug-2012 04:34 PM Michael Jurewitz:
Key Value Observing does not notify the observing object when a weakly held observed object goes to nil as a result of being deallocated.

Steps to Reproduce:
1. Inspect attached sample project
2. Notice that Class A weakly holds an instance of Class B.
3. Instance `a` of Class A is subscribed to watch for changes to instance b's name.
4. Notice `a` is properly notified of changes to `b's name.
5. Notice that once `b's last strong reference goes away, it is deallocated.
6. `a` is never notified via KVO that `b` has gone away or changed after this point.

Expected Results:
KVO should inform observers when observed values change, particularly as a result of being deallocated via the __weak subsystem. This is likely an enormous source of subtle bugs and unexpected behavior. It should be possible to be weakly interested in an object from a graph management perspective and still be informed of changes to that object via KVO. This bug basically makes it impossible to take a KVO interest in a weakly held object.

Actual Results:
KVO actually detects this situation enough to log to the console. However, instead of being of any further help and informing the observing objects, it just silently fails.

31-Aug-2012 04:34 PM Michael Jurewitz:
'WeakTest.zip' was successfully uploaded


Duped as 12479339

By heath.borders at Oct. 11, 2012, 4:30 p.m. (reply...)

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