KVO events are not being fired for certain float/double-type properties

[triplef]

In our app, observing certain properties does not result in a KVO change notification when the property changes. This seems to affect float- and double-type properties only, although not every property of this type seems to be affected. The properties are direct, i.e. not derived from other key paths.

While we haven’t been able to track it down to a test case, here is what we found:

• For affected properties, will/didChangeValueForKey: is called with key == null
• Consequently, calling will/didChangeValueForKey: manually within these methods causes KVO to work for them
• It seems that _keyFromSelector() in NSKVOSwizzling.mm is returning nil here, i.e. the associated key object is not found
• _NSKVOEnsureKeyWillNotify() is called correctly, and the associated key object is set on the object
• However, it seems like the selector pointer is not the same in _NSKVOEnsureKeyWillNotify() and notifyingSetImpl(), which is why the associated key object is not found.

Any ideas on how to debug this further would be appreciated.

[DHowett-MSFT]

Root caused this one: we're using objc associated objects to track selector<->key mappings.
libobjc2's association logic would only search the most shallow association table in the stack.

Therefore, when you associate more than REFERENCE_LIST_SIZE (10, at the moment) keys with an object, certain keys may become inaccessible.

Fix pending investigation; filing a bug over there and working on the association table.