Re: KVO and the observeValueForKeyPath bottleneck

FROM : Chris Kane
DATE : Mon Jul 17 22:42:26 2006

On Jul 17, 2006, at 1:00 PM, Scott Anguish wrote:
> On Jul 17, 2006, at 2:31 PM, Matt Neuburg wrote:

>> <http://developer.apple.com/documentation/Cocoa/Conceptual/KeyValueObserving

>> /Concepts/KVOBasics.html#//apple_ref/doc/uid/20002252-179866>
>>
>> (Interestingly, the comment in that example is completely different 
>> from the
>> comment in the copy on my machine.) The code shown in the example 
>> will break
>> if super is NSObject. So perhaps I should file a bug on the docs.

>
>     Actually, that was why it says "if it implements it".  Now I 
> suppose one could argue that NSObject does implement it, but as 
> Chris said, it just throws.


The problem with that would be that "if it implements it" can change 
over time.  Next month you change a superclass of the class to start 
listening for KVO notifications (perhaps you install a software update 
of the OS!), and ... your hard-coded knowledge of what the super class 
did and didn't do is no longer true.

You can't just blindly call super, and face the exception (hmm, is the 
exception there for a reason? could be a clue), but if you don't you 
cut off your superclasses from receiving necessary KVO notifications, 
given the design.

Nobody's figured out the tricky problem yet ....

[I didn't design KVO, I'm just a bystander.]


Chris Kane
Cocoa Frameworks, Apple