Re: Key Value Observation is over reacting

[Fred Kiefer]

-------- Original-Nachricht --------
> Datum: Fri, 14 Dec 2007 02:11:02 +0100
> Von: Markus Hitter <address@hidden>
> An: Fred Kiefer <address@hidden>
> CC: GNUstep Developer <address@hidden>
> Betreff: Re: Key Value Observation is over reacting

> 
> Am 13.12.2007 um 21:12 schrieb Fred Kiefer:
> 
> >  I just see that this special case is not
> > failing there. Should I set breakpoints on methods of the watched
> > instance and check the back trace for on the fly methods?
> 
> Yes, why not?
> 
> As for the remaining part, I can't see when the getter method would  
> be used inside KVO at all. Notifications are sent on value changes  
> only, which does the setter. Where does the conflict arise?
> 

It is when the KVO code tries to get the value for the key before the change 
(and also after, but it doesn't get that far). To get the value it uses the 
valueForKey: method and this fails with an unknown key (OK, the details are a 
bit more complex, but it boils down to this). 

 
> Until today, I always thought KVO is based on a mini notification  
> center inside NSObject's -setValue:forKey:. This method would try a  
> few ways to get the variable set, then send a notification to all  
> subscribers on success. No need to patch existing classes.
> 
This is only enough for changes done via the setValue:forKey: method, what 
about normal setter methods, the ones I have this issue with? 

> However, this morning I learned, Apple uses something they call "isa- 
> swizzling":
> 
> <http://developer.apple.com/documentation/Cocoa/Conceptual/ 
> KeyValueObserving/Concepts/KVOImplementation.html#//apple_ref/doc/uid/ 
> 20002307>
> 

This is more or less what GNUstep does as well. It is just the details, which 
methods will be replaced at which time that make a difference. 
GNUstep is rather eager at replacing methods. Perhaps we should just replace 
them when they are actually being watched at least on one object?

In the long run it will probably be best to use a better integration with KVO 
inside of KVB. There we already know in advance which keys may be watched, 
perhaps we could do better with that information.

Fred



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