Re: KVO and the observeValueForKeyPath bottleneck

FROM : Chris Kane
DATE : Mon Jul 17 20:24:04 2006

On Jul 17, 2006, at 10:49 AM, Matt Neuburg wrote:
> On Mon, 17 Jul 2006 09:09:04 -0700, Chris Kane <<email_removed>> said:

>> On Jul 17, 2006, at 7:38 AM, Matt Neuburg wrote:
>>

>>> On or about 7/17/06 3:38 AM, thus spake "Jakob Olesen"
>>> <<email_removed>>:
>>>

>>>> - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)
>>>> object change:(NSDictionary *)change context:(void *)context
>>>> {
>>>>      if ([self respondsToSelector:(SEL)context])
>>>>          [self performSelector:(SEL)context withObject:change];
>>>> }

>>>
>>> By an incredible coincidence, yesterday (before this reponse
>>> arrived at my
>>> desk), after considering all the responses I'd gotten, I decided
>>> upon an
>>> approach and implemented it in my app, and it was word for word,
>>> letter for
>>> letter identical with this. m.

>>
>>
>> I'm just curious ... with the trivial approach above, and the other
>> ones involving the context being a keypath and so on ...  has anyone
>> considered what will happen if a superclass or subclass (of whatever
>> class that is) is also doing KVO (on either same or different keys)
>> of the same objects?

>
> I hadn't, but I presume you wouldn't be asking the question unless the
> answer were, "Something bad...?" :) I guess you're hinting that one 
> needs to
> call super.


No, I wasn't thinking that.  Though having to call super could still 
be a valid conclusion.

Simply calling super would  be problematic, because NSObject doesn't 
implement it (well, it throws).

No, I was pondering something more subtle to do with the context.  The 
basic "shunting" pattern here that is somewhat popular is 
fundamentally broken, I think, because people are not considering 
subclassing, and I wonder if the circumlocutions to avoid the problem 
are worth the trouble.


> The thing I'm really wondering is: Why was KVO implemented in this 
> oddly
> bottleneck-based way? There is terrific prior art, in the form of 
> ordinary
> notifications and the notification center (broadcaster-type 
> architecture);
> is there some technical reason why KVO doesn't use that?


I'm going to have to leave you hanging on these...



Chris Kane
Cocoa Framework, Apple