Re: With What does Apple Swizzle the IsA-pointer in KVO?
Re: With What does Apple Swizzle the IsA-pointer in KVO?
- Subject: Re: With What does Apple Swizzle the IsA-pointer in KVO?
- From: Philip Mötteli <email@hidden>
- Date: Fri, 23 Nov 2007 19:09:15 +0100
Am 23.11.2007 um 18:05 schrieb Clark Cox:
On Nov 23, 2007 8:03 AM, Philip Mötteli <email@hidden>
wrote:
Am 23.11.2007 um 15:54 schrieb Clark Cox:
On Nov 23, 2007 5:42 AM, Philip Mötteli <email@hidden>
wrote:
Apple says, that it uses IsA-swizzling as the underlying technology
to
implement KVO.
What are/is the class(es), which replace the original class of the
observed object? Is it an NSProxy subclass? Different NSProxy
subclasses, depending on the observed object? Is it a direct
subclass
of the observed object?
Perhaps I could provide more help if you
stated why it is that you think that you need to know this.
I need to know, when an object has changed and I would prefer to
reuse
as much as possible, of what KVO already offers.
Unfortunately, KVO is based on the internals of an object (which
should be assumed as being hidden in OOP).
KVC (and by extension, KVO) only uses instance variable directly if:
1) There is no appropriate accessor method
2) +accessInstanceVariablesDirectly is not implemented to return NO
I have a problem.
Ideally, I just want to implement a plug-in, that reuses the
functionality, offered by KVO, but extends it, so that the publicly
accessible part of an object can be observed instead of the hidden
internals.
The publicly accessible parts of objects *are* what is observed.
What is the difference between:
- (void)setValue:(id)value forKey:(NSString *)key
and
OBJC_EXPORT void object_setIvar(id obj, Ivar ivar, id value)
In both versions, you have violated, what Apple writes in Attachment:
Object-Oriented Programming with Objective-C- Classes.webloc
Description: application/applefile
:
An object’s data is protected and won’t be touched by any other part
of the program. Methods can therefore trust its integrity. They can
be sure that external access hasn’t put it in an illogical or
untenable state.
So yes, in my eyes, KVC violates OO principles (except, when it uses a
publicly declared accessor).
I'm still not clear as to what you're trying to do. NSString is not
mutable, so attempting to observe one of it's properties doesn't make
sense,
Yes, bad example. Take NSMutableString or NSMutableRLEArray,
NSMutableAttributedString, NSAttributedString, NSMutableCharacterSet,…
and finalize and dealloc are just methods, what would observing
them even mean?
That your object has disappeared, which is also a kind of changement.
But Clarc, tell me, do you know the answer to my initial question:
With What does Apple Swizzle the IsA-pointer in KVO?
Then, please tell me.
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