Re: [Pyobjc-dev] Categories (was: Subclassing NSTextField)

[Peter M. <zig...@po...>]

I don't know if this is what you want but it may help:

http://www.cocoadev.com/index.pl?MethodSwizzling

Peter

On Thursday, December 19, 2002, at 01:41 AM, bb...@ma... wrote:

> On Wednesday, Dec 18, 2002, at 01:32 US/Eastern, Ronald Oussoren wrote:
>> On Tuesday, Dec 17, 2002, at 21:08 Europe/Amsterdam, bb...@ma... 
>> wrote:
>>> On Tuesday, Dec 17, 2002, at 14:25 US/Eastern, Steven D. Arnold 
>>> wrote:
>>>> On a related note, is there any mechanism to implement categories 
>>>> in pyobjc?
>>>> I thought this question had been asked before, but I couldn't find 
>>>> it in my
>>>> archive.
>>>
>>> Good question:   Ronald?
>>
>> Not really. You can of course implemenent methods from a category for 
>> your own subclasses (e.g. like the text* categories Bill mentioned in 
>> the previous message), but you cannot add new methods to existing 
>> Objective-C classes. You can add new methods to subclasses defined in 
>> Python.
>
> Some random notes...  (more Internet as a todo list -- also a lot of 
> this can be answered more specifically by looking at the darwin source 
> -- i don't have net connectivity right now, though).
>
> How are categories found in bundles currently loaded into the runtime? 
>   Specifically, I see a declaration for a category structure in 
> objc.h/objc-load.h/objc-class.h, but the only add methods API is 
> this...
>
> OBJC_EXPORT void class_addMethods(Class, struct objc_method_list *);
> OBJC_EXPORT void class_removeMethods(Class, struct objc_method_list *);
>
> ... which is also interesting in that it appears that removing methods 
> is naturally supported by ObjC?  Didn't know that.
>
>> Some other useful things you cannot do at the moment (using the 
>> Internet as my Todo list :-) are:
>> - Adding new methods to subclassses implemented in Python if those 
>> methods override/extend existing methods. You can do this, but your 
>> method won't be called if someone tries to access this method from 
>> Objective-C.
>
> Is this a matter of invoking class_addMethods() with the appropriate 
> objc_method_list?
>
>> - Removing methods from subclasses implemented in Python, if those 
>> methods override/extend existing methods. Again you can actually do 
>> this, but this doesn't have the right semantics.
>
> Not sure what you mean.   It certainly isn't something that is 
> commonly done in ObjC-- I don't think I have ever seen methods removed 
> from classes in the 15 years I have been doing ObjC-- but the API 
> appears to support this.  This assumes that class_removeMethods() is 
> more than a no-op.
>
>> - Redefine an existing class. This is a feature: Because Objective-C 
>> has a flat namespace and python doesn't you might otherwise 
>> accidently replace and existing class. It is also not really possible 
>> to implement this without memory leaks.
>
> Right.  In this case, enforcing the ObjC semantics of 
> once-a-class-always-the-same-class is the way to go.
>
>> All of these would be usefull for incremental development and may be 
>> added in some future version (that includes support for categories).
>
> It would be useful for more than incremental development.
>
> Categories-- as much as they can be horrendously abused-- are a fairly 
> fundamental part of the ObjC development experience.
>
> If we can redefine existing methods on the fly-- swizzle methods-- 
> then we can build some incredibly powerful introspection and debugging 
> tools.  Namely, we could potentially replace existing ObjC methods in 
> a fashion where the existing functionality remains unchanged but some 
> random method is invoked before and/or after the normal 
> implementation.  For debugging, this would be HUGE!
>
> b.bum
>
>
>
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