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

[<bb...@ma...>]

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