Re: [Pyobjc-dev] Access to 'hidden' class-methods

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

On Sunday, Jan 12, 2003, at 16:53 US/Eastern, Ronald Oussoren wrote:
>> Hmm, another wild idea: would this mean that by declaring a category 
>> on the metaclass you could influence creation of all ObjC objects?
> You probably could, but: the metaclass has the same name as the normal 
> class which makes it hard to define a category on it. But is guess 
> defining a category containing class-mehods on a normal class would do 
> what you want.

Adding methods to a class via a category works just like instance 
methods;  the methods will be added and all of the inheritance based 
method dispatch rules will be followed.

You can add methods in a category to NSObject to influence all classes 
rooted at NSObject (ObjC does not require you to use NSObject as the 
root class or even have a superclasss at all).

Keep in mind that categories that override existing functionality are 
generally evil in any context but in an application-- even then, they 
are dangerous.   Overriding existing functionality means that you are 
changing the behavior of the overridden API in a  fashion where people 
using that API have no way of accessing the original functionality.

--

Posing allows one to access the original implementation.  That is, I 
could override NSObject's -description (or +description) method in a 
subclass, tell that subclass to pose as NSObject, and my functionality 
would override NSObject's implementation.  Unlike categories, I could 
still invoke the original implementation by calling [super description] 
in my description method.

-- 

Method swizzling involves directly editing the objective-c runtime to 
change the behavior of an existing object.   A very handy tool.  For 
example, if the following class is loaded into an application, it will 
cause NSUserDefaults to print every default and its value as the 
application accesses the defaults:

@interface NoisyDefaults:NSUserDefaults
@end
@implementation NoisyDefaults
static IMP ofk;
id objectForKey(self, _cmd, defaultName)
{
     id v = ofk(self, _cmd, defaultName);
    fprintf(stdout, "%s ==> %s\n", [defaultName cString], [[v 
description] cString]);
     return v;
}

+(void) load
{
     Method origM;

     origM = class_getInstanceMethod([NSUserDefaults class], 
@selector(objectForKey:));
     ofk = origM->method_imp;

     origM->method_imp = objectForKey;
}
@end

Swizzling is a handy debugging tool, but should be avoided in real 
world situations.

--

ObjC also provides lower level tools for manipulating the behavior of 
the runtime.   PyObjC uses these APIs to build classes on the fly 
(and-- someday-- categories, too).

OBJC_EXPORT void objc_setClassHandler(int (*)(const char *));
/* overriding the default object allocation and error handling routines 
*/
OBJC_EXPORT id  (*_alloc)(Class, unsigned int);
OBJC_EXPORT id  (*_copy)(id, unsigned int);
OBJC_EXPORT id  (*_realloc)(id, unsigned int);
OBJC_EXPORT id  (*_dealloc)(id);
OBJC_EXPORT id  (*_zoneAlloc)(Class, unsigned int, void *);
OBJC_EXPORT id  (*_zoneRealloc)(id, unsigned int, void *);
OBJC_EXPORT id  (*_zoneCopy)(id, unsigned int, void *);
OBJC_EXPORT void        (*_error)(id, const char *, va_list);