Re: Test fro overridden method

[Fred Kiefer]

Richard Frith-Macdonald schrieb:
> 
> On 29 Dec 2006, at 01:26, Fred Kiefer wrote:
> 
>> Could somebody please verify that this method does what I expect it to
>> do?
>> I need such a test within NSDocument. Here Apple takes special care that
>> if somebody has implemented the old interface methods for NSDocument in
>> a subclass, then these methods are actually called instead of the new
>> interface methods.
>>
>> /*
>>  * Private helper method to check, if the method given via the
>> selector sel
>>  * has been overridden in the current subclass.
>>  */
>> - (BOOL)_hasOverridden: (SEL)sel
>> {
>>   // The actual signature is not important as we wont call the methods.
>>   IMP meth1;
>>   IMP meth2;
>>
>>   meth1 = [self methodForSelector: sel];
>>   meth2 = [[NSDocument class] instanceMethodForSelector: sel];
>>
>>   return (meth1 != meth2);
>> }
> 
> I'm not sure what you mean by 'verify' ... but unless someone has
> overridden any of the three methods you use, I would expect the code to
> return 1 if the method for  _sel has been overridden, 0 otherwise.
> 
> You can bypass issues with those methods being overridden by calling the
> appropriate runtime functions, however if someone has overridden the
> methods they probably did so for a reason, and bypassing them could 
> also cause problems.  So while using the runtime directly can help, it
> can also (if less likely) cause problems.  I would guess it is less
> likely to cause problems than cure them, so runtime use is probably
> better, but it's by no means certain.
> 
> I suppose the most efficient implementation one could do would be
> something like ...
> 
> BOOL
> GSObjCHasOverridden(Class baseClass, NSObject *instance, SEL selector)
> {
>   Class instanceClass = GSObjCClass(instance);
> 
>   if (instanceClass == baseClass
>     || GSGetMethod(instanceClass, selector, YES. YES) ==
> GSGetMethod(base, selector, YES. YES))
>     {
>       return YES;
>     }
>   return NO;
> }
> 
> which might be a candidate for inclusion in GSObjCRuntime.[hm]
> 
> Of course, it might be worth being able to test overriding of class
> methods too, depending on whether you pass an instance or a class ....
> 
> BOOL
> GSObjCHasOverridden(Class baseClass, id object, SEL selector)
> {
>   if (GSObjCIsClass(object))
>     {
>       if ((Class)object == baseClass
>         || GSGetMethod((Class)object, selector, NO. YES) ==
> GSGetMethod(base, selector, NO. YES))
>         {
>           return YES;
>         }
>     }
>   else
>     {
>       Class instanceClass = GSObjCClass(object);
> 
>       if (instanceClass == baseClass
>         || GSGetMethod(instanceClass, selector, YES. YES) ==
> GSGetMethod(base, selector, YES. YES))
>         {
>           return YES;
>         }
>     }
>   return NO;
> }

Thank you very much for this code. This uses some fast but GNU runtime
specific functions. I will stick with my slower method of testing until
this implementation makes it into GSobjCRuntime and we also have an
abstraction that allows similar tests with the Apple runtime.

Cheers,
Fred