BUG?: Multiple Inheritance and DO

FROM : Philip Mötteli
DATE : Sun Apr 24 23:58:02 2005

Hi


Has Apple changed something with their implementation of Distributed 
Objects? Do they always call -methodDescriptionForSelector: before 
calling the real method?
I have an object, that implements multiple inheritance. I have done 
this, by implementing the following methods:

- (void)forwardInvocation:(NSInvocation *)anInvocation;
- (BOOL)respondsToSelector:(SEL)aSelector;
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector;

So far everything worked well. But since recently, I see the 
NSConnection make the following call, before calling the real method:

-[anObject 
methodSignatureForSelector:@selector(methodDescriptionForSelector:)]


Of course this returns something, because 
-methodDescriptionForSelector: is implemented by Apple in NSObject. But 
then in this method they seem to try to extract some information about 
the real method, that was called. Unfortunately, the real method is not 
part of the original object, but part of the multiple inheritance, so 
another object. So I get a 'method not implemented' error.
So I thought, we apparently have to implement another method in order 
to achieve multiple inheritance:

- (struct objc_method_description 
*)methodDescriptionForSelector:(SEL)aSelector
{
   struct objc_method_description    *aDescription = [super 
methodDescriptionForSelector:aSelector];
   if(aDescription)
       return aDescription;

   return [[self otherObject] methodDescriptionForSelector:aSelector];
}


This then gives the following DO error:

 
_preInitWithCoder:signature:valid:wireSignature:target:selector:
argCount: incompatible method params: got NSMethodSignature: 
types=^{objc_method_description=:*}@:: nargs=3 sizeOfParams=160 
returnValueLength=4;  want NSMethodSignature: 
types=^{objc_method_description=}@:: nargs=3 sizeOfParams=160 
returnValueLength=4;


How do I implement multiple inheritance with DO?


Thanks
Phil