Re: Test fro overridden method

[Richard Frith-Macdonald]

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;
}