Key-Value-Observing and Collections

[Philip Mötteli]

Hi,


There is something, I just don't get with KVO. Lets say, I have an  
object


@interface Pupil
{
	NSMutableArray	grades;
}


- (void)setGrades:(NSMutableArray *)theGrades;
- (NSMutableArray *)grades;
- (void)addGrade:(Grade *)aGrade;

@end


Using KVO, I want to be notified not only, when the old grades  
collection object (NSMutableArray) is replaced, but also, when a  
Grade instance is added to the grades array (something like "[grades  
addObject:aGrade]").
The first case is easy, because I know the attribute name (grades),  
which gives the key. But for the second, I don't know the attribute  
name, so I can't add an observer.
Can anybody explain me how to do this?


Thanks
Phil