Question

I’m implementing an application where both instances of a class as well as the class itself have “children” (placed inside a NSMutableArray). It’s a pretty complicated application, but thanks to Objective-C, it’s a breeze: classes are themselves objects and can have methods and “variables” (with the help of static variables and such).

To make it myself easy, I implemented accessor/setter method on both my instances (using -) and classes (using +) to fetch and manipulate the “children” without having direct access to the arrays. I like to have my objects as much as possible closed and the methods do some data validation.

I also implemented a fast enumeration instance method from the NSFastEnumeration protocol. And here comes the question: can I implement a fast enumeration class method and use it with Objective-C’s for...in construct? In order words, can I implement this:

+ (NSUInteger)countByEnumeratingWithState: (NSFastEnumerationState *)state objects: (__unsafe_unretained id [])buffer count: (NSUInteger)len;

And then use it somewhere like that:

for (id child in [MyClass class]) {
    // Do magic stuff…
}

I peeked in GNUStep’s GSFastEnumeration.h file which implements fast enumeration as a macro, which affirms the above is possible, but I’m not sure if Apple does the same.

Even if I can’t associate the NSFastEnumeration protocol to my class object, is fast enumeration without that protocol possible (and future-proof)?

Answer 1

The method -countByEnumeratingWithState:objects:count: is the whole of fast enumeration—the protocol is, I believe, mostly there for description (it’s easier to implement a protocol than declare the method with the correct signature). I would expect it to work just fine, but I don’t have a reference for that. You would probably want to loop over [MyClass class], though.

I would probably consider it future-proof. Note that it’d be really trivial to make a tiny wrapper class around your class object that does nothing but implement NSFastEnumeration and forward the instance method -countByEnumeratingWithState:objects:count: to your class’s method +countByEnumeratingWithState:objects:count:.