Example of adopting/implementing fast enumeration for my class?

Question

I'm trying to understand how to adopt the fast enumeration protocol (under iOS/objective C) for a class I'm creating. I read the section of Apple's docs, but... I don't quite get it!

Anyone have some sample code I could look at?

What I'm trying to do: I have an array of objects over which I want a sender to iterate. The sender wants to use a for-in construct. The catch is I don't want the sender to see all the objects in the array because some of them aren't valid in the context of the application.

In other words, I want my iterator to return a subset of objects in the array matching certain criteria.

I'd prefer not to create any new arrays in the process so as not to slow things down.

Answer 1

Apple's own FastEnumerationSample demonstrates it quite well. From the sounds of it, you may not have seen it yet.

There's also this blog post.

Answer 2

I don't know what you mean by "sender wants to use a for-in construct". The outcome you're asking for would probably be better achieved by using indexesOfObjectsPassingTest:, which will give you an index set of all the items in your array that pass whatever test you have set up for it.

You could also use filteredArrayUsingPredicate: to derive a new array that's a subset of your original -- I know you said you didn't want to create any new arrays, but if you want to return a subset of your array, then you have to create a new array.

Answer 3

Much better explanation here:

http://www.cocoawithlove.com/2008/05/implementing-countbyenumeratingwithstat.html

NB: Apple's source code is technically correct, but poorly explained, and mostly useless unless you enjoy playing "guess what the programmer was thinking / smoking that day".

Liekwise, Mike Ash's post is correct, and useful after you know how to do it, but terrible as a starting point.