Question

To support iCloud, we’re encouraged to use a UIDocument subclass. If I define a new subclass, set the project target version to 3.0, and test using for iOS 5 before using my new subclass, will the code work on iOS 4 or does linking in a subclass break backwards compatibility?

Answer 1

UIKit can be weak-linked, but the results would be undefined if you tried to initialize a UIDocument or UIDocument subclass. You would need something like:

if (NSStringFromClass(@"UIDocument")) 
{
    ...
}

That would make it totally useless for your purposes. So the answer to your question is no, any code involving UIDocument would not run, but you could put conditional checks around such code. You’re better off finding an alternative method for saving data.