Question

How do you programmatically check if UIKit has a given class?

For example, on iOS3 there’s no UINib, but on iOS4 there is. I can build on SDK4, but if I try to run on an iOS3.0 device I’ll meet an exception.

Answer 1

The iOS 4 release notes state:

iOS 4 includes a new UINib class to support rapidly unarchiving nib
files. While this class is new to iOS SDK 4, it was present but
private, in previous releases. Special care needs to be taken when
deploying code that uses the UINib class and also runs on iOS releases
prior to version 4. Specifically, you cannot determine the
availability of the class solely using the NSClassFromString function,
because that check returns a private class on iOS 3.x and earlier.
Instead, after getting the UINib class using NSClassFromString, you
must also use the respondsToSelector: method of the returned class to
see if it responds to the nibWithNibName:bundle: method. If it
responds to that method, you can use the class.