What's the 'non-fragile methods' in Objective-C?

Question

I heard something unknown non-fragile methods in Objective-C. What's this?

Answer 1

There’s no such thing as non-fragile methods in Objective-C.

On the other hand, non-fragile instance variables are a mechanism that allows the layout of a class to be modified whilst keeping compatibility with code that isn’t aware of the layout change.

This is useful for modifying classes in a framework without affecting subclasses residing outside of the framework, and it also allows class extensions to define instance variables, including declared properties.