Question

How does a beginner go about practicing introspection in Cocoa programming (specifically iOS)? In other words, what kind of scenarios should I create inside my practice code that will force me to use stuff like “isKindOfClass”?

Thanks and apologies for the abstract nature of this question. I just couldn’t phrase this better.

Answer 1

In Cocoa (the Mac) and Cocoa Touch (iOS), it’s generally better to use respondsToSelector: or conformsToProtocol: than isKindOfClass:.

The best reason I can think of to use isKindOfClass: is that you’re reading in a property list or JSON data and need to determine what kind of object its root object (or, if you’re paranoid, any other object in it) is. You might do this in an assertion, assuming you’re willing to catch the exception (if the user supplied the file, you’d do better to present an error than let the exception kill the app).

The other two generally are used with delegate protocols. In your delegate property’s setter, you might assert that the new delegate conforms to the delegate protocol. Most commonly, if any of the methods in the protocol are optional, you’ll want to check whether your delegate actually responds to the message before trying to send it.

If you’re looking to deliberately create an application that hits all of these cases, I recommend writing a document-based Cocoa application with a plug-in architecture. Use a protocol to specify your plug-in API, with the conformsToProtocol: and respondsToSelector: checks in the application side. Implement a single plug-in that handles documents in a property-list format and uses isKindOfClass: to verify that the property list it reads in is as it expects.