Another appropriate use of @property design question

Question

This is a followup to Avoiding @property-itis.

UIWebView has the following property declarations:

@property(nonatomic,readonly,getter=canGoBack) BOOL canGoBack;
@property(nonatomic,readonly,getter=canGoForward) BOOL canGoForward;

UIScrollView has these:

@property(nonatomic) BOOL canCancelContentTouches;

Yet, UIResponder has

- (BOOL)isFirstResponder;
- (BOOL)canBecomeFirstResponder;
- (BOOL)canResignFirstResponder;

Is the UIResponder case one where they should have been declared as properties, but, for whatever reason, were not?

Or is it a case where declaring them as properties was inappropriate? If inappropriate, why?

Answer 1

My best guess is that UIResponder is meant to match NSResponder, which of course was designed before Objective-C 2.0 introduced properties. Why UIWebView doesn't do the same with regard to WebView, I don't know. I'd expect properties in Cocoa to be a little schizophrenic in this way for some time, and I wouldn't think into it too much when considering your own code.

Answer 2

Synthesized properties specify how properties are accessed. Whether or not to use synthesized properties is a design decision since properly implemented getter and setter methods will provide identical functionality.

As long as the method(s) in question can be properly implemented using the available property attributes, there is no reason why they might not be rewritten to do so.

Answer 3

A property should map, intuitively if not exactly, to some storage mechanism of your class - be it an ivar declared in your @interface or something synthesized by the runtime. The act of calling a method such as canBecomeFirstResponder need not necessarily query the class for a storage mechanism which holds a trivial BOOL but rather causes some chain of events to fire off that queries the responder tree. That is to say that there is no firstResponder ivar as the value of any of those methods cannot be stored in a cache and must be determined at the time of their execution.