Craig Craig - 1 year ago 137
Objective-C Question

Selectors in Objective C

First, I'm not sure I really understand what a selector is. From my understanding, it's the name of a method, and you can assign it to a class of type 'SEL' and then run methods such as respondToSelector to see if the receiver implements that method. Can someone offer up a better explanation?

Secondly, to this point, I have the following code:

NSString *thing = @"Hello, this is Craig";

SEL sel = @selector(lowercaseString:);
NSString *lower = (([thing respondsToSelector:sel]) ? @"YES" : @"NO");
NSLog (@"Responds to lowercaseString: %@", lower);
if ([thing respondsToSelector:sel]) //(lower == @"YES")
NSLog(@"lowercaseString is: %@", [thing lowercaseString]);

However, even though
is clearly a kind of NSString, and should respond to lowercaseString, I cannot get the 'respondsToSelector' conditional to return "YES"...

Answer Source

You have to be very careful about the method names. In this case, the method name is just "lowercaseString", not "lowercaseString:" (note the absence of the colon). That's why you're getting NO returned, because NSString objects respond to the lowercaseString message but not the lowercaseString: message.

How do you know when to add a colon? You add a colon to the message name if you would add a colon when calling it, which happens if it takes one argument. If it takes zero arguments (as is the case with lowercaseString), then there is no colon. If it takes more than one argument, you have to add the extra argument names along with their colons, as in compare:options:range:locale:.

You can also look at the documentation and note the presence or absence of a trailing colon.