Programming iOS 6, 3rd Edition

Messages as Data Type

Objective-C is so dynamic that it doesn’t have to know until runtime what message to send to an object or what object to send it to. Certain important methods actually accept both pieces of information as parameters. For example, consider this method declaration from Cocoa’s NSNotificationCenter class:

- (void)addObserver:(id)notificationObserver
           selector:(SEL)notificationSelector
               name:(NSString *)notificationName
             object:(id)notificationSender

We’ll discuss later what this method does (when we talk about notifications in Chapter 11), but the important thing to understand here is that it constitutes an instruction to send a certain message to a certain object at some later, appropriate time. For example, our purpose in calling this method might be to arrange to have the message tickleMeElmo: sent at some later, appropriate time to the object myObject.

So let’s consider how we might actually make this method call. The object to which the message will be sent is here called notificationObserver, and is typed as an id (making it possible to specify any type of object to send the message to). So, for the notificationObserver parameter, we’re going to pass myObject. The message itself is the notificationSelector parameter, which has a special data type, SEL (for “selector,” the technical term for a message name). The question now is how to express the message name tickleMeElmo:.

You can’t just put tickleMeElmo: as a bare term; that doesn’t work syntactically. You might ...