Key–Value Coding

The way Cocoa derives the name of an accessor from the name of an instance variable is through a mechanism called key–value coding, or simply KVC. (See also Chapter 5, where I introduced key–value coding.) A key is a string (an NSString) that names the value to be accessed. The basis for key–value coding is the NSKeyValueCoding protocol, an informal protocol (it is actually a category) to which NSObject (and therefore every object) conforms.

The fundamental key–value coding methods are valueForKey: and setValue:forKey:. When one of these methods is called on an object, the object is introspected. In simplified terms, first the appropriate accessor is sought; if it doesn’t exist, the instance variable is accessed directly. So, for example, suppose the call is this:

[myObject setValue:@"Hello" forKey:@"greeting"];

First, a method setGreeting: is sought in myObject; if it exists, it is called, passing @"Hello" as its argument. If that fails, but if myObject has an instance variable called greeting, the value @"Hello" is assigned directly to myObject’s greeting ivar.


The key–value coding mechanism can bypass completely the privacy of an instance variable! Cocoa knows that you might not want to allow that, so a class method accessInstanceVariablesDirectly is supplied, which you can override to return NO (the default is YES).

Both valueForKey: and setValue:forKey: require an object as the value. Your accessor’s signature (or, if there is no accessor, the instance variable ...

Get Programming iOS 4 now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.