Programming iOS 4

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.

Warning

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 ...