iPad Application Development For Dummies, 3rd Edition

Hiding Instance Variables

When properties were first developed, they were looked at as a way to avoid the tedium of writing accessors for instance variable-based properties.

People used to think about properties as a way to access instance variables. In fact, instance variables shouldn’t be equated to properties, and more important, instance variables shouldn’t be made public. (Doing so violates the object-oriented principle of encapsulation, but that’s a conversation for a different time.) In fact, Apple’s new approach is to put instance variable declarations in the implementation file of the class.

Before the new Objective-C compiler that comes with Xcode 4.2 came about, programmers declared instance variables in the header file in the @interface class declaration. In the old times, you would’ve added the following bolded code to the TestDriveController.h file:

@interface TestDriveController : UIViewController

<DestinationControllerDelegate> {

AVAudioPlayer *backgroundAudioPlayer;

SystemSoundID burnRubberSoundID;

BOOL touchInCar;

}

This approach made instance variables (ivars) visible to everyone and everything and was, as I mentioned, at odds with the principle of encapsulation (even if the variables couldn’t be accessed).

Starting with Xcode 4.2, you can now hide instance variables by declaring them in the implementation file in one of two ways. The first is as a class extension, which you create by adding a second interface block in the implementation file followed ...