Programming iOS 6, 3rd Edition

Subclassing

A built-in Cocoa class may define methods that Cocoa itself will call and that you are invited (or required) to override in a subclass, so that your custom behavior, and not (merely) the default behavior, will take place.

An example I gave in Chapter 10 was UIView’s drawRect:. This is what I call a functional event. By overriding drawRect: in a UIView subclass, you dictate the full procedure by which a view draws itself. You don’t know exactly when this method will be called, and you don’t care; when it is, you draw, and this guarantees that the view will always appear the way you want it to. (You never call drawRect: yourself; if some underlying condition has changed and you want the view to be redrawn, you call setNeedsDisplay and let Cocoa call drawRect: in response.)

Built-in UIView subclasses may have other functional event methods you’ll want to customize through subclassing. Typically this will be in order to change the way the view is drawn, without taking command of the entire drawing procedure yourself. In Chapter 10 I gave an example involving UILabel and its drawTextInRect:. Another example is UISlider, which lets you customize the position and size of the slider’s “thumb” by overriding thumbRectForBounds:trackRect:value: (Chapter 25).

UIViewController (Chapter 19) is a good example of a class meant for subclassing. Of the methods listed in the UIViewController class documentation, just about all are methods you might have reason to override. If you create a ...