Programming iOS 9

Chapter 2. Drawing

The views illustrated in Chapter 1 were mostly colored rectangles; they had a backgroundColor and no more. But that, of course, is not what a real iOS program looks like. Everything the user sees is a UIView, and what the user sees is a lot more than a bunch of colored rectangles. That’s because the views that the user sees have content. They contain drawing.

Many UIView subclasses, such as a UIButton or a UILabel, know how to draw themselves. Sooner or later, you’re also going to want to do some drawing of your own. You can prepare your drawing as an image file beforehand. You can draw an image as your app runs, in code. You can display an image in your interface in a UIView subclass that knows how to show an image, such as a UIImageView or a UIButton. A pure UIView is all about drawing, and it leaves that drawing largely up to you; your code determines what the view draws, and hence what it looks like in your interface.

This chapter discusses the mechanics of drawing. Don’t be afraid to write drawing code of your own! It isn’t difficult, and it’s often the best way to make your app look the way you want it to. (For how to draw text, see Chapter 10.)

Images and Image Views

The basic general UIKit image class is UIImage. UIImage can read a file from disk, so if an image does not need to be created dynamically, but has already been created before your app runs, then drawing may be as simple as providing an image file as a resource in your app’s bundle. The system knows ...