Programming iOS 8

Chapter 2. Drawing

Many UIView subclasses, such as a UIButton or a UILabel, know how to draw themselves; sooner or later, though, you’re going to want to do some drawing of your own. You can draw an image in code, and then display it in your interface in a class 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 12.)

UIImage

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 how to work with many standard image file types, such as TIFF, JPEG, GIF, and PNG; when an image file is to be included in your app bundle, iOS has a special affinity for PNG files, and you should prefer them whenever possible. You can also obtain image data in some other way, such as by downloading it, and transform this into a UIImage. Conversely, you can construct your own UIImage for display in your interface or for saving to disk (image file output is ...