Programming iOS 5, 2nd Edition

Chapter 15. Drawing

Many UIView subclasses, such as a UIButton or a UITextField, know how to draw themselves; sooner or later, though, you’re going to want to do some drawing of your own. A class like UIImageView will display a static image; you can generate that image dynamically by drawing it in code. And a pure UIView does little or no drawing of its own; you can draw its appearance.

Drawing is not difficult, but it is a very large topic. This chapter will make you comfortable with the basic principles, so that you can consult and understand Apple’s documentation when you need further details.

UIImage and UIImageView

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. You can also obtain image data in some other way, such as by downloading it, and transform this into a UIImage. Conversely, you can draw your own image for display in your interface or for saving to disk (image file output is discussed in Chapter 36).

In the very simplest case, an image file in your app’s bundle can be obtained through the UIImage class method imageNamed:. This method looks at the top level of your app’s bundle for an image file with the supplied name, including the file extension, ...