Programming iOS 6, 3rd Edition

Graphics Contexts

UIImageView draws an image for you and takes care of all the details; in many cases, it will be all you’ll need. Eventually, though, you may want to create some drawing yourself, directly, in code. To do so, you will always need a graphics context.

A graphics context is basically a place you can draw. Conversely, you can’t draw in code unless you’ve got a graphics context. There are several ways in which you might obtain a graphics context; in this chapter I will concentrate on two, which have proven in my experience to be far and away the most common:

You create an image context

The function UIGraphicsBeginImageContextWithOptions creates a graphics context suitable for use as an image. You then draw into this context to generate the image. When you’ve done that, you call UIGraphicsGetImageFromCurrentImageContext to turn the context into a UIImage, and then UIGraphicsEndImageContext to dismiss the context. Now you have a UIImage that you can display in your interface or draw into some other graphics context or save as a file.

Cocoa hands you a graphics context

You subclass UIView and implement drawRect:. At the time your drawRect: implementation is called, Cocoa has already created a graphics context and is asking you to draw into it, right now; whatever you draw is what the UIView will display. (A slight variant of this situation is that you subclass a CALayer and implement drawInContext:, or make some object the delegate of a layer and implement drawLayer:inContext: ...