Subclasses of UIView can implement their own custom graphics by providing a drawRect method that implements the custom drawing routines. The drawRect method takes a CGRect argument, which indicates the area to draw in, but the actual drawing commands are performed on a Core Graphics context, which is represented by the CGContext class and can be obtained by a call to UIGraphicsGetCurrentContext.

The Core Graphics context represents a drawable area in iOS and is used for printing as well as drawing graphics. Each view is responsible for drawing itself; the rectangle will either be the full area (for example, the first time that a view is drawn) or it might be a subset of the area (for example, when a dialog has been shown and then subsequently removed).

Core Graphics is a C-based interface (rather than Objective-C based), so the API is exposed as a set of functions beginning with the UIGraphics prefix. As with other drawing APIs, the program can set the current...