The functions and types that begin with CG come from the Core Graphics framework, a C language API for 2D drawing. The hub of the Core Graphics framework is CGContextRef: all other Core Graphics functions and types interact with a drawing context in some way, and then the context creates an image.
Let’s look at the Core Graphics functions you used in the implementation of HypnosisView’s drawRect: method. You set up the drawing state with CGContextSetLineWidth and then set its color using CGContextSetRGBStrokeColor.
Next, you added a path to the context using CGContextAddArc. A path is a collection of points that forms a shape and can form anything from a square to a circle to a human outline. There are a number of Core Graphics ...