Core Animation Layer - iOS Programming: The Big Nerd Ranch Guide - page 615

Creating a CALayer
Not all layers are implicit layers. You can create a layer by sending alloc to the class
CALayer . Layers created this way are called explicit layers . In this section, you're going
to create a layer and then make it a sublayer of the implicit layer of your HypnosisView
( Figure 22.3 ).
Figure 22.3 Object diagram
In HypnosisView.h , import the QuartzCore framework and add an instance variable to
hold on to the layer object you are about to create:
#import <Foundation/Foundation.h>
#import <QuartzCore/QuartzCore.h>
@interface HypnosisView : UIView
{