Animation is a hallmark of the iOS interface. When used properly, it gives the user visual cues about the application’s workflow. The classes and functions needed to animate an application’s interface are in the Core Animation API. To use any part of Core Animation, you need to add the QuartzCore framework to your project.

Open your HypnoTime project and select the project from the project navigator. Then, select the HypnoTime target and the Build Phases pane. Add QuartzCore.framework to Link Binary With Libraries, as shown in Figure 22.1.

There are two classes that make Core Animation work: CALayer and CAAnimation.