iOS Programming: The Big Nerd Ranch Guide, Second Edition

For the More Curious: The Presentation Layer and the Model Layer

You can think of an instance of CALayer as having two parts: the content that gets composited onto the screen, and a set of parameters that describe how it should be composited: opacity, transform, position, etc. When a layer is being animated, it actually has two copies of these parameters: the model version and the presentation version. The presentation parameters are the ones that are being smoothly changed by the animation object. The model parameters are the persistent ones – the ones that will be used once the animation is over.

So, when a layer is being animated, its content is composited to the screen using the presentation parameters. When it is animation-less, the model ...