withAppearance

Q. Each page needs an Appearance, so why are we using WithAppearance instead?

A. We can supply a NinePatch (which is not an Appearance For example, one page of the animation could gradually change color (by changing the actor's tint), or shake (by changing the actor's angle back and forth)...

This can be null, which means the appearance shouldn't change. It might seem strange to have a page which doesn't change the appearance, until you take tick and onPageFinished into account. For example, suppose we have a flip-book animation of an actor dying. The first few pages could be him falling to the floor, and the final "pages" don't change the appearance, but just fade out (by changing Actor.tint).