For the sake of completeness, I will now explain how implicit animation works — that is, how implicit animation is turned into explicit animation behind the scenes. The basis of implicit animation is the action mechanism.
What an Action Is
An action is an object that adopts the CAAction protocol. This simply means that it implements
The action object could do anything in response to this message. The notion of an action is completely general. However, in real life, the only class that adopts the CAAction protocol is CAAnimation. So, an animation is a special case of an action, but in fact it is also the only case of an action.
What an animation does when it receives
runActionForKey:object:arguments: is to assume that the second parameter, the
object, is a layer, and to add itself to that layer’s animations list. Thus, for an animation, receiving the
runActionForKey:object:arguments: message is like being told: “Play yourself!”
You would never send
runActionForKey:object:arguments: to an animation directly. Rather, this message is sent to an animation for you, as the basis of implicit animation.
The Action Search
When you set a property of a layer and trigger an implicit animation, you are actually triggering the action search. This basically means that the layer searches for an action object to which it can send the
runActionForKey:object:arguments: message; because that action object will be an animation, and because it will respond to this message ...