Graphics Programs Reference
In-Depth Information
Typically, the key path matches the name of the property. For example, a property anima-
tion that will animate a layer's opacity property will have a key path of opacity .
Sometimes a property whose type is a structure (like position , whose type is
CGPoint ) can have each of its members accessed by a key path. (The available options
for this are in the documentation under Core Animation Extensions To Key-Value Cod-
ing.)
However, like CAAnimation , CAPropertyAnimation is an abstract superclass. To
create an animation object that modifies a property of a layer, you use one of the two con-
crete subclasses of CAPropertyAnimation : CABasicAnimation and CAKey-
frameAnimation . Most of the time you will spend with Core Animation will involve
these two classes.
CABasicAnimation is the simpler of the two classes. It has two properties:
fromValue and toValue , and it inherits CAAnimation 's duration property.
When a basic animation is added to a layer, the property to be animated is set to the value
in fromValue . Over the time specified by duration , the value of the property is inter-
polated linearly from fromValue to toValue , as shown in Figure 23.3 .
Figure 23.3 Interpolating a CABasicAnimation that animates the position of a layer
 
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