Explicit Animations - iOS Core Animation - page 183

The odd thing about the transform.rotation property is that it doesn't really exist. It
can't exist because CATransform3D isn't an object; it's a struct and so cannot have KVC
(Key-Value Coding) compliant properties. transform.rotation is actually a virtual
property that CALayer provides to simplify the process of animating transforms.
You cannot set properties like transform.rotation or transform.scale directly; they
can only be used for animation. When you animate these properties, Core Animation
automatically updates the transform property with the actual value that your changes
necessitate by using a class called CAValueFunction .
CAValueFunction is used to convert the simple floating-point value that we assign to the
virtual transform.rotation property into the actual CATransform3D matrix value that is
needed to position the layer. You can change the value function used by a given
CAPropertyAnimation by setting its valueFunction property. The function you specify
will override the default.
CAValueFunction seems like it could be a useful mechanism for animating properties that
cannot naturally be summed together or interpolated (such as transform matrices), but
because the implementation details of CAValueFunction are private, it's not currently
possible to subclass it to create new value functions. You can only use the functions that
Apple already make available as constants (which currently all relate to the transform
matrix's virtual properties and are therefore somewhat redundant since the default actions
for those properties already use the appropriate value functions).
Animation Groups
Although CABasicAnimation and CAKeyframeAnimation only target individual properties,
multiple such animations can be gathered together using a CAAnimationGroup .
CAAnimationGroup is another concrete subclass of CAAnimation that adds an animations
array property, to be used for grouping other animations. Let's test this out by grouping the
keyframe animation in Listing 8.6 together with another basic animation that adjusts the
layer background color (see Listing 8.10). Figure 8.3 shows the result.
Adding an animation group to a layer is not fundamentally different from adding the
animations individually, so it's not immediately clear when or why you would use this
class. It provides some convenience in terms of being able to collectively set animation
durations, or add and remove multiple animations from a layer with a single command, but
it's usefulness only really becomes apparent when it comes to hierarchical timing , which is
explained in Chapter 9.
Listing 8.10 Grouping a Keyframe and Basic Animation Together
- ( void )viewDidLoad
{
[ super viewDidLoad ];