Implicit Animations - iOS Core Animation - page 158

So to recap, we've learned the following:
▪ UIView backing layers do not have implicit animation enabled. The only ways to
animate the properties of a backing layer are to use the UIView animation methods
(instead of relying on CATransaction ), to subclass UIView itself and override
the -actionForLayer:forKey: method, or to create an explicit animation (see
Chapter 8 for details).
▪ For hosted (that is, nonbacking) layers, we can control the animation that will be
selected for an implicit property animation by either implementing the
-actionForLayer:forKey: layer delegate method, or by providing an
actions dictionary.
Let's specify a different action for our color fade example. We'll modify Listing 7.1 by
setting a custom actions dictionary for colorLayer . We could implement this using
the delegate instead, but the actions dictionary approach requires marginally less code.
So how can we create a suitable action object?
Actions are usually specified using an explicit animation object that will be called implicitly
by Core Animation when it is needed. The animation we are using here is a push transition ,
which is implemented with an instance of CATransition (see Listing 7.6).
Transitions are explained fully in Chapter 8, but suffice to say for now that
CATransition conforms to the CAAction protocol, and can therefore be used as a
layer action. The result is pretty cool; whenever we change our layer color, the new value
slides in from the left instead of using the default crossfade effect (see Figure 7.3).
Listing 7.6 Implementing a Custom Action
@interface ViewController ()
@property ( nonatomic , weak ) IBOutlet UIView *layerView;
@property ( nonatomic , weak ) IBOutlet CALayer *colorLayer;
@end
@implementation ViewController
- ( void )viewDidLoad
{
[ super viewDidLoad ];
//create sublayer
self .colorLayer = [ CALayer layer ];
self .colorLayer. frame = CGRectMake ( 50.0f , 50.0f , 100.0f , 100.0f );
self .colorLayer. backgroundColor = [ UIColor blueColor ]. CGColor ;