Other Useful Layers - Core Animation - page 172

subclassing UIView is so we can override the +layerClass method and dictate what kind
of layer this UIView will be backed with, as shown in Listing 10-6.

LISTING 10-6

Overriding the +layerClass Method

+ ( Class )layerClass
{

return [ CAReplicatorLayer class ];

}

This class method is called whenever an instance of the ReplicatorView is initialized.
Instead of having a CALayer backing the view, we will automatically have
CAReplicatorLayer as the backing layer.

Because we are subclassing the UIView , add the setup code shown in Listing 10-7 to the
-initWithFrame: method of the ReplicatorView class. This setup code tells
CAReplicatorLayer what to do with the sublayers (and subviews) it is about to receive.

LISTING 10-7 Overriding the +layerClass Method

- ( id )initWithFrame:( CGRect )frame
{

if (!( self = [ super initWithFrame :frame])) return nil ;

CGFloat reflectionBase = 230.0f;

CATransform3D transform = CATransform3DMakeScale (1.0, -1.0, 1.0);
transform = CATransform3DTranslate (transform, 0.0, frame. size . height
- 2.0 * reflectionBase, 0.0);
CAReplicatorLayer *replicatorLayer = ( CAReplicatorLayer *)[ self layer];
[replicatorLayer setInstanceTransform :transform];
[replicatorLayer setInstanceRedOffset :-0.5];
[replicatorLayer setInstanceGreenOffset :-0.5];
[replicatorLayer setInstanceBlueOffset :-0.5];
[replicatorLayer setInstanceAlphaOffset :-0.45];
[replicatorLayer setInstanceCount :2];

return self ;

}

After calling super , a transform is used to both flip the layer it is associated with and to
shift that layer down 230 pixels. The reason for shifting the layer down 230 pixels is
because we know the ReplicatorView will be 220 pixels high, so we want to position the
layer 10 pixels below the top of its parent view.