Specialized Layers - iOS Core Animation - page 133

NSInteger x = floor(bounds.origin.x / layer.tileSize.width * scale);
NSInteger y = floor (bounds.origin.y / layer.tileSize.height * scale);
Correcting the scale in this way also means that our snowman image will be rendered at
half the size on Retina devices (at a total size of only 1024×1024 points instead of
2048×2048 as before). This usually doesn't matter for the types of images normally
displayed with CATiledLayer (such photographs and maps, which are designed to be
zoomed and viewed at various scales), but it's worth bearing in mind.
CAEmitterLayer
In iOS 5, Apple introduced a new CALayer subclass called CAEmitterLayer .
CAEmitterLayer is a high-performance particle engine designed to let you create real-
time particle animations such as smoke, fire, rain, and so on.
CAEmitterLayer acts as a container for a collection of CAEmitterCell instances
that define a particle effect. You will create one or more CAEmitterCell objects as
templates for the different particle types, and the CAEmitterLayer is responsible for
instantiating a stream of particles based on these templates.
A CAEmitterCell is similar to a CALayer : It has a contents property that can be
set using a CGImage , as well as dozens of configurable properties that control the
appearance and behavior of the particles. We won't attempt to describe each and every
property in detail, but they are well documented in the CAEmitterCell class header file.
Let's try an example: We'll create a fiery explosion effect by emitting particles in a circle
with varying velocity and alpha. Listing 6.13 contains the code for generating the
explosion. You can see the result in Figure 6.13.
Listing 6.13 Creating an Explosion Effect with CAEmitterLayer
#import "ViewController.h"
#import <QuartzCore/QuartzCore.h>
@interface ViewController ()
@property ( nonatomic , weak ) IBOutlet UIView *containerView;
@end
@implementation ViewController
- ( void )viewDidLoad
{
[ super viewDidLoad ];