Layer Performance - iOS Core Animation - page 327

from having to do something much more complex later on, and avoid us having to
unnecessarily create and position layer objects that we don't need.
Let's refactor our app so that layers are instantiated dynamically as the view is scrolled
instead of all being allocated in advance. That way, we can calculate whether they are
needed before ever creating them. Next, we'll add some code to calculate the visible area so
that we can eliminate layers that are outside the field of view. Listing 15.4 shows the
updated code.
Listing 15.4 Eliminating Layers Outside of the Screen Bounds
#import "ViewController.h"
#import <QuartzCore/QuartzCore.h>
#define WIDTH 100
#define HEIGHT 100
#define DEPTH 10
#define SIZE 100
#define SPACING 150
#define CAMERA_DISTANCE 500
#define PERSPECTIVE(z) (float)CAMERA_DISTANCE/(z + CAMERA_DISTANCE)
@interface ViewController () < UIScrollViewDelegate >
@property ( nonatomic , weak ) IBOutlet UIScrollView *scrollView;
@end
@implementation ViewController
- ( void )viewDidLoad
{
[ super viewDidLoad ];
//set content size
self . scrollView . contentSize = CGSizeMake (( WIDTH - 1 )* SPACING ,
( HEIGHT - 1 )* SPACING );
//set up perspective transform
CATransform3D transform = CATransform3DIdentity ;
transform. m34 = - 1.0 / CAMERA_DISTANCE ;
self . scrollView . layer . sublayerTransform = transform;
}