Layer Performance - iOS Core Animation - page 321

It can sometimes be beneficial to enable rasterization as an optimization for layers that
require offscreen rendering, but only if the layer/sublayers do not need to be redrawn
frequently.
For layers that require offscreen rendering and need to animate (or which have animated
sublayers), you may be able to use CAShapeLayer , contentsCenter , or shadowPath to
achieve a similar appearance with less of a performance impact.
CAShapeLayer
Neither cornerRadius nor masksToBounds impose any significant overhead on their own,
but when combined, they trigger offscreen rendering. You may sometimes find that you
want to display rounded corners and clip sublayers to the layer bounds, but you don't
necessarily need to clip to the rounded corners, in which case you can avoid this overhead
by using CAShapeLayer .
You can get the effect of rounded corners and still clip to the (rectangular) bounds of the
layer without incurring a performance overhead by drawing the rounded rectangle using the
handy +bezierPathWithRoundedRect:cornerRadius: constructor for UIBezierPath (see
Listing 15.1). This is no faster than using cornerRadius in itself, but means that the
masksToBounds property no longer incurs a performance penalty.
Listing 15.1 Drawing a Rounded Rectangle Using CAShapeLayer
#import "ViewController.h"
#import <QuartzCore/QuartzCore.h>
@interface ViewController ()
@property ( nonatomic , weak ) IBOutlet UIView *layerView;
@end
@implementation ViewController
- ( void )viewDidLoad
{
[ super viewDidLoad ];
//create shape layer
CAShapeLayer *blueLayer = [ CAShapeLayer layer ];
blueLayer. frame = CGRectMake ( 50 , 50 , 100 , 100 );
blueLayer. fillColor = [ UIColor blueColor ]. CGColor ;
blueLayer. path = [ UIBezierPath bezierPathWithRoundedRect :
CGRectMake ( 0 , 0 , 100 , 100 ) cornerRadius : 20 ]. CGPath ;