Animating layer contents when bounds changes

gistfile1.txt

@interface AnimatedContentsLayer: CALayer

@property (nonatomic, assign) CGSize animatedSize;

@end


@implementation AnimatedContentsLayer

@dynamic animatedSize;

+ (BOOL)needsDisplayForKey:(NSString *)key
{
    if ([key isEqualToString:@"animatedSize"])
    {
        return YES;
    }
    return [super needsDisplayForKey:key];
}

- (void)display
{
    CGSize size = ((AnimatedContentsLayer *)self.presentationLayer).animatedSize;
    UIGraphicsBeginImageContext(size);
    [[UIColor colorWithRed:arc4random_uniform(255)/255.0 green:arc4random_uniform(255)/255.0 blue:arc4random_uniform(255)/255.0 alpha:1] setFill];
    UIRectFill((CGRect){CGPointZero, size});
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    self.contents = (id)image.CGImage;
}

- (void)setBounds:(CGRect)bounds
{
    super.bounds = bounds;
    self.animatedSize = bounds.size;
}

@end