MCSpriteLayer, core component to create sprite sheet based animation. See http://mysterycoconut.com/blog/2011/01/cag1/

MCSpriteLayer.h

#import <QuartzCore/QuartzCore.h>

@interface MCSpriteLayer : CALayer {
    unsigned int sampleIndex;
}

@property (readwrite, nonatomic) unsigned int sampleIndex;

MCSpriteLayer.m

#import "MCSpriteLayer.h"

@implementation MCSpriteLayer

@synthesize sampleIndex;

+ (BOOL)needsDisplayForKey:(NSString *)key;
{
    return [key isEqualToString:@"sampleIndex"];
}

- (void)display;
{
    unsigned int currentSampleIndex = ((MCSpriteLayer*)[self presentationLayer]).sampleIndex;
    if (!currentSampleIndex)
        return;
    
    CGSize sampleSize = self.contentsRect.size;
    self.contentsRect = CGRectMake(
        ((currentSampleIndex - 1) % (int)(1/sampleSize.width)) * sampleSize.width, 
        ((currentSampleIndex - 1) / (int)(1/sampleSize.width)) * sampleSize.height, 
        sampleSize.width, sampleSize.height
    );
}

@end

I don't own this code, just took it from the Mystery Coconut Blog. All credits to them!