Ultra chicken CAEmitterLayer particle system

SFSChickenScreen.m

//
// Created by tdimson on 9/5/12.

#import <QuartzCore/QuartzCore.h>
#import "SFSChickenScreen.h"


@implementation SFSChickenScreen {
    __weak CAEmitterLayer *_chickenEmitter;
    CGFloat _decayAmount;
}

- (id) initWithFrame:(CGRect)frame chickenPosition:(CGPoint)position {
    if((self = [super initWithFrame:frame])) {
        self.userInteractionEnabled = NO;
        self.backgroundColor = [UIColor clearColor];
        _chickenEmitter = (CAEmitterLayer*)self.layer;
        _chickenEmitter.emitterPosition = position;
        _chickenEmitter.emitterSize = self.bounds.size;
        _chickenEmitter.emitterShape = kCAEmitterLayerPoint;

        CAEmitterCell *chicken = [CAEmitterCell emitterCell];
        chicken.contents = (__bridge id)[[UIImage imageNamed:@"ArgChicken.png"] CGImage];
        chicken.name = @"chicken";
        chicken.birthRate = 5;
        chicken.lifetime = 5.0;
        chicken.color = [[UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:0.0] CGColor];
        chicken.alphaSpeed = 10.0;

        chicken.velocity = 200;
        chicken.velocityRange = 50;
        chicken.emissionRange = (CGFloat) M_PI_2;
        chicken.emissionLongitude = (CGFloat)(M_PI_2 + M_PI_4);
        chicken.scale = 0.9;
        chicken.scaleRange = 0.2;
        chicken.spin = 0;
        chicken.spinRange = 10;
        chicken.yAcceleration = 250;
        _chickenEmitter.emitterCells = [NSArray arrayWithObject:chicken];
    }

    return self;
}

- (void)stopEmitting {
    _chickenEmitter.birthRate = 0.0;
}

static NSTimeInterval const kDecayInterval = 0.1;

- (void) decayStep {
    _chickenEmitter.birthRate -=_decayAmount;
    if (_chickenEmitter.birthRate < 0) {
        _chickenEmitter.birthRate = 0;
    } else {
        [self performSelector:@selector(decayStep) withObject:nil afterDelay:kDecayInterval];
    }
}

- (void) decayOverTime:(NSTimeInterval)interval {
    _decayAmount = (CGFloat) (_chickenEmitter.birthRate /  (interval / kDecayInterval));
    [self decayStep];
}

+ (Class) layerClass {
    return [CAEmitterLayer class];
}

@end