Generic Bounce CAKeyfameAnimation

EPFBounceAnimation.h

//
//  EPFBounceAnimation.h
//
//  Created by Chris Cieslak on 12/22/11.
//

#import <QuartzCore/QuartzCore.h>

@interface EPFBounceAnimation : CAKeyframeAnimation

+ (id) animationWithKeyPath:(NSString *)path startValue:(double)startValue endValue:(double)endValue steps:(NSUInteger)steps;

@end

EPFBounceAnimation.m

//
//  EPFBounceAnimation.m
//
//  Created by Chris Cieslak on 12/22/11.
//  Derived from http://cocoawithlove.com/2008/09/parametric-acceleration-curves-in-core.html
//  Created by Matt Gallagher on 7/09/08.
//  Copyright 2008 Matt Gallagher. All rights reserved.
//
//  Permission is given to use this source code file without charge in any
//  project, commercial or otherwise, entirely at your risk, with the condition
//  that any redistribution (in part or whole) of source code must retain
//  this copyright and permission notice. Attribution in compiled projects is
//  appreciated but not required.
//
#import "EPFBounceAnimation.h"

@interface EPFBounceAnimation()

- (double)calculatedValueForStep:(double)step;

@end

@implementation EPFBounceAnimation

+ (id) animationWithKeyPath:(NSString *)path startValue:(double)startValue endValue:(double)endValue steps:(NSUInteger)steps {
    
    EPFBounceAnimation *animation = [EPFBounceAnimation animationWithKeyPath:path];
    
    NSUInteger count = steps + 2;
	
	NSMutableArray *valueArray = [NSMutableArray arrayWithCapacity:count];
    
	double progress = 0.0;
	double increment = 1.0 / (double)(count - 1);
    
	for (int i = 0; i < count; i++) {
		double value = startValue + [animation calculatedValueForStep:progress] * (endValue - startValue);
		[valueArray addObject:[NSNumber numberWithDouble:value]];
		
		progress += increment;
	}

    animation.values = valueArray;
    return animation;
    
}

- (double)calculatedValueForStep:(double)step {
    double omega = 20.0;
    double zeta = 0.53;
    double beta = sqrt(1 - zeta * zeta);
	double phi = atan(beta / zeta);
	double result = 1.0 + -1.0 / beta * exp(-zeta * omega * step) * sin(beta * omega * step + phi);
	return result; 
}

@end