Two alternative solutions for you:

First, You can also animate the transform property.

Using Brad s code, but using @"transform" for the keypath. The primary advantage being that you do not have to calculate the actual frame, but instead provide a simple scaling factor:

Objective-C:

CAKeyframeAnimation *boundsOvershootAnimation = [CAKeyframeAnimation animationWithKeyPath:@"transform"];

CATransform3D startingScale = CATransform3DScale (layer.transform, 0, 0, 0);
CATransform3D overshootScale = CATransform3DScale (layer.transform, 1.2, 1.2, 1.0);
CATransform3D undershootScale = CATransform3DScale (layer.transform, 0.9, 0.9, 1.0);
CATransform3D endingScale = layer.transform; 

NSArray *boundsValues = [NSArray arrayWithObjects:[NSValue valueWithCATransform3D:startingScale],
                                                  [NSValue valueWithCATransform3D:overshootScale],
                                                  [NSValue valueWithCATransform3D:undershootScale],
                                                  [NSValue valueWithCATransform3D:endingScale], nil];
[boundsOvershootAnimation setValues:boundsValues];

NSArray *times = [NSArray arrayWithObjects:[NSNumber numberWithFloat:0.0f],
                  [NSNumber numberWithFloat:0.5f],
                  [NSNumber numberWithFloat:0.9f],
                  [NSNumber numberWithFloat:1.0f], nil];    
[boundsOvershootAnimation setKeyTimes:times];


NSArray *timingFunctions = [NSArray arrayWithObjects:[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut], 
                            [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut],
                            [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut],
                            nil];
[boundsOvershootAnimation setTimingFunctions:timingFunctions];
boundsOvershootAnimation.fillMode = kCAFillModeForwards;
boundsOvershootAnimation.removedOnCompletion = NO;

Swift 4:

let boundsOvershootAnimation = CAKeyframeAnimation(keyPath: "transform")

let startingScale = CATransform3DScale(layer.transform, 0, 0, 0)
let overshootScale = CATransform3DScale(layer.transform, 1.2, 1.2, 1.0)
let undershootScale = CATransform3DScale(layer.transform, 0.9, 0.9, 1.0)
let endingScale = layer.transform

boundsOvershootAnimation.values = [startingScale, overshootScale, undershootScale, endingScale]

boundsOvershootAnimation.keyTimes = [0.0, 0.5, 0.9, 1.0].map { NSNumber(value: $0) }

boundsOvershootAnimation.timingFunctions = [
    CAMediaTimingFunction(name:kCAMediaTimingFunctionEaseOut),
    CAMediaTimingFunction(name:kCAMediaTimingFunctionEaseInEaseOut),
    CAMediaTimingFunction(name:kCAMediaTimingFunctionEaseInEaseOut)
]

boundsOvershootAnimation.fillMode = kCAFillModeForwards
boundsOvershootAnimation.isRemovedOnCompletion = false

Second, and likely easier, is using FTUtils, an open source wrapper for core animation. It includes a stock "springy" animation.

You can get it at: http://github.com/neror/ftutils

transform.scale, anyone?

let anim = CAKeyframeAnimation(keyPath: "transform.scale")
anim.values = [0, 1, 0, 1, 0, 1, 0]
anim.duration = 4.0

smallView.layer.addAnimation(anim, forKey: "what")

Totally unrelated, if you are gonna use floats in the values array, you must add them as NSNumbers, otherwise they d just end up as 0s!

anim.values = [0.0, 1.2, 0.9, 1.0].map { NSNumber(double: $0) }

Rather than setting the path of your CAKeyframeAnimation, you ll want to set the keyframes themselves. I ve created a "pop-in" effect before by animating the size of the bounds of a layer:

CAKeyframeAnimation *boundsOvershootAnimation = [CAKeyframeAnimation animationWithKeyPath:@"bounds.size"];
CGSize startingSize = CGSizeZero;
CGSize overshootSize = CGSizeMake(targetSize.width * (1.0f + POPINOVERSHOOTPERCENTAGE), targetSize.height * (1.0f + POPINOVERSHOOTPERCENTAGE));
CGSize undershootSize = CGSizeMake(targetSize.width * (1.0f - POPINOVERSHOOTPERCENTAGE), targetSize.height * (1.0f - POPINOVERSHOOTPERCENTAGE));
NSArray *boundsValues = [NSArray arrayWithObjects:[NSValue valueWithCGSize:startingSize],
                         [NSValue valueWithCGSize:overshootSize],
                         [NSValue valueWithCGSize:undershootSize],
                         [NSValue valueWithCGSize:targetSize], nil];
[boundsOvershootAnimation setValues:boundsValues];

NSArray *times = [NSArray arrayWithObjects:[NSNumber numberWithFloat:0.0f],
                  [NSNumber numberWithFloat:0.5f],
                  [NSNumber numberWithFloat:0.9f],
                  [NSNumber numberWithFloat:1.0f], nil];    
[boundsOvershootAnimation setKeyTimes:times];


NSArray *timingFunctions = [NSArray arrayWithObjects:[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut], 
                            [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut],
                            [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut],
                            nil];
[boundsOvershootAnimation setTimingFunctions:timingFunctions];
boundsOvershootAnimation.fillMode = kCAFillModeForwards;
boundsOvershootAnimation.removedOnCompletion = NO;

where POPINOVERSHOOTPERCENTAGE is the fraction by which I wanted to overshoot the target size of the layer.

I don t know if you can use a CAKeyframeAnimation for animating the scale of a UIView, but you can do it with a CABasicAnimation and setting the fromValue and toValue properties, and using that to animate the transform property:

- (CAAnimation*)monInAnimation 
{
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"transform"];
    animation.fromValue = [NSValue valueWithCATransform3D:CATransform3DMakeScale(1.0, 1.0, 1.0)];
    animation.toValue = [NSValue valueWithCATransform3D:CATransform3DMakeScale(3.0, 3.0, 3.0)];
    [animation setDuration:1.5];
    [animation setAutoreverses:YES];
    return animation;
}

- (IBAction)monBtnIn 
{
    [monButton.layer addAnimation:[self monInAnimation] forKey:@"transform"];
}