SKBounceAnimation is a CAKeyframeAnimation subclass that creates an animation for you based on start and end values and a number of bounces. It’s based on the math and technology in this blog post, which in turn was based partially on Matt Gallagher’s work here.

Check it out on GitHub:


Basic code is simple:

SKBounceAnimation *bounceAnimation = [SKBounceAnimation animationWithKeyPath:@"position.y"];  
bounceAnimation.fromValue = [NSNumber];  
bounceAnimation.toValue = [NSNumber numberWithFloat:300];  
bounceAnimation.duration = 0.5f;  
bounceAnimation.delegate = self;  
bounceAnimation.numberOfBounces = 2;  
[bouncingView.layer setValue:animation.toValue forKeyPath:animation.keyPath];  
[view.layer addAnimation:bounceAnimation forKey:@"someKey"];  

SKBounceAnimation is an explicit animation, so you have to give the layer its final value before you add the animation. If you don’t, the layer will snap back to its original state after the animation is over.


The math is simple. Check out the blog post and the informational post preceding it for exact details, but essentially the system behaves with oscillating exponential decay in the form of the equation: x = Ae^(-αt)•cos(ωt) + B.

A is the difference between start and end values, B is the end value, α is determined by the number of frames required to get the exponential decay portion to close enough to 0, and ω is determined by the number of periods required to get the desired number of bounces.


shouldOvershoot is a property that you can change. It defaults to YES; if you set it to NO, the animation will bounce as if it were hitting a wall, instead of overshooting the target value and bouncing back. It looks a lot like the Anvil effect in Keynote.

I’d like to add another property that would make the object stretch away from its original location and then bounce back, the way the photo button in iOS 5.1+ works, but I haven’t had a chance to implement yet.

Demo app

The demo app contains demos for several different animations that are supported by SKBounceAnimation.

  • One-axis animation: Using a keypath like position.x, we can animate along one axis.
  • Two-axis animation: Using a keypath like position, SKBounceAnimation will generate a path, and your layer will follow it.
  • Size: Using the bounds keypath, we can make the size increase. The center of the size increase is determined by anchorPoint, which can be moved. It defaults to the center of the layer.
  • Color: I have no idea why anyone would want to bounce a color animation, but I was feeling whimsical, so I added support for this as well.
  • Scale: Using a CATransform3D struct and the transform keypath, we can scale objects. This is very useful to create an effect like UIAlerts bouncing in.
  • Scale & Rotate: Using multiple CATransform3Ds on top of each other, we can do super weird effects like scale and rotating. They look really cool.
  • Rect: The last demo creates two SKBounceAnimations with two different keyPath s ( position and bounds ) but attaches them to the same layer. The effect looks like a frame animation.

Future work

SKBounceAnimation doesn’t support the byValue property yet. I also would like to add a property like stretchAndBounceBack.