JNWSpringAnimation is a subclass of CAKeyframeAnimation that adds support for creating damped harmonic animations.

Although JNWSpringAnimation is a subclass of CAKeyframeAnimation, it should be treated as if it were a subclass of CABasicAnimation.

To get started, copy the four source files into your project.

The animation can be created by using the dedicated initializer, +animationWithKeyPath:. Alternatively, the animation can be created using +animation and by setting the key path afterwards. The full list of currently-compatible animatable properties is available in the header.

JNWSpringAnimation *animation = [JNWSpringAnimation animationWithKeyPath:@"position.x"];

Next, the fromValue and toValue properties must be set for the interpolated values to be calculated correctly.

animation.toValue = @(toX);
animation.fromValue = @(currentX);

Finally, the values for the spring constants can optionally be changed. Currently, stiffness, damping, and mass are available for modification.

animation.mass = 30; // this will move extremely slowly
// and so on

The animation itself can be applied like any other subclass of CAAnimation, namely -addAnimation:forKey: on any CALayer.

This was created in my desire to have an open-source version of the (currently private) CASpringAnimation which was discovered in iOS 6.

Spring animations, when used appropriately, can really enhance the way your app feels to the user by connecting in physical simulations with an app's interface. And besides, who doesn't love messing around with springs?

I created this as a weekend project, so the implementation is still somewhat unfinished. Compared to CASpringAnimation, the velocity property is still unimplemented. Pull requests are welcome.

JNWSpringAnimation is licensed under the MIT license. See LICENSE.md.

You can follow me on Twitter as @willing, email me at the email listed on my GitHub profile, or read my blog at jwilling.com.