Create CASpringAnimation instance without accessing undocumented APIs

CASpringAnimationFactory

import UIKit

private let SharedCASpringAnimationFactory = CASpringAnimationFactory()

public class CASpringAnimationFactory {
    private var dummyView: UIView

    private init() {
        dummyView = UIView(frame: CGRect.zeroRect)
    }

    private class var shared: CASpringAnimationFactory {
        return SharedCASpringAnimationFactory
    }

    public class func animation(#keyPath: String, dumping: CGFloat, initialSpringVelocity: CGFloat) -> CABasicAnimation {
        UIView.animateWithDuration(0.3, delay: 0.0, usingSpringWithDamping: dumping, initialSpringVelocity: initialSpringVelocity, options: nil,
            animations: { () -> Void in
                CASpringAnimationFactory.shared.dummyView.bounds = CGRect(origin: CGPoint.zero, size: CGSize(width: 100, height: 100))
            }, completion: nil)

        let arrestedAnimation = shared.dummyView.layer.animationForKey("bounds").copy() as CABasicAnimation

        arrestedAnimation.duration = CATransaction.animationDuration()
        arrestedAnimation.keyPath = keyPath
        arrestedAnimation.fromValue = nil
        arrestedAnimation.toValue = nil

        shared.dummyView.layer.removeAllAnimations()
        shared.dummyView.bounds = CGRect.zeroRect

        return arrestedAnimation
    }
}