How to derive CASpringAnimation properties from simpler UISpringTimingParameter initializer?

Question

I have a spring animation curve specified using a UISpringTimingParameters with damping ratio and initial velocity, and I use it with a UIViewPropertyAnimator with its duration specified. Elsewhere in my app, I need to express the same animation using CASpringAnimation. But CASpringAnimation can't be initialized with just a damping ratio and initialVelocity - it requires all the different spring parameters and derives its settlingDuration from them.

Given a duration, damping ratio, and initial velocity vector, how do I create a CASpringAnimation?

Answer 1

I found the answer on a blog post. In short:

let spring = CASpringAnimation(keyPath: somePropertyPath)

//Set initial velocity and desired duration
let initialVelocity: CGFloat = desiredInitialVelocity
let relaxationTime: CGFloat = desiredDurationInSeconds

//Spring constants
let dampingRatio: CGFloat = 0.9
//Only allow damping ratio between just above 0 and 1 (critically damped)
let clippedDampingRatio: CGFloat = min(1, max(dampingRatio, 0.01))
let mass: CGFloat = 1
let fractionOfAmplitude: CGFloat = 1500 //A spring never gets to 0 amplitude, it gets infinitely smaller. This fraction represents the perceived 0 point.
let logOfFraction: CGFloat = log(fractionOfAmplitude)
let stiffness: CGFloat = (mass * pow(logOfFraction, 2)) / (pow(relaxationTime, 2) * pow(clippedDampingRatio, 2))
let angularFrequency: CGFloat = sqrt(stiffness/mass)
let damping: CGFloat = 2 * mass * angularFrequency * clippedDampingRatio

spring.initialVelocity = initialVelocity
spring.mass = mass
spring.stiffness = stiffness
spring.damping = damping

Answer 2

Sorry I can't help you with the math, but perhaps you could use the a dummy property animator to compute what you need? See https://developer.apple.com/videos/play/wwdc2016/216/?time=2521

func animationDuration() -> TimeInterval {
    return propertyAnimator().duration
}

func propertyAnimator(initialVelocity: CGVector = .zero) -> UIViewPropertyAnimator {
    let timingParameters = UISpringTimingParameters(mass: 4.5, stiffness: 1300, damping: 95, initialVelocity: initialVelocity)
    return UIViewPropertyAnimator(duration: assetTransitionDuration, timingParameters:timingParameters)
}