Spring Animation Playground - Calculating stiffness and damping with a given duration and dampingRatio.

SpringAnimation.swift

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white
        self.view = view

        //Edit these
        let duration: CGFloat = 0.35
        let dampingRatio: CGFloat = 1

        let purpleSquare = UIView()
        purpleSquare.frame = CGRect(x: 135, y: 200, width: 50, height: 50)
        purpleSquare.backgroundColor = .systemPurple
        view.addSubview(purpleSquare)

        let purpleAnimation = UIViewPropertyAnimator(duration: TimeInterval(duration), dampingRatio: dampingRatio, animations: {
            purpleSquare.frame.origin.y += 200
        })

        let blueSquare = UIView()
        blueSquare.frame = CGRect(x: purpleSquare.frame.origin.x + 55, y: purpleSquare.frame.origin.y, width: 50, height: 50)
        blueSquare.backgroundColor = .systemBlue
        view.addSubview(blueSquare)

        //Formula for calculating damping and stiffness given duration and dampingRatio (value from >0 to 1)
        let fractionOfAmplitude: CGFloat = 1/10000
        let mass: CGFloat = 1
        let damping: CGFloat = (-2 * mass * log(fractionOfAmplitude)) / duration
        let stiffness: CGFloat = pow(damping,2) / (4 * pow(dampingRatio, 2)) * mass

        let blueAnimation = UIViewPropertyAnimator(duration: 0, timingParameters: UISpringTimingParameters(mass: 1, stiffness: stiffness, damping: damping, initialVelocity: CGVector(dx: 0, dy: 0)))
        blueAnimation.addAnimations {
            blueSquare.frame.origin.y += 200
        }

        purpleAnimation.startAnimation(afterDelay: 2)
        blueAnimation.startAnimation(afterDelay: 2)
    }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()

Purple square uses the regular UIViewPropertyAnimator that you can use in most cases.
The blue square uses a spring, which you might need to use if you must animate a CALayer. The formulas on lines 33 and 34 show you how to calculate the damping and stiffness, given a dapingRatio and duration.

This really helped me today when trying to make a SwiftUI spring animation match a UIKit spring animation. Thank you for this!