Spring Animator

SpringAnimator.swift

//: Playground - noun: a place where people can play

import UIKit

//Changing values
var initialVelocity: CGFloat = 0
var runningTime: CGFloat = 0

var myProperty: CGFloat = 0

//Resting position of spring
let endNumberValue: CGFloat = 300

//Set relaxation time (duration in seconds)
let relaxationTime: CGFloat = 1

//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

internal func springFunction(_ currentValue: CGFloat, _ length: CGFloat, _ velocity: inout CGFloat) -> CGFloat {
    let displacement         = currentValue - length
    let springForce          = -stiffness * displacement  // Hooke's Law (F = -kx) (where k is stiffness constant and x is displacement value)
    let forceDamper          = -damping * velocity        // F = -dv (where d is damping constant and v is velocity)
    let dampedForce          = springForce + forceDamper
    let acceleration         = dampedForce / mass         // a = F / m
    let refreshRate: CGFloat = 1/60                       //Hz

    velocity += acceleration * refreshRate

    let position = velocity * refreshRate

    return position
}

public func animateProperty(_ property: inout CGFloat, _ runningTime: CGFloat) {
    if runningTime < relaxationTime {
        property += springFunction(property, endNumberValue, &initialVelocity)
    } else {
        //Set final value to exact value
        property = endNumberValue
    }
}

while runningTime <= relaxationTime {
    //Put this inside a CADisplayLink updating the running time and your view origin will animate in a spring
    animateProperty(&myProperty, runningTime)

    runningTime += 1/60
    print(myProperty)
}