InterpolatingAnimator.swift

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true


extension Comparable {
    /// Clamp the value between `low` and `high`
    func clamp(_ low: Self, _ high: Self) -> Self {
        return min(high, max(low, self))
    }
}

class InterpolatingAnimator: NSObject {

    enum Interpolation {
        case linear
        case easeOut
        case custom((CGFloat) -> CGFloat)

        func transform(_ t: CGFloat) -> CGFloat {
            switch self {
            case .linear: return t
            case .easeOut: return t * (2 - t)
            case .custom(let transform): return transform(t)
            }
        }
    }

    typealias InterpolationUpdate = (CGFloat, Bool) -> Void

    private var displayLink: CADisplayLink!
    private let interpolation: Interpolation
    private let update: InterpolationUpdate
    private let duration: TimeInterval
    private let from: CGFloat
    private let to: CGFloat
    private var timeRunning: CFTimeInterval = 0

    init(_ interpolation: Interpolation, duration: TimeInterval, fromValue: CGFloat, toValue: CGFloat, execute: @escaping
        InterpolationUpdate) {
        self.interpolation = interpolation
        self.duration = duration
        from = fromValue
        to = toValue
        update = execute
        super.init()
        self.displayLink = CADisplayLink(target: self, selector: #selector(updateProgress))
    }

    private dynamic func updateProgress() {
        timeRunning += displayLink.duration
        let progress = CGFloat(timeRunning / duration).clamp(0, 1)
        let value = from + abs(to - from) * interpolation.transform(progress)
        let finished = progress >= 1
        update(value, finished)
        if finished {
            displayLink.remove(from: .current, forMode: .commonModes)
        }
    }

    public func start() {
        update(from, false)
        displayLink.add(to: .current, forMode: .commonModes)
    }

}



let label = UILabel()
label.frame = CGRect(x: 0, y: 0, width: 100, height: 50)
label.backgroundColor = .white

let formatter = NumberFormatter()
formatter.minimumIntegerDigits = 1
formatter.minimumFractionDigits = 2
formatter.maximumFractionDigits = 2

var animator: InterpolatingAnimator?
animator = InterpolatingAnimator(.easeOut, duration: 2, fromValue: 3, toValue: 4) { value, finished in
    guard let text = formatter.string(from: NSNumber(value: Double(value))) else { return }
    label.text = text
    print(text)

    if finished {
        PlaygroundPage.current.finishExecution()
        animator = nil
    }
}


animator?.start()
PlaygroundPage.current.liveView = label