Lottie subclass for partial and reverse playback on iOS

STAnimationView.swift

//
//  STAnimationView.swift
//  Allan Weir
//
//  Created by Allan Weir on 06/02/2017.
//  Copyright © 2017 Allan Weir. All rights reserved.
//  http://www.studiousdesigns.com
//

import UIKit
import Lottie

class STAnimationView: LOTAnimationView {

    private var displayLink: CADisplayLink?
    private var lastUpdateTime: CGFloat = 0
    private var endTime: CGFloat = 1

    private var lastTimeStamp: Date = Date()

    private(set) var isPlayingReverse: Bool = false

    class func animation(named animationName: String) -> STAnimationView {
        let urlPath = Bundle.main.path(forResource: animationName, ofType: "json")!
        return STAnimationView(contentsOf: URL(fileURLWithPath: urlPath))
    }

    func play(fromPosition: CGFloat, toPosition: CGFloat) {

        if let oldLink = self.displayLink {
            oldLink.invalidate()
        }

        self.displayLink = CADisplayLink(target: self, selector: #selector(STAnimationView.updateDisplayLink(link:)))
        self.displayLink?.add(to: RunLoop.main, forMode: .defaultRunLoopMode)

        self.isPlayingReverse = (toPosition < fromPosition)

        self.lastUpdateTime = fromPosition
        self.endTime = toPosition
        self.animationProgress = fromPosition
        self.lastTimeStamp = Date()
    }

    internal func updateDisplayLink(link: CADisplayLink) {

        let newTime = Date()
        let timePassed = newTime.timeIntervalSince(self.lastTimeStamp)
        let deltaProgress = CGFloat(timePassed) * (self.animationSpeed / self.animationDuration)
        var newProgress = !self.isPlayingReverse ? self.animationProgress + deltaProgress : self.animationProgress - deltaProgress

        if !self.isPlayingReverse {
            if newProgress >= self.endTime {
                newProgress = self.endTime
                self.displayLink?.invalidate()
                self.displayLink = nil
            }
        } else {
            if newProgress <= self.endTime {
                newProgress = self.endTime
                self.displayLink?.invalidate()
                self.displayLink = nil
            }
        }

        self.animationProgress = newProgress
        self.lastTimeStamp = newTime

    }

}