SWLayer.swift

SWLayer.swift

class SWLayer: CALayer {
    @NSManaged var currentX: CGFloat
    
    override init() {
        super.init()
    }
    
    override init(layer: Any) {
        super.init(layer: layer)
        if let layer = layer as? SWLayer {
            currentX = layer.currentX
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    
    private class func isCustomAnimKey(key: String) -> Bool {
        return key == "currentX"
    }
    
    override class func needsDisplay(forKey key: String) -> Bool {
        if self.isCustomAnimKey(key: key) {
            return true
        }
        return super.needsDisplay(forKey: key)
    }
}

SWView.swift

class SWView : UIView {
    var currentX: CGFloat {
        set {
            if let layer = layer as? SWLayer {
                layer.currentX = newValue
            }
        }
        get {
            if let layer = layer as? SWLayer {
                return layer.currentX
            }
            return 0.0
        }
    }
    
    override class var layerClass : AnyClass {
        return SWLayer.self
    }
    
    override func display(_ layer: CALayer) {
        print("DISPLAYING")
        print(self.currentX)
    }
}

ViewController.swift

class Test : UIViewController {
  override func viewDidLoad() {
        super.viewDidLoad()
    
        let viewWithCustomLayer = SWView()
        viewWithCustomLayer.frame = CGRect(x: 0, y: 0, width: 150, height: 150)
        
        viewWithCustomLayer.currentX = 0
        UIView.animate(withDuration: 2, animations: {
            viewWithCustomLayer.currentX = 100.0
        })
    }
}