UIView Animate Gradient

Gradient.swift

class GradientView: UIView {
    var color: UIColor
    
    init(_ color: UIColor) {
        self.color = color
        super.init(frame: .zero)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    var middleAlpha: CGFloat {
        get {
            if let layer = layer as? UIViewAnimatableGradientLayer {
                return layer.middleAlpha
            }
            return 0
        }
        set {
            if let layer = layer as? UIViewAnimatableGradientLayer {
                layer.middleAlpha = newValue
                // Change model layer property
                layer.colors = [
                    color.withAlphaComponent(0).cgColor,
                    color.withAlphaComponent(newValue).cgColor,
                    color.cgColor,
                ]
            }
        }
    }
    
    override class var layerClass: AnyClass {
        UIViewAnimatableGradientLayer.self
    }
}

class UIViewAnimatableGradientLayer: CAGradientLayer {
    @NSManaged var middleAlpha: CGFloat
    
    override init() {
        super.init()
    }
    
    override init(layer: Any) {
        super.init(layer: layer)
        if let layer = layer as? UIViewAnimatableGradientLayer {
            self.middleAlpha = layer.middleAlpha
        }
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }
    
    private class func isCustomAnimKey(key: String) -> Bool {
        return key == "middleAlpha"
    }
    
    override class func needsDisplay(forKey key: String) -> Bool {
        if self.isCustomAnimKey(key: key) {
            return true
        } else {
            return super.needsDisplay(forKey: key)
        }
    }
    
    override func action(forKey event: String) -> CAAction? {
        if UIViewAnimatableGradientLayer.isCustomAnimKey(key: event) {
            // Use a normal animatable property name to retrieve a CABasicAnimation, we need timing curve and duration info from it.
            if let infoAction = super.action(forKey: "backgroundColor") as? CABasicAnimation {
                infoAction.keyPath = "colors"
                if let pLayer = presentation() {
                    infoAction.fromValue = pLayer.colors
                }
                infoAction.toValue = nil
                return infoAction
            }
            setNeedsDisplay()
            return nil
        }
        return super.action(forKey: event)
    }
}