A custom CALayer (review from NSScreencast)

CustomCALayer.swift

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

import UIKit
import PlaygroundSupport

private final class ProgressLayer: CALayer {
    @NSManaged var progress: CGFloat
    
    private let fillColor = UIColor.blue.cgColor
    
    override class func needsDisplay(forKey key:String) -> Bool {
        
        if key == "progress" {
            return true
        }
        
        return super.needsDisplay(forKey: key)
    }
    
    override func action(forKey key: String) -> CAAction? {
        if key == "progress" {
            let animation = CABasicAnimation(keyPath: key)
            animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
            animation.fromValue = presentation()?.value(forKey: key)
            return animation
        }
        
        return super.action(forKey: key)
    }
    
    override init() {
        super.init()
    }
    
    override init(layer: Any) {
        super.init(layer: layer)
        
        guard let progressLayer = layer as? ProgressLayer else { return }
        
        progress = progressLayer.progress
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }
    
    override func draw(in context: CGContext) {
        context.setFillColor(fillColor)
        
        let progress = presentation()?.progress ?? 0
        
        var rect = bounds
        rect.size.width *= progress
        context.fill(rect)
    }
}

final class ProgressView: UIView {
    var progress: CGFloat {
        get {
            return progressLayer?.progress ?? 0
        }
        set {
            progressLayer?.progress = newValue
        }
    }
    
    override class var layerClass: AnyClass {
        return ProgressLayer.self
    }
    
    private var progressLayer: ProgressLayer? {
        return layer as? ProgressLayer
    }
}

final class ViewController: UIViewController {
    
    let progressView = ProgressView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .white
        
        progressView.progress = 0.31
        progressView.frame = view.bounds
        progressView.backgroundColor = .clear
        progressView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        view.addSubview(progressView)
        
        
        let tap = UITapGestureRecognizer(target: self, action: #selector(increment))
        view.addGestureRecognizer(tap)
    }
    
    @objc private func increment() {
        progressView.progress += 0.1
    }
}

PlaygroundPage.current.liveView = ViewController()