观察派生属性的变化:CALayer KVO示例

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let layer = CALayer()
        layer.frame = CGRect(x: 0, y: 0, width: 100, height: 100)
        layer.backgroundColor = UIColor.red.cgColor
        view.layer.addSublayer(layer)
        
        layer.addObserver(self, forKeyPath: "bounds", options: .new, context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "bounds" {
            print("Layer bounds changed: \(change?[.newKey])")
        }
    }
}