KVO

KVO（Key-Value Observing）监听属性值，可以用来动态更新页面

import UIKit

//属性前加上@objc dynamic实现动态派发
class Preson: NSObject {
    @objc dynamic var age = 0
}

class ViewController: UIViewController {

    let p = Preson()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        //为p对象的age属性设置监听。
        //options为枚举数组，传入了new和old两个值，表示我们需要监听改变前和改变后的值。
        //context是Any类型，可以传入任意类型数据，方便在监听到变化时获取该数据。
        p.addObserver(self, forKeyPath: "age", options: [.new, .old], context: nil)
    }
    
    //监听
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "age" {
            print("age被改变了")
            print(change as Any)
        }
    }
    
    //释放监听
    deinit { 
        p.removeObserver(self, forKeyPath: "age")
    }
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        p.age = 20
    }

}