开篇提醒：OC中的KVO及其KVO的基础知识可参见：深入runtime探究KVO
KVO本质上是基于runtime的动态分发机制，通过key来监听value的值。
OC能够实现监听因为都遵守了NSKeyValueCoding协议
OC所有的类都是继承自NSObject，其默认已经遵守了该协议，但Swift不是基于runtime的， Swift 中的属性处于性能等方面的考虑默认是关闭动态分发的，只有在属性前加 dynamic才会开启运行时，允许监听属性的变化。
OC中，所有的类继承自 NSObject ，它对 KVO提供了默认实现,但Swift不是。
原因有二：
class Person：NSObject {
//name不支持KVO监听，age支持KVO
var name: String
dynamic var age: Int = 0
init(name: String,age: Int) {
self.name = name
self.age =age
}
譬如：
class Person {
var name: String?
var age: Int = 0
init(name: String,age: Int) {
self.name = name
self.age =age
}
该类根本无法调用
open func addObserver(_ observer: NSObject, forKeyPath keyPath: String, options: NSKeyValueObservingOptions = [], context: UnsafeMutableRawPointer?)
方法，所以肯定无法使用KVO观察者模式，但Swift中提供了属性观察器（didSet,willSet）来解决这种问题；
class Father: NSObject {
var firstName: String = "First" {
willSet { //新值设置之前被调用
print("willSet的新值是\(newValue)")
}
didSet { //新值设置之后立即调用
print("didSet的新值是\(oldValue)")
}
}
}
第一步：注册
open func addObserver(_ observer: NSObject, forKeyPath keyPath: String, options: NSKeyValueObservingOptions = [], context: UnsafeMutableRawPointer?)
第二步：监听
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?)
第三步：移除
open func removeObserver(_ observer: NSObject, forKeyPath keyPath: String)
import UIKit
//监听UISlider的滑动，把滑动的结果传递给UIProgressView，以显示滑动进度
class viewController: UIViewController {
@IBOutlet weak var slider: UISlider!
@IBOutlet weak var progressView: UIProgressView!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
self.progressView.addObserver(self, forKeyPath: "progress", options: .new, context: nil)
self.slider.addObserver(self, forKeyPath: "value", options: .new, context: nil)
}
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
if keyPath == "value" {
if let value = change?[NSKeyValueChangeKey.newKey] as? Float {
self.progressView.progress = value/self.slider.maximumValue
view.alpha = CGFloat(self.progressView.progress)
self.textLabel.font = UIFont.systemFont(ofSize: view.alpha * 20)
self.textField.text = self.father?.firstName
}
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
deinit {
self.progressView.removeObserver(self, forKeyPath: "progress")
self.slider.removeObserver(self, forKeyPath: "value")
}
}