一.概述
KVO,即：Key-Value Observing，它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

二.使用的步骤
1.注册，指定被观察者和观察的对象

   objectToObserve.addObserver(self, forKeyPath: "contentArray", options: .new, context: &myContext)

2.实现属性变化的回调

  override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if context == &myContext {

       } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
    }

3.移除观察

   deinit {
        objectToObserve.removeObserver(self, forKeyPath: "contentArray", context: &myContext)
    }

三.完整的代码

@objcMembers class MyObjectToObserve: NSObject {
    dynamic var contentArray = [String]()
}
private var myContext = 0
class PlaySpeechString: NSObject,AVSpeechSynthesizerDelegate {
    static let shared = PlaySpeechString()
    var objectToObserve = MyObjectToObserve()

    // Make sure the class has only one instance
    // Should not init or copy outside
    private override init() {
        super.init()
        objectToObserve.addObserver(self, forKeyPath: "contentArray", options: .new, context: &myContext)
    }
    
    override func copy() -> Any {
        return self // SingletonClass.shared
    }
    
    override func mutableCopy() -> Any {
        return self // SingletonClass.shared
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if context == &myContext {

        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }

    }

    deinit {
        objectToObserve.removeObserver(self, forKeyPath: "contentArray", context: &myContext)
        }
    }

注意：
1.被观察属性所在的类需要用 @objcMembers 关键字修饰 要不会报error
fatal error: Could not extract a String from KeyPath
Swift.ReferenceWritableKeyPath
2.被观察的属性需要用dynamic修饰，否则也无法观察到。