虽然模型不了解视图的任何详细信息，但是模型和视图之间存在“观察者”关系。换句话说，视图保留对模型的引用并将其自身注册到模型以接收更改通知。接收到通知后，视图将从模型中获取新数据并对 UI 进行更新。(Although the model does not understand any of the details of the view, there is an observer relationship between the model and the view. In other word