观察者在 Ruby 中的实现
Observer is a behavioral design pattern that allows one objects to notify other objects about changes in their state.
The Observer pattern provides a way to subscribe and unsubscribe to and from these events for any object that implements a subscriber interface.
在 Ruby 中使用模式
使用示例： The Observer pattern is pretty common in Ruby code, especially in the GUI components. It provides a way to react to events happening in other objects without coupling to their classes.
识别方法： The pattern can be recognized by subscription methods, that store objects in a list and by calls to the update method issued to objects in that list.