The observer pattern
Let's first see the language-agnostic definition of the observer pattern. The GOF book, Design Patterns: Elements of Reusable Object-Oriented Software, defines the observer pattern as follows:
One or more observers are interested in the state of a subject and register their interest with the subject by attaching themselves. When something changes in our subject that the observer may be interested in, a notify message is sent which calls the update method in each observer. When the observer is no longer interested in the subject's state, they can simply detach themselves.
In the observer design pattern, the subject keeps a list of objects depending on it (called observers) and notifies them when the state changes. The subject uses a broadcast to the observers to inform them of the change. Observers can remove themselves from the list once they no longer wish to be notified. Based on this understanding, we can define the participants in this pattern:
Subject: This keeps...
