To move things along, we need to remind ourselves about a particular pretty old and well-known design pattern—the Observer pattern. That is one of the twenty-three famous GoF (Gang of Four) design patterns. At first glance, it may appear that the Observer pattern is not related to reactive programming. However, as we will see later, with some small modifications, it defines the foundations of reactive programming.
To read more about GoF design patterns, please refer to Design
Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (https://en.wikipedia.org/wiki/Design_Patterns).
The Observer pattern involves a subject that holds a list of its dependants, called ...
