The Observer pattern in reactive programming

The Observer pattern (covered in Chapter 5, Behavioral Design Patterns) is useful for notifying an object or a group of objects when the state of a given object changes. This type of traditional Observer allows us to react to some object change events. It provides a nice solution for many cases, but in a situation where we must deal with many events, some depending on each other, the traditional way could lead to complicated, difficult-to-maintain code. That is where another paradigm called reactive programming gives us an interesting option. In simple terms, the concept of reactive programming is to react to many events (streams of events) while keeping our code clean.

Let’s focus on ReactiveX (http://reactivex.io), which is a part of reactive programming. At the heart of ReactiveX is a concept known as an Observable. According to its official website, ReactiveX is about providing an API for asynchronous programming with what...