The Subscriber from RxKotlin 1.x,  essentially became an Observer in RxKotlin 2.x. There is an Observer interface in RxKotlin 1.x, but Subscriber is what you pass to the subscribe() method, and it implements Observer. However, In RxJava 2.x, Subscriber only exists when talking about Flowables, which we will cover in Chapter 4, Introduction to Backpressure and Flowables.

As you can see in the previous examples in this chapter, Observer is an interface with four methods in it—onNext(item:T), onError(error:Throwable), onComplete(), and onSubscribe(d:Disposable). As stated earlier, when we connect Observable to Observer, it looks for these four methods in Observer and calls them. So, the following is a short description of the four methods:

• onNext: Observable calls this method of Observer to pass each of the items one by one.
• onComplete: When Observable wants to denote, it's done with passing items to the onNext method, and it calls the onComplete method of Observer.
• onError: When Observable faces any error, it calls the onError method to deal with the error if defined in the Observer, otherwise, it throws the exception.
• onSubscribe: This method is called whenever a new Observable subscribes to the Observer.