Scala Reactive Programming

Observer

Observer provides a technique for receiving push-based notifications. It will subscribe with an Observable using its subscribe() method.

Once an Observer makes a call to an Observable.subscribe() method and the subscription is done, the Observable calls the Observer.onNext() method to provide notifications to that Observer.

Asynchronous data stream processing may finish either successfully or with some errors. If it's done successfully, Observable will make a call to the Observer.onCompleted() method. If it's done with errors, Observable will make a call to the Observer.onError() method.

A well-behaved Observable will call an Observer's onCompleted or onError methods exactly once.