Observer

Probably one of the highlights of this chapter, this design pattern provides us with a bridge to the following chapters, which are dedicated to functional programming.

So, what is the Observer pattern all about? You have one publisher, which may also be called a subject, that may have many subscribers, also known as observers. Each time something interesting happens with the publisher, all of its subscribers should be updated.

This may look a lot like the Mediator design pattern, but there's a twist. Subscribers should be able to register or unregister themselves at runtime.

In the classical implementation, all subscribers/observers need to implement a particular interface for the publisher to update them. But since Kotlin has higher-order functions, we can omit this part. The publisher will still have to provide a means for observers to be able to subscribe and unsubscribe.

This may have sounded a bit complex, so let's take a look at the following...