Observer 

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

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

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 certain interface in order for the publisher to be able to update them. But since Kotlin has higher-order functions, we can omit this part. The publisher will still have to provide means for observers to be able to subscribe and unsubscribe.

Animal Choir

So, animals have decided to have a choir of their own. The cat was elected as the conductor...