How does a mediator object work? What is the idea behind it?

Question

I'm interested in the mediator object because it sounds useful, but deciphering code examples in order to learn how to interact with and build that object escapes me. I love code examples if they come with some explanations, however short. Would someone be able to just explain what I'm building when I build a mediator object?

Would a mediator object be a way to handle action events sent between classes? or does the mediator object simply serve better for consolidating like-code into one handy place?

I don't know if it's practical for convenience or if it's practical because there is no other way to do what it does. Any details, however "dumbed down", would be most excellent. Thanks in advance.

Answer 1

The mediator object is intended to do nothing itself. You should not move any logic that you already have into it, except maybe for some multiplexing/demultiplexing (when one object sends the same message to multiple other objects). The mediator is just an external interface (if it simultaneously serves as a facade), and definitely a message passing channel between pre-existing objects.

Likewise, a mediator should not be created until you are already perceiving the need for such a message passing channel. How does such a need look like? You already have a set of objects that start calling each other in increasingly complex ways. Those objects are storing references to each other; the number of such references is already getting bigger than the number of such objects themselves.

So instead of each object talking to each object (with a quadratic number of references and complicated graph of interactions) you introduce a star topology to interactions; everybody directly talks just to the mediator. It is then easier to instantiate, monitor, debug, extend, polymorphize...

Do not start introducing mediators too early or the overall complexity will grow instead of dropping.