Event Queue, Message Bus and Dispatcher

NOTE: I will use the terms event and message interchangeably. In some contexts there may be a minor differentiation but for this discussion it will be ignored.

Event Queue or Message Bus

Definition:

Decouple when a message or event is sent from when it is processed. - R. Nystrom

What's the easiest way to manage the connections between event listeners and event producers? You guessed it, “Event Queues”.

When an event occurs, such as user input, or an entity sending an event to another entity, it needs to be stored somewhere such that it's not lost while in transit between the source that reported the event and when the program gets around to respond to it. That “somewhere” is a queue.

New events are added (aka enqueue) to a queue of unprocessed events. And at a later time when it's convenient in the application we...

Observer Design Pattern

Observer is a behavioral software design pattern, that is being extensively utilized in almost every single self-respecting codebase as a means to efficiently communicate between objects in an event-based fashion. It wouldn't be an understatement to say it is the foundation of event based programming.

In fact, underlying the ubiquitous, in network programming, “Model View Controller” (MVC) design pattern, is the observer pattern.

Observers are used in every single game engine - they're known there mostly as “Event dispatchers”.

Alternative terminology for this pattern includes the following: - Publish-Subscribe, Listener-Reporter.

Design

• a class, called the Subject, maintains a list of...