Overview

ECS (Entity Component System) is an architectural pattern where computation is defined as a list of systems operating on a set of entities, each of which consists of a dynamic set of pure data components. Systems select the entities to process via means of persistent, efficient queries over the entities' component "shapes".

Here's a short glossary cribbed from the getting started guide (which you should probably read first):

• entities: an object with a unique ID that can have multiple components attached to it.
• components: different facets of an entity, e.g. geometry, physics, hit points. Data is only stored in components.
• systems: pieces of code that do the actual work within an application by processing entities and modifying their components.
• queries: used by systems to determine which entities they are interested in, based on the components attached to the entities.
• world: a container for entities, components, systems and queries.

The usual workflow when building an ECS based program:

1. Create the component types that shape the data you need to use in your application.
2. Create entities and attach components to them.
3. Create the systems that will use these components to read and transform the data of entities selected by a query.
4. Execute all the systems each frame.

Example

Let's say we want to create a game where the player fights with wolves and dragons. We will start by defining components that will be attached to entities:

• Walker and Flyer for entities that will walk and fly (respectively).
• Enemy for enemy entities.
• Model3D for all the entities that will have a 3D Model.

Then we use these components to define our main entities:

• wolf: It's an Enemy, can walk and has a model3D.
• dragon: It's an Enemy, can fly and has a model3D.
• player: It's an Player, can walk and has a model3D.

And finally we define the systems that will add the logic to the game:

• Walk: It will modify the Walker entities (Player and Wolf) moving them around.
• Fly: It will modify the Flyer entities (Dragon) moving them around in the sky.
• AI_Walk: It will modify the Enemy and Walker entities (Wolf) using AI techniques to compute the path they will follow.
• Attack: It will implement all the logic for attacks between Enemy and Player entities.
• Draw: It will draw all the entities that has Model3D component on the screen.