What are Entity and Component

An Entity is a unique identifier that usually has a number of Components associated, these components are nothing more than simple data containers. For example a Player entity could have the attributes Health and Position associated. It is also common for entities to have a marker component associated, the marker is used to identify and describe the type of an entity. In the case of a Player it would be a struct Player; that is used to mark the entity as a Player.

Modeling a Player in ECS starts by defining some components that holds attributes for the player.

// Component used to mark an entity as a player
struct Player;

// Component storing health
struct Health(f64);

// Component storing position
struct Position {
    x: f64,
    y: f64,
    z: f64,
}

Here Health and Postion attributes could be shared across multiple types of entities, but the player marker would only exist for a Player entity. We could define a separate Zombie marker for a Zombie and have it share Health and Postion components with the Player.

// Component used to mark an entity as a zombie
struct Zombie;

In ECS each entity and its associated components are stored in a world, we can imagine the world being a large matrix where each column is a component, each row a unique entity, and each cell component data associated with the entity.

Now that we have defined two marker components Player and Zombie, and the associated attribute components Health and Postion, we can create Player and Zombie as entities and spawn them into the ECS world.

use fecs::EntityBuilder;

EntityBuilder::new()
    .with(Player)
    .with(Health(20.0))
    .with(Postion {
        x: 0.0,
        y: 0.0,
        z: 0.0,
    })
    .build()
    .spawn_in(&mut world);

EntityBuilder::new()
    .with(Zombie)
    .with(Health(15.0))
    .with(Position {
        x: 0.0,
        y: 0.0,
        z: 0.0,
    })
    .build()
    .spawn_in(&mut world);