ecs-lib is a tiny and easy to use ECS (Entity Component System) library for game programming. It's written in Typescript but you can use on node.js an web browser too.

TLDR; Take a look at the example and its source code

npm install --save ecs-lib

Table of contents

• Documentation
  • World
  • Component
    • Raw data access
    • Secondary attributes
  • Entity
    • Adding and removing from the world
    • Adding and removing components
    • Subscribing to changes
    • Accessing components
  • System
    • Adding and removing from the world
    • Limiting frequency (FPS)
    • Time Scaling - Slow motion effect
      • Pausing
    • Global systems - all entities
    • Before and After update
    • Enter - When adding new entities
    • Change - When you add or remove components
    • Exit - When removing entities
• API
  • ECS
  • Component
    • Component<T>
  • Entity
  • System
• Feedback, Requests and Roadmap
• Contributing
  • Translating and documenting
  • Reporting Issues
  • Fixing defects and adding improvements
• License

Documentation

Entity-Component-System (ECS) is a distributed and compositional architectural design pattern that is mostly used in game development. It enables flexible decoupling of domain-specific behaviour, which overcomes many of the drawbacks of traditional object-oriented inheritance.

For further details:

• Entity Systems Wiki
• Evolve Your Hierarchy
• ECS on Wikipedia
• Entity Component Systems in Elixir

World

A ECS instance is used to describe you game world or Entity System if you will. The World is a container for Entities, Components, and Systems.

import ECS from "ecs-lib";

const world = new ECS();

Component

Represents the different facets of an entity, such as position, velocity, geometry, physics, and hit points for example. Components store only raw data for one aspect of the object, and how it interacts with the world.

In other words, the component labels the entity as having this particular aspect.

import {Component} from "ecs-lib";

export type Box = {
    width: number;
    height: number;
    depth: number;
}

export const BoxComponent = Component.register<Box>();

The register method generates a new class that represents this type of component, which has a unique identifier. You also have access to the type id from the created instances.

const boxCmp = new BoxComponent({ width:10, height:10, depth:10 });

// prints true, in this case type = 1
console.log(BoxComponent.type === boxCmp.type);

You can also have access to the Component class from ECS (ECS.Component.register)

Raw data access

Component instance displays raw data by property data

boxCmp.data.width = 33;
console.log(boxCmp.data.width);

Secondary attributes

A component can have attributes. Attributes are secondary values used to save miscellaneous data required by some specialized systems.

boxCmp.attr.specializedSystemMetadata = 33;
console.log(boxCmp.attr.specializedSystemMetadata);

Entity

The entity is a general purpose object. An entity is what you use to describe an object in your game. e.g. a player, a gun, etc. It consists only of a unique ID and the list of components that make up this entity.

import {Entity} from "ecs-lib";
import {ColorComponent} from "../component/ColorComponent";
import {Box, BoxComponent} from "../component/BoxComponent";

export default class CubeEntity extends Entity {

    constructor(cube: Box, color: string) {
        super();

        this.add(new BoxComponent(cube));
        this.add(new ColorComponent(color));
    }
}

You can also have access to the Entity class from ECS (ECS.Entity)

Adding and removing from the world

You can add multiple instances of the same entity in the world. Each entity is given a unique identifier at creation time.

const cubeEnt = new CubeEntity({
    width: 10,
    height: 10,
    depth: 10
}, '#FF0000');

console.log(cubeEnt, cubeEnt.id);

world.addEntity(cubeEnt);

world.removeEntity(cubeEnt);
world.removeEntity(cubeEnt.id);

Adding and removing components

At any point in the entity's life cycle, you can add or remove components, using add and remove methods.

cubeEnt.add(boxCmp);
cubeEnt.remove(boxCmp);

ecs-lib entities can have more than one component per type, it is up to the programmer to control the addition and removal of entity components.

cubeEnt.add(new BoxComponent({ width:10, height:10, depth:10 }));
cubeEnt.add(new BoxComponent({ width:20, height:20, depth:20 }));

Subscribing to changes

In ecs-lib you can be informed when a component is added or removed from an entity by simply subscribing to the entity.

To unsubscribe, simply invoke the function you received at the time of subscription.

const cancel = cubeEnt.subscribe((entity)=>{
    console.log(entity === cubeEnt);
});

cancel();

Accessing components

To gain access to the components of an entity, simply use the allFrom and oneFrom methods of the Component class to get all or the first instance of this component respectively.

BoxComponent.allFrom(cubeEnt)
    .forEach((boxCmp)=>{
        console.log(boxCmp.data.height);
    });

const boxCmp = BoxComponent.oneFrom(cubeEnt);
console.log(boxCmp.data.height);

System

Represents the logic that transforms component data of an entity from its current state to its next state. A system runs on entities that have a specific set of component types.

Each system runs continuously (as if each system had its own thread).

In ecs-lib, a system has a strong connection with component types. You must define which components this system works on in the System abstract class constructor.

If the update method is implemented, it will be invoked for every update in the world. Whenever an entity with the characteristics expected by this system is added or removed on world, or it components is changed, the system is informed via the enter, change and exit methods.

import {Entity, System} from "ecs-lib";
import KeyboardState from "../utils/KeyboardState";
import {BoxComponent} from "../component/BoxComponent";
import {Object3DComponent} from "../component/Object3DComponent";

export default class KeyboardSystem extends System {

    constructor() {
        super([
            Object3DComponent.type,
            BoxComponent.type
        ]);
    }

    update(time: number, delta: number, entity: Entity): void {
        let object3D = Object3DComponent.oneFrom(entity).data;
        if (KeyboardState.pressed("right")) {
            object3D.translateX(0.3);
        } else if (KeyboardState.pressed("left")) {
            object3D.translateX(-0.3);
        } else if (KeyboardState.pressed("up")) {
            object3D.translateZ(-0.3);
        } else if (KeyboardState.pressed("down")) {
            object3D.translateZ(0.3);
        }
    }
}

Adding and removing from the world

To add or remove a system to the world, simply use the addSystem and removeSystem methods.

const keyboardSys = new KeyboardSystem();
world.addSystem(keyboardSys);
world.removeSystem(keyboardSys);

Limiting frequency (FPS)

It is possible to limit the maximum number of invocations that the update method can perform per second (FPS) by simply entering the frequency parameter in the class constructor. This control is useful for example to limit the processing of physics systems to a specific frequency in order to decrease the processing cost.

export default class PhysicsSystem extends System {

    constructor() {
        super([
            Object3DComponent.type,
            VelocityComponent.type,
            PositionComponent.type,
            DirectionComponent.type
        ], 25);  // <- LIMIT FPS
    }

    // Will run at 25 FPS
    update(time: number, delta: number, entity: Entity): void {
       //... physics stuff
    }
}

Time Scaling - Slow motion effect

A very interesting feature in ecs-lib is the TIMESCALE. This allows you to change the rate that time passes in the game, also known as the timescale. You can set the timescale by changing the timeScale property of the world.

A time scale of 1 means normal speed. 0.5 means half the speed and 2.0 means twice the speed. If you set the game's timescale to 0.1, it will be ten times slower but still smooth - a good slow motion effect!

The timescale works by changing the value returned in the time and delta properties of the system update method. This means that the behaviors are affected and any movement using delta. If you do not use delta in your motion calculations, motion will not be affected by the timescale! Therefore, to use the timescale, simply use the delta correctly in all movements.

ATTENTION! The systems continue to be invoked obeying their normal frequencies, what changes is only the values received in the time and delta parameters.

export default class PhysicsSystem extends System {

    constructor() {
        super([
            Object3DComponent.type,
            VelocityComponent.type
        ], 25);
    }

    update(time: number, delta: number, entity: Entity): void {
        let object = Object3DComponent.oneFrom(entity).data;
        let velocity = VelocityComponent.oneFrom(entity).data;
        object.position.y += velocity.y * delta;   
    }
}

world.timeScale = 1; // Normal speed
world.timeScale = 0.5; // Slow motion

Pausing

You can set the timescale to 0. This stops all movement. It is an easy way to pause the game. Go back to 1 and the game will resume.

You may find that you can still do things like shoot using the game controls. You can get around this by placing your main game events in a group and activating / deactivating that group while pausing and not pausing.

It's also a good way to test if you used delta correctly. If you used it correctly, setting the timescale to 0 will stop everything in the game. If you have not used it correctly, some objects may keep moving even if the game should be paused! In this case, you can check how these objects are moved and make sure you are using delta correctly.

Global systems - all entities

You can also create systems that receive updates from all entities, regardless of existing components. To do this, simply enter [-1] in the system builder. This functionality may be useful for debugging and other rating mechanisms for your game.

import {Entity, System} from "ecs-lib";

export default class LogSystem extends System {

    constructor() {
        super([-1], 0.5); // Logs all entities every 2 seconds (0.5 FPS)
    }

    update(time: number, delta: number, entity: Entity): void {
        console.log(entity);
    }
}

Before and After update

If necessary, the system can be informed before and after executing the update of its entities in this interaction (respecting the execution frequency defined for that system).

import {Entity, System} from "ecs-lib";

export default class LogSystem extends System {

    constructor() {
        super([-1], 0.5); // Logs all entities every 2 seconds (0.5 FPS)
    }

    beforeUpdateAll(time: number): void {
        console.log('Before update');
    }
    
    update(time: number, delta: number, entity: Entity): void {
        console.log(entity);
    }
    
    afterUpdateAll(time: number, entities: Entity[]): void {
        console.log('After update');
    }
}

Enter - When adding new entities

Invoked when:

1. An entity with the characteristics (components) expected by this system is added in the world;
2. This system is added in the world and this world has one or more entities with the characteristics expected by this system;
3. An existing entity in the same world receives a new component at runtime and all of its new components match the standard expected by this system.

It can be used for initialization of new components in this entity, or even registration of this entity in a more complex management system.

import {Entity, System} from "ecs-lib";
import {BoxGeometry, Mesh, MeshBasicMaterial} from "three";
import {BoxComponent} from "../component/BoxComponent";
import {ColorComponent} from "../component/ColorComponent";
import {Object3DComponent} from "../component/Object3DComponent";

export default class CubeFactorySystem extends System {

    constructor() {
        super([
            ColorComponent.type,
            BoxComponent.type
        ]);
    }
    
    enter(entity: Entity): void {
        let object = Object3DComponent.oneFrom(entity);
        if (!object) {
            const box = BoxComponent.oneFrom(entity).data;
            const color = ColorComponent.oneFrom(entity).data;

            const geometry = new BoxGeometry(box.width, box.height, box.depth);
            const material = new MeshBasicMaterial({color: color});
            const cube = new Mesh(geometry, material);

            // Append new component to entity
            entity.add(new Object3DComponent(cube));
        }
    }
}

Change - When you add or remove components

A system can also be informed when adding or removing components of an entity by simply implementing the "change" method.

import {Entity, System, Component} from "ecs-lib";

export default class LogSystem extends System {

    constructor() {
        super([-1], 0.5); // Logs all entities every 2 seconds (0.5 FPS)
    }

    change(entity: Entity, added?: Component<any>, removed?: Component<any>): void {
        console.log(entity, added, removed);
    }
}

Exit - When removing entities

Invoked when:

1. An entity with the characteristics (components) expected by this system is removed from the world;
2. This system is removed from the world and this world has one or more entities with the characteristics expected by this system;
3. An existing entity in the same world loses a component at runtime and its new component set no longer matches the standard expected by this system;

Can be used to clean memory and references.

import {Scene} from "three";
import {Entity, System} from "ecs-lib";
import {Object3DComponent} from "../component/Object3DComponent";

export default class SceneObjectSystem extends System {

   private scene: Scene;

   constructor(scene: Scene) {
       super([
           Object3DComponent.type
       ]);

       this.scene = scene;
   }

   exit(entity: Entity): void {
       let model = Object3DComponent.oneFrom(entity);
       this.scene.remove(model.data);
   }
}

API

Feedback, Requests and Roadmap

Please use GitHub issues for feedback, questions or comments.

If you have specific feature requests or would like to vote on what others are recommending, please go to the GitHub issues section as well. I would love to see what you are thinking.

Contributing

You can contribute in many ways to this project.

Translating and documenting

I'm not a native speaker of the English language, so you may have noticed a lot of grammar errors in this documentation.

You can FORK this project and suggest improvements to this document (https://github.com/nidorx/ecs-lib/edit/master/README.md).

If you find it more convenient, report a issue with the details on GitHub issues.

Reporting Issues

If you have encountered a problem with this component please file a defect on GitHub issues.

Describe as much detail as possible to get the problem reproduced and eventually corrected.

Fixing defects and adding improvements

1. Fork it (https://github.com/nidorx/ecs-lib/fork)
2. Commit your changes (git commit -am 'Add some fooBar')
3. Push to your master branch (git push)
4. Create a new Pull Request

License

This code is distributed under the terms and conditions of the MIT license.