index-ecs

Entity-Component-System for Node.js

Motivation

Entity-Component-System (ECS) is a software architecture pattern that originated in the game industry. It emphasizes separation of concerns to make software more maintainable for software engineers, and more flexible for game designers.

Object-Oriented Programming (OOP) has been the gold standard in software architecture for a long time. In complicated and creative endeavors like games, software engineers noticed that OOP class hierarchies became difficult to modify the larger and more coupled they grew. Moreover, game designers often needed code changes in order to modify the game content.

The ECS pattern emphasizes separation of concerns by organizing things into three distinct areas:

• Component - A set of data that defines an aspect / attribute / facet of existence. Things that exist somewhere might have a "position" component with data "x", "y", and "z" components. Things that have health and can be damaged and/or killed might have a "health" component with a data element "hit-points". Components do not contain code, they are simply a type-named collection of related data.

• Entity - An identified collection of Components; typically any object in a game. These could be a wall, a rock, a tree, a power-up, the player, the enemies, the weather, etc. Entities contain no code, they have some ID value, and reference a collection of Components. All of the entities that exist are said to live in a World.

• System - Code that operates on Entities and Components. All of the application code lives here, and uses the existence of components to decide which entities to operate upon. For example, a Mover system might look for all entities with "position" and "velocity" components and then update the positions according to the velocities. A System operates only on the entities that contain relevant Components, and ignores the others.

Usage

This library provides support for the Entity and Component aspects of an ECS implementation. A System is simply code, and it is assumed that developers know how to write and organize functions for themselves.

index-ecs provides one class: World

var World = require('index-ecs').World;

This component World provides several methods. The purpose of these methods are summarized below. API details along with examples are covered in the section to follow.

• addComponent - Add a Component to an Entity
• createEntity - Create a new Entity and add it to the World
• find - Find Entities by the Component(s) they contain
• findAll - Find ALL Entities contained in the World
• findById - Find a specific Entity by its UUID
• loadEntity - Import an existing object as an Entity in the World
• remove - Remove ALL Entities from the World
• removeComponent - Remove a Component from an Entity
• removeEntity - Remove an Entity from the World
• size - Determine how many Entities exist in the World

API: World

addComponent(entity, component, data) -> entity

Add a component to an entity, optionally providing data with which to populate the component.

• entity Object: The entity to which to add the component
• component String: The name of the component to add
• data Object [Optional]: The data with which to populate the component

The provided entity is returned by the call.

Example:

var world = new World();
var dog = world.createEntity();
// this poor doggie is lost
world.addComponent(dog, "position");
// dog.position = {}

Example 2:

var world = new World();
var dog = world.createEntity();
world.addComponent(dog, "position", {
  x: -120,
  y: 50,
  z: 2
});
// dog.position = { x:-120, y:50, z:2 }

createEntity(id) -> entity

Create a new entity, and add that entity to the world.

• id String (UUID v4) [Optional]: The ID to be used by the Entity.

The id value is optional, and supplying it is intended only for very advanced use-cases. It is recommended that you allow the World to generate an ID for the entity.

The created entity is returned by the call.

Example:

var world = new World();
var dog = world.createEntity();
// dog.uuid = "<some generated UUID v4 value>"

find(components) -> [entity]

Find entities that contain the all of the provided components.

• components String or [String]: The components an entity must contain

An array of [Entity] (possibly empty) is returned by the call. All of the entities contained in the array will have all of components specified in the call to find.

Example:

// some other code has added entities and game has been going for awhile
var world = new World();

// this is a Mover system that uses two components: position and velocity
//
// position:
//   x: the current x position of the entity
//   y: the current y position of the entity
//
// velocity:
//   dx: delta-x, how fast the x position of the entity is changing
//   dy: delta-y, how fast the y position of the entity is changing

var i, len, movingThing, movingThings, pos, vel;

// let's find all of the things that need to move
movingThings = world.find(["position", "velocity"]);

// now let's update the "position" component of all of them
for (i = 0, len = movingThings.length; i < len; i++) {
  movingThing = movingThings[i];
  pos = movingThing.position;
  vel = movingThing.velocity;
  pos.x = pos.x + vel.dx;
  pos.y = pos.y + vel.dy;
}

// all moving entities have had their positions updated
// now we might run a system to check for collisions?

findAll() -> [entity]

Find all of the entities contained in the World.

An array of [Entity] (possibly empty) is returned by the call. The entities contained in the array have no specific components.

Example:

var world = new World();
var everything = world.findAll();
// everything = [] // we haven't added any entities yet!

This method is intended for advanced use-cases only; perhaps debugging, logging, metrics, monitoring, serialization, etc. Note that it is an anti-pattern to obtain all of the entities and iterate over each one looking for specific components. Use the find() method instead.

findById(id) -> entity

Find the identified entity.

• id UUID v4: The UUID that identifies the entity

An Entity or undefined is returned by the call. If an entity with the provided UUID is found, it will be returned. Otherwise the call will return undefined to indicate that the entity does not exist.

Example:

var world = new World();
var dog = world.createEntity();
var dogTag = dog.uuid;
// ... in some other part of the code
var myDog = world.findById(dogTag);
// myDog === dog

loadEntity(object) -> entity

Import an existing object as an Entity in the World.

• object: An object to import as an Entity

This method converts an existing object into an Entity in the World. The components are a shallow clone (copied references) of the original object. Modification of the original object is not recommended after calling loadEntity.

If the provided object has a uuid property, that UUID will be used as the identifier of the Entity in the World. If the provided object does not contain a uuid property, the World will provide one for the newly created Entity.

The Entity created by the world to clone the provided object will be returned from the call.

This method is intended for use by serialization frameworks rather than applications, so no example is provided.

remove() -> World

Remove all entities from the World.

The world object is returned from this call.

Example:

var world = new World();
world.remove();

This method is pretty extreme and intended for advanced use-cases only. A faster way to obtain an empty World object would simply be to call the constructor and make a fresh one.

removeComponent(entity, component) -> entity

Remove a component from an entity.

• entity Object: The entity from which to remove the component
• component String: The name of the component to be removed

The provided entity is returned by the call.

Example:

var world = new World();
var dog = world.createEntity();
world.addComponent(dog, "breed", {
  type: "Corgi",
  color: "tuxedo"
});
// dog.breed = { type: "Corgi", color: "tuxedo" }
world.removeComponent(dog, "breed");
// dog.breed = undefined

removeEntity(entity) -> World

Removes an entity from the World.

• entity Object: The entity to remove from the World.

The world object is returned by the call.

Example:

var world = new World();
var dog = world.createEntity();
world.addComponent(dog, "breed", {
  type: "Corgi",
  color: "tuxedo"
});
// dog.breed = { type: "Corgi", color: "tuxedo" }
world.removeEntity(dog);
// gone to doggie heaven

size() -> number

Determine how many entities exist in the World.

The number returned is the number of entities in the world.

Example:

var world = new World();
var dog = world.createEntity();
var count = world.size();
// count = 1

Events: World

For advanced use-cases, one can listen on a World object for events involving Entities and Components. World is an EventEmitter object.

component-added -> (entity, component)

Fired when a Component is added to an Entity.

• entity Object: The entity to which a Component was added.
• component String: The name of the Component added to the Entity

Example:

var world = new World();
world.on("component-added", function(entity, component) {
  if (component === "breed") {
    return console.log("Breed is: " + entity.breed.type);
  }
});
var dog = world.createEntity();
world.addComponent(dog, "breed", {
  type: "Corgi",
  color: "tuxedo"
});
// Breed is: Corgi

component-removed -> (entity, component)

Fired when a Component is removed from an Entity.

• entity Object: The entity from which a Component was removed
• component String: The name of the Component removed from the Entity

Example:

var world = new World();
world.on("component-removed", function(entity, component) {
  if (component === "breed") {
    return console.log("Breed is: " + entity.breed.type);
  }
});
var dog = world.createEntity();
world.addComponent(dog, "breed", {
  type: "Corgi",
  color: "tuxedo"
});
world.removeComponent(dog, "breed");
// Breed is: Corgi

entity-created -> (entity)

Fired when an entity is created in the world.

• entity Object: The entity which is newly created

Example:

var world = new World();
world.on("entity-created", function(entity) {
  return console.log("ID is: " + entity.uuid);
});
var dog = world.createEntity();
// ID is: <some generated UUID v4 value>

entity-removed -> (entity)

Fired when an entity is removed from the world.

• entity Object: The entity which is removed from the world

Example:

var world = new World();
world.on("entity-removed", function(entity) {
  return console.log("Goodbye ID " + entity.uuid);
});
var dog = world.createEntity();
// Goodbye ID <some generated UUID v4 value>

Development

In order to make modifications to index-ecs, you'll need to establish a development environment:

git clone https://github.com/blinkdog/node-ecs.git
cd node-ecs
npm install
node_modules/.bin/cake rebuild

Code Coverage

You can see the istanbul coverage report for index-ecs with a task in the cake file:

cake coverage

This task will attempt to open the coverage report in a new tab in Mozilla Firefox. If you use another browser, you'll need to modify the Cakefile to specify your preferred command for viewing the coverage report.

Source files

The source files are located in src/main/coffee.

The test source files are located in src/test/coffee.

License

index-ecs
Copyright 2017 Patrick Meade.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License along with this program. If not, see http://www.gnu.org/licenses/.