(wee) Entity Component System

HEADS UP: Documentation is out of date for v8, these docs are for <8.0.0

A tiny Entity Component System for Javascript

Features

• Super small & simple
• Flexible - Use ECS or Observer pattern to interact with entities, any class can be a component.
• Performant - Doesn't re-implement a GC, entities are deleted when they have no components and fall out of scope, avoids unnecessary iteration.

• (wee) Entity Component System
  • Features
  • Installation
  • Quick Start
  • API
    • World
      • constructor(config?: Config)
      • addComponent<T>(entity: Entity, Component: Component<T>, ...args: any[])
      • addComponents(entity: Entity, components: [Component<unknown>, ...any[]][])
      • createEntity(components: [Component<unknown>, ...any[]][]): Entity
      • query(components: Component<unknown>[], persist?: Boolean): Entity[]
      • register(system: Function, components: Component<unknown>[])
      • removeComponent<T>(entity: Entity, component: Component<T>)
      • removeComponents(entity: Entity, components: Component<unknown>[])
      • async run(...args: any[]): Promise<void>
      • subscribe(components: Component<unknown>[], callback: QueryCallback, emit?: boolean): Function
      • makeSubscription(components: IComponent<unknown>[], emit?: boolean): (cb: QueryCallback) => () => void
      • unsubscribe(components: Component<unknown>[], callback: QueryCallback)
      • updateComponent<T>(entity: Entity, Component, update: any | ComponentUpdater<T>):
    • Component<T>
    • ID
    • getID(entity: Entity): string | number
    • getComponent<T>(entity: Entity, Component: Component<T>): T
    • hasComponent<T>(entity: Entity, components: Component<T>)
    • hasComponents(entity: Entity, components: Component<unknown>[])

Installation

yarn add wecs

Quick Start

For a more complete example, see the examples which don't exist yet

import { World, ID, Component, getComponent } from 'wecs'

// instantiate the world
const world = new World()

// create a component
class Movable {} // Functions as a tag
class Velocity extends Component<{ val: number }> { } // Use a shorthand generic base class

class Position { // use your own class
  val: number
  constructor(pos) {
    this.val = pos
  }
}

// create a system that is called with entities every time world.run is called
function System(entities) {
  entities.forEach(e => {
    world.updateComponent(e, Position, c => {
      c.val += getComponent(e, Velocity).val
    })
  })
}

// register the System to receive entities with both position and velocity components
world.register(
  System,
  [
    Movable,
    Position,
    Velocity
  ]
)

// create an entity, this one can move, has a position, and a velocity
world.createEntity([
  [ID, { id: 'foo' }],
  [Movable],
  [Position, 0],
  [Velocity, { val: 2 }]
])

// execute all systems in parallel
world.run()

API

World

World is the default export class containing entities

constructor(config?: Config)

You can construct World with the following config shape. All properties are optional.

Key	Type	Description
parallel	boolean	Run all systems in parallel
id	() => string	Function used to generate entity IDs, defaults to a fast and good enough emplementation, but cuid would be good too if you prefer.
onBefore	(...args: any[]) => Promise<void>	A function called with an optional message before all systems are run.`
onAfter	(...args: any[]) => Promise<void>	A function called with an optional message after all systems are run.`

addComponent<T>(entity: Entity, Component: Component<T>, ...args: any[])

Add a single component to a given entity

class Position {
  constructor(number, bar) {
    console.assert(number === 0)
    console.assert(foo === 'foo')
  }
}

world.addComponent(e, Position, 0, 'foo')

addComponents(entity: Entity, components: [Component<unknown>, ...any[]][])

Adds components to a given entity

world.addComponents(
  e,
  [
    [Position, 0, 'foo'],
    [Velocity, 2, 'bar']
  ]
)

createEntity(components: [Component<unknown>, ...any[]][]): Entity

Creates an entity with the provided Components and values.

class Velocity {
  constructor(number, bar) {
    console.assert(number === 2)
    console.assert(foo === 'bar')
  }
}

const entity = world.createEntity([
  [Position, 0, 'foo'],
  [Velocity, 2, 'bar']
])

query(components: Component<unknown>[], persist?: Boolean): Entity[]

Query all entities that meet the component criteria, optionally saving the query for faster retrieval later.

const entities = world.query([Position, Velocity], true)

register(system: Function, components: Component<unknown>[])

Register a system function to be called with entities that meet all of the component criteria:

class Component {
  constructor(counter) {
    this.foo = 'bar'
  }
}

function System(entities) {
  entities.forEach(e => world.updateComponent(e, Component, c => {
    c.foo = 'baz'
  })
}

world.register(System, [Component, OtherComponent, ThirdComponent])

removeComponent<T>(entity: Entity, component: Component<T>)

Removes a single component from a given entity

world.removeComponent(
  e,
  Position
)

removeComponents(entity: Entity, components: Component<unknown>[])

Removes components from a given entity

world.removeComponents(
  e,
  [Position, Velocity]
)

async run(...args: any[]): Promise<void>

Executes all the registered systems. If args are present (typically a ms delta of now minus last run, but can be whatever arguments you want), systems will be called with system(...args, entities) as opposed to just system(entities).

function System(delta, time, entities) {
  // do stuff
}

function run() {
  // Compute delta and elapsed time
  var time = performance.now()
  var delta = time - lastTime

  // Run all the systems
  world.run(delta, time)

  lastTime = time
  requestAnimationFrame(run)
}

var lastTime = performance.now()
run()

Note: Dynamic length arguments passed to .run at runtime will likely fuxx with your program so try to stick to one function signature for all of your system functions. For example, the following is bad:

// entities won't always be what you want it to be!
function System(entities) {}
function SystemTwo(delta, entities) {}

// this sucks don't do this
if(foo) world.run(delta)
else world.run()

subscribe(components: Component<unknown>[], callback: QueryCallback, emit?: boolean): Function

Subscribe to updates with a callback function that gets executed when:

• a new entity meeting the component criteria gets created
• an entity gets a new component that meets the criteria
• an entity has a component removed that makes it no longer meet the criteria
• an entity's component is updated via world.updateComponent

The third emit argument, when true, will immediately call the callback with relevant entities.

This method will also return a function you can use to unsubscribe.

const unsubscribe = world.subscribe(
  [Position, Velocity],
  (entities) => console.log(entities),
  true
)

unsubscribe()

makeSubscription(components: IComponent<unknown>[], emit?: boolean): (cb: QueryCallback) => () => void

Instead of taking a callback, create a factory function that can be used to create subscriptions. Returns a function that expects a single callback function as an argument that plays nicely with other reactive frameworks.

//Kefir/Rx
var stream = Kefir.fromCallback(world.makeSubscription([Position, Velocity], true));

// svelte
function useEntities(components) {
  return {
    subscribe: world.makeSubscription(components, true)
  }
}

unsubscribe(components: Component<unknown>[], callback: QueryCallback)

Another way to unsubscribe, handy for rxjs

import { fromEventPattern } from 'rxjs';
 
const addHandler = (components) => (handler) => {
  world.subscribe(components, handler, true)
}
 
const removeHandler = (components) => (handler) => {
  world.unsubscribe(components, handler)
}
 
const entities = fromEventPattern(
  addHandler([Position, Velocity]),
  removeHandler([Position, Velocity])
)

entities.subscribe(entities => console.log(entities))

updateComponent<T>(entity: Entity, Component, update: any | ComponentUpdater<T>):

Takes an entity, a component, and a either callback function that is called with value of the entity's component, or a new value for the component.

If the callback returns a value, the entity's component will be set to that value.

Afterwards, trigger all relevant subscriptions

class Component {
  constructor(multiplier = 2) {
    this.values = [1, 2, 3].map(v => v * multiplier)
  }
}

world.subscribe([Component], (entities) => {
  // this will be called three times
})

const entity = world.createEntity([
  [Component, 1]
])

world.updateComponent(entity, Component, c => {
  // mutate the component directly
  c.values = []
}

world.updateComponent(e, Component, c => {
    // reset the value
  return new Component(3)
})

// reset the value
world.updateComponent(e, Component, new Component(3))

Component<T>

The exported Component class is a generic class that expects to be constructed with an object of shape T, and assigns properties of the object T to its context.

Component is mainly a convenient shorthand to build typed, easily serializable components.

import { Component } from 'wecs'

class Position extends Component<{ x: number, y: number }> { }

const p = new Position({ x: 0, y: 0 })
console.assert(p.value.x === 0)
console.assert(p.value.y === 0)

ID

It's common practice when using ECS to uniquely identify your entities. The exported ID class extends Component and has elevated component privileges. Every entity is given an ID component, and ID components can not be removed from entities, but ID components can be updated.

// Entities created without an ID component will use a short randomly generated id
import { ID } from 'wecs'
const entity = world.createEntity([Component])

// Creating an entity with a custom ID component is fine too
const entity = world.createEntity([
  [ID, 'foo']
])
getID(entity) === 'foo'

getID(entity: Entity): string | number

Retrieves a given entity's unique identifier from its ID component.

const entity = world.createEntity([
  [ID, 'foo']
])
getID(entity) === 'foo'

getComponent<T>(entity: Entity, Component: Component<T>): T

Given an entity, gets the given component or null if the entity doesn't have it.

import { getComponent } from 'wecs'

const entity = world.createEntity([
  [Position, 0],
  [Velocity, 2]
])

const position = getComponent(entity, Position)

hasComponent<T>(entity: Entity, components: Component<T>)

Given an entity, gets the given component or null if the entity doesn't have it.

import { hasComponent } from 'wecs'

const entity = world.createEntity([
  [Position, 0],
  [Velocity, 2]
])

console.assert(hasComponent(entity, Position))

hasComponents(entity: Entity, components: Component<unknown>[])

Given an entity, gets the given component or null if the entity doesn't have it.

import { hasComponents } from 'wecs'

const entity = world.createEntity([
  [Position, 0],
  [Velocity, 2]
])

console.assert(hasComponents(entity, [Position, Velocity]))