recs

functional entity-component-system experiment

recs allows for the definitions of components, systems, and message handlers on entities in a way that reduces shared state and promotes isolation.

To read more about the ECS pattern, take a look at nano-ecs, the underlying module that powers recs. It's a really useful pattern for writing the core logic in video games.

Usage

var recs = require('recs')()

function GravityWell () {
  this.power = 1000
}

function Body () {
  this.x = 0
  this.y = 0
  this.xv = 0
  this.yv = 0
}

var Blackhole = [Body, GravityWell]
var Ship = [Body]

recs.system(Ship, function (d) {
  d.body.x += d.body.xv
  d.body.y += d.body.yv

  console.log('Physics', d.body.x, d.body.y)
})

recs.system(Blackhole, Ship, function (g, d) {
  var dx = g.body.x - d.body.x
  var dy = g.body.y - d.body.y
  var len = Math.sqrt(dx*dx + dy*dy) + 0.0001
  var force = g.gravityWell.power / (len * len)
  d.body.xv += force * dx / len
  d.body.yv += force * dy / len

  console.log('Suction')
})

recs.entity(Blackhole, function (e) {
  e.body.x = 30
  e.body.y = 0

  console.log('blackhole init')
})

recs.entity(Ship, function (e) {
  e.body.x = 0
  e.body.y = 0
  e.body.xv = 0
  e.body.yv = 0

  console.log('ship init')
})

recs.tick()
recs.tick()
recs.tick()

outputs

blackhole init
ship init
Physics 30 0
Physics 0 0
Suction
Physics 30 0
Physics 1.1111000000740736 0
Suction
Physics 30 0
Physics 3.4204114875106733 0
Suction

You can see a similar graphical demo in action, too.

Core Ideas

Components

Components are any named Javascript functions. For example:

function Health () {
  this.maxHp = 100
  this.hp = this.maxHp
}

Components define data and not behaviour.

Entities

Entities are a collection of component instances. On their own, they have no data and no functionality. They are created using recs.entity():

recs.entity([Physics, Health, Sprite], function init (e) {
  e.physics.velocity = [10, -4]
  e.health.maxHp = 500
  e.sprite.image = imageFromFile('ship.png')
})

Systems

Systems are stateless functions that operate on all entities that have at least the system's required components:

recs.system([Physics], function process (e) {
  e.physics.velocity[0] += e.physics.position[0]
  e.physics.velocity[1] += e.physics.position[1]
})

Messages

Messages can be published, in an EventEmitter-style, on a specific entity. Message receivers specify what components they require in order to be received: all applicable receivers are fired.

recs.recv([Health], 'damage', function (e, amount) {
  e.health.hp -= amount
})

recs.recv([Human], 'damage', function (e) {
  console.log('ouch!')
})

recs.system([Health, Human], function onFire (e) {
  e.send('damage', 25)
})

recs.system([Health, Cyborg], function onFire (e) {
  e.send('damage', 1)
})

API

var RECS = require('recs')
var recs = RECS()

// or

var recs = require('recs')()

recs.system([name,] components, func)

Defines a system that runs on all entities that have components. Runs the function func with an entity as its sole parameter.

recs.system([name,] componentsA, componentsB, func)

Defines a system that runs on the cartesian product of all entities that have componentsA in one group, and all entities with componentsB in the other group. Runs the function func with signature function (entityA, entityB) {}.

These systems are particularly useful when two types of entities need to interact regularly, like collision detection:

recs.system([Player, BoundingBox], [Projectile, BoundingBox], function (plr, proj) {
  if (collides(plr.boundingBox, proj.boundingBox)) {
    plr.send('collision', plr)
    plr.send('collision', proj)
  }
})

recs.recv(components, msg, cb)

Registers a message receiver, which fires if a message is sent to an entity that has the listed components and the message name msg. cb is called with an entity as its first parameter; any other params passed on send are included as subsequent arguments.

recs.recv([Balloon], 'burst', function (e, adverb) {
  console.log('the balloon bursts ' + adverb)
})

recs.system([Balloon], function (e) {
  e.balloon.lifetime--

  if (e.balloon.lifetime === 0) {
    e.send('burst', 'loudly')
  }
})

recs.entity([name,] components[, cb])

Creates a new entity with components components. cb is called with a reference to the brand new entity for any initialization you'd like to do.

recs.entity([Balloon], function (e) {
  e.balloon.lifetime = 16
})

recs.tick()

Runs all systems on all applicable entities exactly once. Any arguments passed into tick() will be available as additional parameters to all systems called.

This is useful for providing systems with e.g. a time delta.

Install

With npm installed, run

$ npm install recs

Acknowledgments

recs was largely inspired by the regl project.

License

ISC