index.js

var inherits = require('inherits')
var EventEmitter = require('events').EventEmitter
var now = require('right-now')
var raf = require('raf')

module.exports = Engine
function Engine(fn) {
    if (!(this instanceof Engine)) 
        return new Engine(fn)
    this.running = false
    this.last = now()
    this._frame = 0
    this._tick = this.tick.bind(this)

    if (fn)
        this.on('tick', fn)
}

inherits(Engine, EventEmitter)

Engine.prototype.start = function() {
    if (this.running) 
        return
    this.running = true
    this.last = now()
    this._frame = raf(this._tick)
    return this
}

Engine.prototype.stop = function() {
    this.running = false
    if (this._frame !== 0)
        raf.cancel(this._frame)
    this._frame = 0
    return this
}

Engine.prototype.tick = function() {
    this._frame = raf(this._tick)
    var time = now()
    var dt = time - this.last
    this.emit('tick', dt)
    this.last = time
}