tween.js

var ease = require('./ease');
var Emitter = require('./emitter');

function extend(obj, src) {
  for (var key in src) {
    if (src.hasOwnProperty(key)) obj[key] = src[key];
  }
  return obj;
}

function Tween(obj) {
  if (!(this instanceof Tween)) return new Tween(obj);
  this._from = obj;
  this.ease('linear');
  this.duration(500);
}

Emitter(Tween.prototype);

Tween.prototype.reset = function(){
  this.isArray = Object.prototype.toString.call(this._from) === '[object Array]';
  this._curr = extend({}, this._from);
  this._done = false;
  this._start = Date.now();
  return this;
};

Tween.prototype.to = function(obj){
  this.reset();
  this._to = obj;
  return this;
};

Tween.prototype.duration = function(ms){
  this._duration = ms;
  return this;
};

Tween.prototype.ease = function(fn){
  fn = 'function' == typeof fn ? fn : ease[fn];
  if (!fn) throw new TypeError('invalid easing function');
  this._ease = fn;
  return this;
};

Tween.prototype.stop = function(){
  this.stopped = true;
  this._done = true;
  this.emit('stop');
  this.emit('end');
  return this;
};

Tween.prototype.step = function(){
  if (this._done) return;

  var duration = this._duration;
  var now = Date.now();
  var delta = now - this._start;
  var done = delta >= duration;

  if (done) {
    this._from = this._to;
    this._update(this._to);
    this._done = true;
    this.emit('end');
    return this;
  }

  var from = this._from;
  var to = this._to;
  var curr = this._curr;
  var fn = this._ease;
  var p = (now - this._start) / duration;
  var n = fn(p);

  if (this.isArray) {
    for (var i = 0; i < from.length; ++i) {
      curr[i] = from[i] + (to[i] - from[i]) * n;
    }

    this._update(curr);
    return this;
  }

  for (var k in from) {
    curr[k] = from[k] + (to[k] - from[k]) * n;
  }

  this._update(curr);
  return this;
};

Tween.prototype.update = function(fn){
  if (0 == arguments.length) return this.step();
  this._update = fn;
  return this;
};

module.exports = Tween;