Creates a computed property that automatically increments itself at the specified interval in milliseconds. Useful when used as a dependent property to properties that need to changed as time progresses.

ember.computed.timer.js

/*
  Creates a computed property that automatically increments itself at the
  specified interval in milliseconds. Useful when used as a dependent
  property to properties that need to changed based on time.

  App.PostController = Ember.ObjectController.extend({
    '@timer': Ember.computed.timer(1000),

    sinceCreated: function() {
      return new Date().getTime() - this.get('createdAt').getTime();
    }.property('createdAt', '@timer')
  });
*/

void function() {

var DEFAULT_INTERVAL = 5000; // 5 seconds

var cancel = Em.run.cancel,
    later  = Em.run.later,
    get    = Em.get,
    set    = Em.set;

function Timer(interval) {
  this.interval = (interval || DEFAULT_INTERVAL);
}

Timer.prototype = {
  didTick:   Em.K,
  isActive:  false,
  currentInterval: 0,

  resume: function() {
    if (!this.isActive) {
      this.isActive = true;
      this._tick();
    }
  },

  _tick: function() {
    var nextTick = later(this, function() {
      this._tick();
      this.currentInterval++;
      this.didTick(this.currentInterval);
    }, this.interval);
    this._nextTick = nextTick;
  }
};

Em.computed.timer = function(interval) {
  var timer = new Timer(interval);
  return Em.computed(function(key) {
    var obj = this;
    timer.didTick = function() {
      obj.notifyPropertyChange(key);
    }
    timer.resume();
    return timer.currentInterval;
  });
};

}.call(this);