docs: function ProgressBar (opts) {
      ProgressBar.superclass.constructor.call(this, opts)
      var size = new geo.Size(272, 32)
      this.contentSize = size
      this.anchorPoint = new geo.Point(0.5, 0.5)
  
      var s
      if (opts.emptyImage) {
          s = new Sprite({file: opts.emptyImage, rect: new geo.Rect(0, 0, size.width, size.height)})
          s.anchorPoint = new geo.Point(0, 0)
          this.emptySprite = s
          this.addChild({child: s})
      }
      if (opts.fullImage) {
          s = new Sprite({file: opts.fullImage, rect: new geo.Rect(0, 0, 0, size.height)})
          s.anchorPoint = new geo.Point(0, 0)
          this.fullSprite = s
          this.addChild({child: s})
      }
  
      events.addPropertyListener(this, 'maxValue', 'change', this.updateImages.bind(this))
      events.addPropertyListener(this, 'value',    'change', this.updateImages.bind(this))
  
      this.updateImages()
  }