code: function(){
          this.frameCount++;
          /** @private */
          var now = Date.now();
          this.__lastTime = this.__lastTime || now;
          this.deltaTime = (this.fixedDelta || exports.globalFixedDelta) ? 1000/Math.min(60, this.__fps) : now - this.__lastTime;
          this.elapsedTime += this.deltaTime;
          this.__lastTime = now;
  
          this.emit('update', this.deltaTime, this.elapsedTime, this.frameCount);
          return this;
      }
docs: update the animation loop once
  
  @emit Animitter#update
  @return {Animitter}