code: function step(){
  
          var time;
  
          window.requestAnimationFrame( step );
  
          if (!active){
              return;
          }
  
          time = now();
  
          if (!time){
              return;
          }
  
          ps.emit( 'tick', time );
      }
docs: /*
  step( time )
  - time (Number): The current time
  
  Publish a tick to subscribed callbacks