code: function(dt){
              var space = this.space;
              dt = dt > 32?16:dt;
  
              if(space.needReindexStatic){
                  space.reindexStatic();
                  space.needReindexStatic = false;
              }
              space.step(dt * .001);
  
              //delete bodies and shapes
              for(var i = this._deleteBodies.length - 1;i >= 0;i --){
                  var body = this._deleteBodies[i];
                  var shapeList = body.shapeList;
                  for(var j = shapeList.length - 1;j >= 0;j --){
                      space.removeShape(shapeList[j]);
                  }
                  space.removeBody(body);
              }
  
              for(var i = this._deleteShapes.length - 1;i >= 0;i --){
                  var shape = this._deleteShapes[i];
                  space.removeShape(shape);
              }
  
              this._deleteBodies.length = 0;
              this._deleteShapes.length = 0;
          }
docs: tick方法，供Hilo.Ticker调用
  @private
  @param  {Number} dt 间隔