code: function(state, length) {
          if(state && !this.next.active) {
              this.next.activate();
          } else if(!state && this.next.active) {
              this.next.deactivate();
          }
      }
docs: Method: onNextChange
  Called when the next history stack changes.
  
  Parameters:
  state - {Object} An object representing the state to be restored
  if next is triggered again or null if no next states remain.
  length - {Integer} The number of remaining next states that can
  be restored.