code: function setBeforeRouteChangeHandler(handler) {
              if (null == handler) {
                  return _beforeRouteChange;
              }
              else {
                  var oldHandler = _beforeRouteChange;
                  _beforeRouteChange = handler;
                  return oldHandler;
              }
          }
docs: @en Setup "before route change" handler.
  @ja "before route change" ハンドラ設定
  
  @internal
  
  @param
  - `en` handler function.
  - `ja` handler 指定.
  @returns
  - `en` old handler function.
  - `ja` 以前の handler.