code: function (len) {
              var result = '';
              if (len) {
                  result = this.source.substr(this.position, len);
              } else {
                  result = this.source.substr(this.position);
              }
              return result;
          }
docs: 从当前位置开始读取指定长度字符。Read text from current position.
  @returns {string}