code: function() {
      if (_xhr.readyState === 4) {
        var result = Result(_xhr);
        // ready
        __callback_func('ready')(result, _this);
        // success
        if (_xhr.status === 200) __callback_func('success')(result, _this);
        // fail
        else __callback_func('fail')(result, _this);
      }
    }
docs: Result: the function to get `Result` instance. The request result object.
  - xhrObj: the xhr obj which can get the xhr result.
  
  How to use it? if `r` is the instance of `Result`.
  var r_json = r.json();
  var status = r.status_code;
  var headers = r.headers;