prototype.js 1.4 原代码阅读

      Ajax.Responders.dispatch('onCreate', this, this.transport);

   /**
      * 此处好像强制使用了异步方式，而不是依照 this.options.asynchronous 的值
      */
      this.transport.open(this.options.method, this.url,
        this.options.asynchronous);

   /**
      * 这里提供了 XmlHttp 传输过程中每个步骤的回调函数
      */
      if (this.options.asynchronous) {
        this.transport.onreadystatechange = this.onStateChange.bind(this);
        setTimeout((function() {this.respondToReadyState(1)}).bind(this), 10);
      }

      this.setRequestHeaders();

      var body = this.options.postBody ? this.options.postBody : parameters;
      this.transport.send(this.options.method == 'post' ? body : null);

    } catch (e) {
      this.dispatchException(e);
    }
  },

  setRequestHeaders: function() {
    var requestHeaders =
      ['X-Requested-With', 'XMLHttpRequest',
       'X-Prototype-Version', Prototype.Version,
       'Accept', 'text/javascript, text/html, application/xml, text/xml, */*'];

    if (this.options.method == 'post') {
      requestHeaders.push('Content-type', this.options.contentType);

      if (this.transport.overrideMimeType)
        requestHeaders.push('Connection', 'close');
    }

    if (this.options.requestHeaders)
      requestHeaders.push.apply(requestHeaders, this.options.requestHeaders);

    for (var i = 0; i < requestHeaders.length; i += 2)
      this.transport.setRequestHeader(requestHeaders[i], requestHeaders[i+1]);
  },

  onStateChange: function() {
    var readyState = this.transport.readyState;
 /**
    * 如果不是 Loading 状态，就调用回调函数
    */
    if (readyState != 1)
      this.respondToReadyState(this.transport.readyState);
  },

  header: function(name) {
    try {
      return this.transport.getResponseHeader(name);
    } catch (e) {}
  },

  evalJSON: function() {
    try {
      return eval('(' + this.header('X-JSON') + ')');
    } catch (e) {}
  },