oadPanelIFrameExecutor非常简单，只是定义了一些私有变量：

AspNetAjaxExtensions.UpdatePanelIFrameExecutor = function(sourceElement)
{
AspNetAjaxExtensions.UpdatePanelIFrameExecutor.initializeBase(this);

// for properties
this._started = false;
this._responseAvailable = false;
this._timedOut = false;
this._aborted = false;
this._responseData = null;
this._statusCode = null;

// the element initiated the async postback
this._sourceElement = sourceElement;
// the form in the page.
this._form = Sys.WebForms.PageRequestManager.getInstance()._form;
}

　　对于大部分属性来说，它们的实现只不过是对上面这些私有变量进行读取或写入而已，在此不提。而一个Executor最重要的莫过于它的executeRequest方法，一些回调函数，还有过期定时器之类的逻辑：

executeRequest : function()
{
this._addAdditionalHiddenElements();

var onSuccess = Function.createDelegate(this, this._onSuccess);
var onFailure = Function.createDelegate(this, this._onFailure);

this._started = true;

var timeout = this._webRequest.get_timeout();
if (timeout > 0)
{
this._timer = window.setTimeout(
Function.createDelegate(this, this._onTimeout), timeout);
}

AspNetAjaxExtensions.UpdatePanelIFrameExecutor._executeForm(
this._form, onSuccess, onFailure);
},

_addAdditionalHiddenElements : function() { },

_removeAdditionalHiddenElements : function() { },

_onSuccess : function(responseData)
{
this._clearTimer();
if (this._aborted || this._timedOut) return;

this._statusCode = 200;
this._responseAvailable = true;
this._responseData = responseData;

this._removeAdditionalHiddenElements();
this.get_webRequest().completed(Sys.EventArgs.Empty);
},

_onFailure : function()
{
this._clearTimer();
if (this._aborted || this._timedOut) return;

this._statusCode = 500;
this._responseAvailable = false;

this._removeAdditionalHiddenElements();
this.get_webRequest().completed(Sys.EventArgs.Empty);
},

abort : function()
{
this._aborted = true;
this._clearTimer();

this._removeAdditionalHiddenElements();
},

_onTimeout : function()
{
this._timedOut = true;

this._statusCode = 500;
this._responseAvailable = false;

this._removeAdditionalHiddenElements();
this.get_webRequest().completed(Sys.EventArgs.Empty);
},

_clearTimer : function()
{
if (this._timer != null)
{
window.clearTimeout(this._timer);
delete this._timer;
}
}

　　如果您了解Executor的功能，那么应该很容易看懂上面的代码：executeRequest方法用于发出请求，在executeRequest方法中还会打开一个监听是否超时的定时器，当得到回复或超时后就会调用WebRequest的completed方法（在_onSuccess和_onFailure方法内）进行通知。不过上面这段代码中还有两个特别的方法“_addAddtionalHiddenElements”和“removeAdditionalHiddenElements，从名称上就能看出，这是为这次“异步提交”而准备的额外元素。

　　那么我们该创建哪些附加的隐藏输入元素呢？自然我们表示“异步回送”的自定义标记是其中之一，那么剩下的还需要哪些呢？似乎我们只能通过阅读PageRequestManager的代码来找到问题的答案。还好，似乎阅读下面的代码并不困难：

function Sys$WebForms$PageRequestManager onFormSubmit(evt)
{
//

// Construct the