ASP.NET AJAX UpdatePanel 控件实现剖析

2021-08-14 02:55:13

使用ASP.NET AJAX框架我们可以搭建快速响应、具有丰富的用户体验的AJAX Web应用程序，而该框架的UpdatePanel控件则提供了一种非常简单的方式来实现Web页面的局部更新，我们不需要在每次回发的时候都加载整个页面。

那这个控件是如何实现这种局部刷新的哪，透过其实现机制我们可以更清楚其优缺点，便于我们确定其使用场合。本文将重点阐述ASP.NET AJAX控件UpdatePanel的实现机制。

1． ASP.NET AJAX 简介 ASP.NET AJAX是微软在ASP.NET 2.0之上对AJAX技术的一个封装，为Web应用程序提供完整的AJAX解决方案。ASP.NET AJAX有两种编程模型：部分更新和远程服务。

部分更新使得用户可以用传统的ASP.NET 2.0应用程序的方式来搭建AJAX应用，具体就是使用UpdatePanel控件来实现无闪烁页面更新。而远程服务则是直接通过前端JavaScript来调用的服务器端服务，前段获取数据后，进行页面更新，这就要求服务器端代码必须分解为特定于应用程序的服务，这是与传统的ASP.NET应用程序完全不同的体系结构。

部分更新着重于对现有应用程序进行渐进式增强，帮助用户逐渐转换到纯粹的AJAX应用。本文主要对部分更新编程模型中核心控件UpdatePanel的实现进行剖析，讲述其背后的故事。

ASP.NET AJAX框架分为客户端以及服务器端两个部分，基于客户端的 Microsoft AJAX Library包含了对浏览器兼容性、网络访问以及客户端控件组件等支持，而服务器端则包括了服务器控件，Web 服务等等。

见下图：

Microsoft Ajax Library就是ASP.NET AJAX的客户端脚本库，其中MicrosoftAjax.js包含了ASP.NET AJAX的核心内容，包括跨浏览器的支持、基于面向对象对JavaScript的扩展以及网络访问等等。MicrosoftAjaxWebForm.js文件则是完全服务于ASP.NET AJAX页面局部更新这样一个功能的，在该文件中定义了一个客户端对象PageRequestManager，该对象将会负责客户端异步回送的全过程。

2． ScriptManager 和 UpdatePanel ScriptManager和UpdatePanel是ASP.NET AJAX服务器端中最重要的两个控件，ScriptManager控件用来管理ASP.NET页面中的客户端脚本，生成及注册所需要的客户端脚本，通过UpdatePanel控件可以更新页面的指定部分而无需加载整个页面。

看个例子：

<form id="form1" runat="server">
   <asp:ScriptManager ID="ScriptManager1" runat="server">
   </asp:ScriptManager>
   <div>
      <asp:UpdatePanel ID="UpdatePanel1" runat="server">
           <ContentTemplate>
               <%= DateTime.Now %>
                <br />
               <asp:Button ID="Button1" runat="server" Text="Button" />
           </ContentTemplate>
       </asp:UpdatePanel>
   </div>
</form>

构建如上代码所示的页面，在Runtime点击UpdatePanel中的Button控件，则不会引起整个页面刷新，只是用来显示当前时间的Label得到更新。

这是如何实现的哪？

3． ASP.NET AJAX部分呈现剖析

3.1 先从客户端讲起

看一下上面的示例代码在客户端的HTML代码，这里只列出核心部分，其他全部隐去。

<script type="text/javascript">
//<![CDATA[
Sys.WebForms.PageRequestManager._initialize('ScriptManager1', document.getElementById('form1'));
Sys.WebForms.PageRequestManager.getInstance()._updateControls(['tUpdatePanel1'], [], [], 90);
//]]>
</script>

<div id="UpdatePanel1">    
     7/25/2008 4:54:36 PM
     <br />
     <input type="submit" name="Button1" value="Button" id="Button1" />            
</div>

看一下上面的两句JavaScript代码，第一句代码中的_initialize 方法是客户端PageRequestManager对象上的静态方法，它会创建一个 PageRequestManager 类的全局实例，并将其初始化。在这个初始化函数中，ageRequestManager对象注册了当前表单对象的submit事件，以及window对象的load和unload事件。

而第二句代码则是通过PageRequestManager的getInstance方法来检索其唯一实例，得到该实例后调用_updateControls方法来注册UpdatePanel以及其Trigger控件。

我们可以从MicrosoftAjaxWebForm.js文件中得到_updateControls方法的声明：

function Sys$WebForms$PageRequestManager$_updateControls(updatePanelIDs, asyncPostBackControlIDs, postBackControlIDs, asyncPostBackTimeout) {}

由其中第一个参数代表了当前页面上所有的UpdatePanel控件的ID集合，如果该UpdatePanel的ChildrenAsTrigger为True的话，应在ID前添加字符't'，否则添加字符'f'；而第二个参数是所有引发异步回送的控件ID；第三个参数是所有引发同步回送的控件ID；第四个参数设定了异步回送的Timeout时间，单位为秒。于PageRequestManager对象注册了当前表单的submit时间，所以每当页面有提交动作的时候，PageRequestManager对象就会介入，看一下PageRequestManager对象页面提交处理函数_onFormSubmit(evt)。

如果需要执行一次异步回送的话，会中止原有的普通浏览器会回发，代之使用XMLHttpRequest进行AJAX回发。在封装这个请求的时候，当前页面的所有字段以及视图状态都会被打包在请求中，另外还设置了这次Request的HTTP头：request.get_headers()['X-MicrosoftAjax'] = 'Delta=true'；

在服务器端将会根据这个HTTP头标记来判定是否为一次AJAX异步回发。

_onFormSubmit(evt)函数代码：

function Sys$WebForms$PageRequestManager$_onFormSubmit(evt) {
        var continueSubmit = true;
        var isCrossPost = this._isCrossPost;
        this._isCrossPost = false;
        if (this._onsubmit) {
            continueSubmit = this._onsubmit();
        }
        if (continueSubmit) {
            for (var i = 0; i < this._onSubmitStatements.length; i++) {
                if (!this._onSubmitStatements[i]()) {
                    continueSubmit = false;
                    break;
                }
            }
        }
        if (!continueSubmit) {
            if (evt) {
                evt.preventDefault();
            }
            return;
        }
        var form = this._form;
        if (isCrossPost) {
            return;
        }
        if (this._activeDefaultButton && !this._activeDefaultButtonClicked) {
            this._onFormElementActive(this._activeDefaultButton, 0, 0);
        }
        if (!this._postBackSettings.async) {
            return;
        }
        var formBody = new Sys.StringBuilder();
        formBody.append(encodeURIComponent(this._scriptManagerID) + '=' + encodeURIComponent(this._postBackSettings.panelID) + '&');
        var count = form.elements.length;
        for (var i = 0; i < count; i++) {
            var element = form.elements[i];
            var name = element.name;
            if (typeof(name) === "undefined" || (name === null) || (name.length === 0)) {
                continue;
            }
            var tagName = element.tagName;
            if (tagName === 'INPUT') {
                var type = element.type;
                if ((type === 'text') ||
                    (type === 'password') ||
                    (type === 'hidden') ||
                    (((type === 'checkbox') || (type === 'radio')) && element.checked)) {
                    formBody.append(encodeURIComponent(name));
                    formBody.append('=');
                    formBody.append(encodeURIComponent(element.value));
                    formBody.append('&');
                }
            }
            else if (tagName === 'SELECT') {
                var optionCount = element.options.length;
                for (var j = 0; j < optionCount; j++) {
                    var option = element.options[j];
                    if (option.selected) {
                        formBody.append(encodeURIComponent(name));
                        formBody.append('=');
                        formBody.append(encodeURIComponent(option.value));
                        formBody.append('&');
                    }
                }
            }
            else if (tagName === 'TEXTAREA') {
                formBody.append(encodeURIComponent(name));
                formBody.append('=');
                formBody.append(encodeURIComponent(element.value));
                formBody.append('&');
            }
        }
        if (this._additionalInput) {
            formBody.append(this._additionalInput);
            this._additionalInput = null;
        }
        
        var request = new Sys.Net.WebRequest();
        var action = form.action;
        if (Sys.Browser.agent === Sys.Browser.InternetExplorer) {
            var queryIndex = action.indexOf('?');
            if (queryIndex !== -1) {
                var path = action.substr(0, queryIndex);
                if (path.indexOf("%") === -1) {
                    action = encodeURI(path) + action.substr(queryIndex);
                }
            }
            else if (action.indexOf("%") === -1) {
                action = encodeURI(action);
            }
        }
        request.set_url(action);
        request.get_headers()['X-MicrosoftAjax'] = 'Delta=true';
        request.get_headers()['Cache-Control'] = 'no-cache';
        request.set_timeout(this._asyncPostBackTimeout);
        request.add_completed(Function.createDelegate(this, this._onFormSubmitCompleted));
        request.set_body(formBody.toString());
        var handler = this._get_eventHandlerList().getHandler("initializeRequest");
        if (handler) {
            var eventArgs = new Sys.WebForms.InitializeRequestEventArgs(request, this._postBackSettings.sourceElement);
            handler(this, eventArgs);
            continueSubmit = !eventArgs.get_cancel();
        }
        if (!continueSubmit) {
            if (evt) {
                evt.preventDefault();
            }
            return;
        }
        this._scrollPosition = this._getScrollPosition();
        this.abortPostBack();
        handler = this._get_eventHandlerList().getHandler("beginRequest");
        if (handler) {
            var eventArgs = new Sys.WebForms.BeginRequestEventArgs(request, this._postBackSettings.sourceElement);
            handler(this, eventArgs);
        }
        
        if (this._originalDoCallback) {
            this._cancelPendingCallbacks();
        }
        this._request = request;
        request.invoke();
        if (evt) {
            evt.preventDefault();
        }
    }

我们可以发现AJAX回发所提交的数据量其实跟普通回发过程中提交的数据量是一样的，并且还附加了一些额外信息。

3.2 服务器端的处理

AJAX回发请求到达服务器之后，当前页面的生命周期跟普通回发引起的请求是一样的，页面的Init、Load和Render等等事件都会被触发，差别只是在于AJAX回发使用了不同的呈现画法。

AJAX回发引起的请求生命周期：

从上图我们可以看到，页面的生命周期与普通回发是一样的，同样页面上的控件也会经历相应的生命周期。

先了解一下ScriptManager控件在服务器端的处理：

- OnInit：在Init事件中，ScriptManager控件会注册页面的InitComplete, PreRenderComplete以及PreRender事件，另外还会根据本次请求的HTTP头来设定一个标记以确定本次回发是否为Ajax异步更新所引起的回发。

见下面的代码：

protected internal override void OnInit(EventArgs e)
{
    base.OnInit(e);
    if (this.EnableHistory)
    {
        this.RegisterAsyncPostBackControl(this);
    }
    if (!base.DesignMode)
    {
        IPage iPage = this.IPage;
        if (GetCurrent(this.Page) != null)
        {
            throw new InvalidOperationException(AtlasWeb.ScriptManager_OnlyOneScriptManager);
        }
        iPage.Items[typeof(IScriptManager)] = this;
        iPage.Items[typeof(ScriptManager)] = this;
        iPage.InitComplete += new EventHandler(this.OnPageInitComplete);
        iPage.PreRenderComplete += new EventHandler(this.OnPagePreRenderComplete);
        if (iPage.IsPostBack)
        {
            this._isInAsyncPostBack = PageRequestManager.IsAsyncPostBackRequest(iPage.Request.Headers);
            if (this.EnableHistory)
            {
                this._isNavigating = iPage.Request["__EVENTTARGET"] == this.UniqueID;
            }
        }
        this.PageRequestManager.OnInit();
        iPage.PreRender += new EventHandler(this.ScriptControlManager.OnPagePreRender);
    }
}

- OnPagePreRenderComplete，在PagePreRenderComplete事件中，ScriptManager控件会注册脚本文件以及Services代理脚本，MicrosoftAjax.js和MicrosoftAjaxWebForm.js就是在这个阶段被注册到客户端的。

见下面的代码：

private void OnPagePreRenderComplete(object sender, EventArgs e)
{
    if (!this.IsInAsyncPostBack)
    {
        if (this.SupportsPartialRendering)
        {
            this.IPage.ClientScript.GetPostBackEventReference(new PostBackOptions(this, null, null, false, false, false, false, true, null));
        }
        this.RegisterGlobalizationScriptBlock();
        this.RegisterScripts();
        this.RegisterServices();
        if (this.EnableHistory)
        {
            JavaScriptSerializer serializer = JavaScriptSerializer.CreateInstance();
            string[] strArray = new string[] { "\r\nSys.Application.setServerId(", serializer.Serialize(this.ClientID), ", ", serializer.Serialize(this.UniqueID), ");\r\n", ((this._initialState != null) && (this._initialState.Count != 0)) ? ("  Sys.Application.setServerState('" + this.GetStateString() + "');\r\n") : "\r\n" };
            string script = string.Concat(strArray);
            RegisterStartupScript(this, typeof(ScriptManager), "HistoryStartup", script, true);
        }
    }
    else
    {
        this.RegisterScripts();
        if (this.EnableHistory)
        {
            if ((this._initialState != null) && (this._initialState.Count == 0))
            {
                this._initialState = null;
            }
            if (this._newPointCreated)
            {
                this.RegisterDataItem(this, "'" + this.GetStateString() + "'", true);
            }
        }
    }
}

- OnPreRender，在PreRender事件中如果判定本次回发为AJAX回发，则会调用PageRequestManager对象的OnPreRender方法。而PageRequestManager对象则会调用Page对象的SetRenderMethodDelegate方法来代理Page的画法，PageRequestManager对象会真正负责本次AJAX回发最终的HTML代码。

见下面的代码：

public class ScriptManager : Control, 
{
   protected internal override void OnPreRender(EventArgs e)
   {
       base.OnPreRender(e);
       if (this.IsInAsyncPostBack)
       {
           this.PageRequestManager.OnPreRender();
       }
   }
}

internal sealed class PageRequestManager
{
   internal void OnPreRender()
   {
        this._owner.IPage.SetRenderMethodDelegate(new RenderMethod(this.RenderPageCallback));
   }
}

PageRequestManager的RenderPageCallback方法最终处理了AJAX回发所需要的HTML代码，在这个方法中会遍历页面上所有涉及到的UpdatePanel控件，得到其更新后的HTML代码后，与隐藏字段还有一些额外信息一起打包，然后传递给客户端。

见下面的代码：

private void RenderPageCallback(HtmlTextWriter writer, Control pageControl)
{
    this.ProcessUpdatePanels();
    IHttpResponse response = this._owner.IPage.Response;
    response.ContentType = "text/plain";
    response.Cache.SetNoServerCaching();
    IHtmlForm form = this._owner.IPage.Form;
    form.SetRenderMethodDelegate(new RenderMethod(this.RenderFormCallback));
    this._updatePanelWriter = writer;
    ParserStringWriter writer2 = new ParserStringWriter();
    ParserHtmlTextWriter writer3 = new ParserHtmlTextWriter(writer2);
    writer2.ParseWrites = true;
    form.RenderControl(writer3);
    writer2.ParseWrites = false;
    foreach (KeyValuePair<string, string> pair in writer2.HiddenFields)
    {
        if (ControlUtil.IsBuiltInHiddenField(pair.Key))
        {
            EncodeString(writer, "hiddenField", pair.Key, pair.Value);
        }
    }
    EncodeString(writer, "asyncPostBackControlIDs", string.Empty, this.GetAsyncPostBackControlIDs(false));
    EncodeString(writer, "postBackControlIDs", string.Empty, this.GetPostBackControlIDs(false));
    EncodeString(writer, "updatePanelIDs", string.Empty, this.GetAllUpdatePanelIDs());
    EncodeString(writer, "childUpdatePanelIDs", string.Empty, this.GetChildUpdatePanelIDs());
    EncodeString(writer, "panelsToRefreshIDs", string.Empty, this.GetRefreshingUpdatePanelIDs());
    EncodeString(writer, "asyncPostBackTimeout", string.Empty, this._owner.AsyncPostBackTimeout.ToString(CultureInfo.InvariantCulture));
    if (writer3.FormAction != null)
    {
        EncodeString(writer, "formAction", string.Empty, writer3.FormAction);
    }
    if (this._owner.IPage.Header != null)
    {
        string title = this._owner.IPage.Title;
        if (!string.IsNullOrEmpty(title))
        {
            EncodeString(writer, "pageTitle", string.Empty, title);
        }
    }
    this.RenderDataItems(writer);
    this.ProcessScriptRegistration(writer);
    this.ProcessFocus(writer);
}

3.3 客户端更新

当服务器端相应完毕后，客户端会得到响应信息，然后调用客户端对象PageRequestManager的_onFormSubmitCompleted方法来进行页面局部更新，最终会调用_updatePanel方法来更新UpdatePanel控件。

参见_onFormSubmitCompleted方法的代码：

function Sys$WebForms$PageRequestManager$_onFormSubmitCompleted(sender, eventArgs) {
        this._processingRequest = true;
        var delimitByLengthDelimiter = '|';
        if (sender.get_timedOut()) {
            this._endPostBack(this._createPageRequestManagerTimeoutError(), sender);
            return;
        }
        if (sender.get_aborted()) {
            this._endPostBack(null, sender);
            return;
        }
        if (!this._request || sender.get_webRequest() !== this._request) {
            return;
        }
        var errorMessage;
        var delta = [];
        if (sender.get_statusCode() !== 200) {
            this._endPostBack(this._createPageRequestManagerServerError(sender.get_statusCode()), sender);
            return;
        }
        var reply = sender.get_responseData();
        var delimiterIndex, len, type, id, content;
        var replyIndex = 0;
        var parserErrorDetails = null;
        while (replyIndex < reply.length) {
            delimiterIndex = reply.indexOf(delimitByLengthDelimiter, replyIndex);
            if (delimiterIndex === -1) {
                parserErrorDetails = this._findText(reply, replyIndex);
                break;
            }
            len = parseInt(reply.substring(replyIndex, delimiterIndex), 10);
            if ((len % 1) !== 0) {
                parserErrorDetails = this._findText(reply, replyIndex);
                break;
            }
            replyIndex = delimiterIndex + 1;
            delimiterIndex = reply.indexOf(delimitByLengthDelimiter, replyIndex);
            if (delimiterIndex === -1) {
                parserErrorDetails = this._findText(reply, replyIndex);
                break;
            }
            type = reply.substring(replyIndex, delimiterIndex);
            replyIndex = delimiterIndex + 1;
            delimiterIndex = reply.indexOf(delimitByLengthDelimiter, replyIndex);
            if (delimiterIndex === -1) {
                parserErrorDetails = this._findText(reply, replyIndex);
                break;
            }
            id = reply.substring(replyIndex, delimiterIndex);
            replyIndex = delimiterIndex + 1;
            if ((replyIndex + len) >= reply.length) {
                parserErrorDetails = this._findText(reply, reply.length);
                break;
            }
            content = reply.substr(replyIndex, len);
            replyIndex += len;
            if (reply.charAt(replyIndex) !== delimitByLengthDelimiter) {
                parserErrorDetails = this._findText(reply, replyIndex);
                break;
            }
            replyIndex++;
            Array.add(delta, {type: type, id: id, content: content});
        }
        if (parserErrorDetails) {
            this._endPostBack(this._createPageRequestManagerParserError(String.format(Sys.WebForms.Res.PRM_ParserErrorDetails, parserErrorDetails)), sender);
            return;
        }
        var updatePanelNodes = [];
        var hiddenFieldNodes = [];
        var arrayDeclarationNodes = [];
        var scriptBlockNodes = [];
        var scriptStartupNodes = [];
        var expandoNodes = [];
        var onSubmitNodes = [];
        var dataItemNodes = [];
        var dataItemJsonNodes = [];
        var scriptDisposeNodes = [];
        var asyncPostBackControlIDsNode, postBackControlIDsNode,
            updatePanelIDsNode, asyncPostBackTimeoutNode,
            childUpdatePanelIDsNode, panelsToRefreshNode, formActionNode;
        for (var i = 0; i < delta.length; i++) {
            var deltaNode = delta[i];
            switch (deltaNode.type) {
                case "updatePanel":
                    Array.add(updatePanelNodes, deltaNode);
                    break;
                case "hiddenField":
                    Array.add(hiddenFieldNodes, deltaNode);
                    break;
                case "arrayDeclaration":
                    Array.add(arrayDeclarationNodes, deltaNode);
                    break;
                case "scriptBlock":
                    Array.add(scriptBlockNodes, deltaNode);
                    break;
                case "scriptStartupBlock":
                    Array.add(scriptStartupNodes, deltaNode);
                    break;
                case "expando":
                    Array.add(expandoNodes, deltaNode);
                    break;
                case "onSubmit":
                    Array.add(onSubmitNodes, deltaNode);
                    break;
                case "asyncPostBackControlIDs":
                    asyncPostBackControlIDsNode = deltaNode;
                    break;
                case "postBackControlIDs":
                    postBackControlIDsNode = deltaNode;
                    break;
                case "updatePanelIDs":
                    updatePanelIDsNode = deltaNode;
                    break;
                case "asyncPostBackTimeout":
                    asyncPostBackTimeoutNode = deltaNode;
                    break;
                case "childUpdatePanelIDs":
                    childUpdatePanelIDsNode = deltaNode;
                    break;
                case "panelsToRefreshIDs":
                    panelsToRefreshNode = deltaNode;
                    break;
                case "formAction":
                    formActionNode = deltaNode;
                    break;
                case "dataItem":
                    Array.add(dataItemNodes, deltaNode);
                    break;
                case "dataItemJson":
                    Array.add(dataItemJsonNodes, deltaNode);
                    break;
                case "scriptDispose":
                    Array.add(scriptDisposeNodes, deltaNode);
                    break;
                case "pageRedirect":
                    if (Sys.Browser.agent === Sys.Browser.InternetExplorer) {
                        var anchor = document.createElement("a");
                        anchor.style.display = 'none';
                        anchor.attachEvent("onclick", cancelBubble);
                        anchor.href = deltaNode.content;
                        document.body.appendChild(anchor);
                        anchor.click();
                        anchor.detachEvent("onclick", cancelBubble);
                        document.body.removeChild(anchor);
                        
                        function cancelBubble(e) {
                            e.cancelBubble = true;
                        }
                    }
                    else {
                        window.location.href = deltaNode.content;
                    }
                    return;
                case "error":
                    this._endPostBack(this._createPageRequestManagerServerError(Number.parseInvariant(deltaNode.id), deltaNode.content), sender);
                    return;
                case "pageTitle":
                    document.title = deltaNode.content;
                    break;
                case "focus":
                    this._controlIDToFocus = deltaNode.content;
                    break;
                default:
                    this._endPostBack(this._createPageRequestManagerParserError(String.format(Sys.WebForms.Res.PRM_UnknownToken, deltaNode.type)), sender);
                    return;
            }
        }
        var i;
        if (asyncPostBackControlIDsNode && postBackControlIDsNode &&
            updatePanelIDsNode && panelsToRefreshNode &&
            asyncPostBackTimeoutNode && childUpdatePanelIDsNode) {
            this._oldUpdatePanelIDs = this._updatePanelIDs;
            var childUpdatePanelIDsString = childUpdatePanelIDsNode.content;
            this._childUpdatePanelIDs = childUpdatePanelIDsString.length ? childUpdatePanelIDsString.split(',') : [];
            var asyncPostBackControlIDsArray = this._splitNodeIntoArray(asyncPostBackControlIDsNode);
            var postBackControlIDsArray = this._splitNodeIntoArray(postBackControlIDsNode);
            var updatePanelIDsArray = this._splitNodeIntoArray(updatePanelIDsNode);
            this._panelsToRefreshIDs = this._splitNodeIntoArray(panelsToRefreshNode);
            for (i = 0; i < this._panelsToRefreshIDs.length; i++) {
                var panelClientID = this._uniqueIDToClientID(this._panelsToRefreshIDs[i]);
                if (!document.getElementById(panelClientID)) {
                    this._endPostBack(Error.invalidOperation(String.format(Sys.WebForms.Res.PRM_MissingPanel, panelClientID)), sender);
                    return;
                }
            }
            var asyncPostBackTimeout = asyncPostBackTimeoutNode.content;
            this._updateControls(updatePanelIDsArray, asyncPostBackControlIDsArray, postBackControlIDsArray, asyncPostBackTimeout);
        }
        this._dataItems = {};
        for (i = 0; i < dataItemNodes.length; i++) {
            var dataItemNode = dataItemNodes[i];
            this._dataItems[dataItemNode.id] = dataItemNode.content;
        }
        for (i = 0; i < dataItemJsonNodes.length; i++) {
            var dataItemJsonNode = dataItemJsonNodes[i];
            this._dataItems[dataItemJsonNode.id] = Sys.Serialization.JavaScriptSerializer.deserialize(dataItemJsonNode.content);
        }
        var handler = this._get_eventHandlerList().getHandler("pageLoading");
        if (handler) {
            handler(this, this._getPageLoadingEventArgs());
        }
        if (formActionNode) {
            this._form.action = formActionNode.content;
        }
        
        
        Sys._ScriptLoader.readLoadedScripts();
        Sys.Application.beginCreateComponents();
        var scriptLoader = Sys._ScriptLoader.getInstance();
        this._queueScripts(scriptLoader, scriptBlockNodes, true, false);
        
        this._updateContext = {
            response: sender,
            updatePanelNodes: updatePanelNodes,
            scriptBlockNodes: scriptBlockNodes,
            scriptDisposeNodes: scriptDisposeNodes,
            hiddenFieldNodes: hiddenFieldNodes,
            arrayDeclarationNodes: arrayDeclarationNodes,
            expandoNodes: expandoNodes,
            scriptStartupNodes: scriptStartupNodes,
            onSubmitNodes: onSubmitNodes
        };
        scriptLoader.loadScripts(0,
            Function.createDelegate(this, this._scriptIncludesLoadComplete),
            Function.createDelegate(this, this._scriptIncludesLoadFailed),
            null);
}

4．结语

使用UpdatePanel是给已经存在的ASP.NET应用程序添加AJAX体验的最快捷方式，对于应用程序的架构也不会有影响，我们可以使用它来逐步的提高应用程序的用户体验。但是其性能与纯粹的AJAX方式相比较，还是比较差的。

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