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。