qr code c# source Modifying OnPreRender of the RSS Web Part in Visual C#.NET

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Listing 10.2 Modifying OnPreRender of the RSS Web Part
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protected override void OnPreRender(EventArgs e) { XElement feed = XElement.Load(this.RssFeedUrl); var items = feed.Descendants("item").Take(this.ItemCount); foreach (var item in items) {
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Panel panel = new Panel(); panel.Controls.Add(new HyperLink() { Text = item.Descendants("title").First().Value, NavigateUrl = item.Descendants("link").First().Value }); updatePanel.ContentTemplateContainer.Controls.Add(panel); } updatePanel.ContentTemplateContainer.Controls.Add( new LiteralControl("Last updated ")); updatePanel.ContentTemplateContainer.Controls.Add( new LiteralControl(DateTime.Now.ToString())); base.OnPreRender(e); }
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The feed is retrieved using the same methods you used earlier but the feed items are added to the ContentTemplateContainer control collection of the UpdatePanel object. This template container is dynamically refreshed in the web interface. Finally, two controls are added so that you can see the time when the feed was last retrieved. If you now deploy and add the Web Part to a page, it will look like figure 10.1. Click the Refresh button and notice that only the update panel reloads, not the entire page. The ASP.NET AJAX Extensions can do a whole lot more for your Web Part interfaces. For example, the extensions contain a Timer control that performs postbacks at regular intervals and an UpdateProgress control that shows status information about partial-page updates.
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The Microsoft Ajax extender controls enhance the client capabilities of standard ASP.NET controls. One common extender control library is the Ajax Control Toolkit, which is a community-supported extension from Microsoft. You can download the Toolkit from http://ajaxcontroltoolkit.codeplex.com/.
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10.1.2 JavaScripts in Web Parts JavaScript has evolved; once a web browser feature that made simple calculations or changes to the page, it s now a service that you can use to take load off the server. Most web applications built today rely heavily on JavaScript, and SharePoint is one of them. The ASP.NET AJAX Extensions use JavaScript to do partial postbacks and updates. Using JavaScript in Web Parts is a great way to accomplish a dynamic interface that interacts with other items. There are two basic ways to add JavaScript to a Web Part: either using an external JavaScript (.js) file or using inline JavaScripts. Using external JavaScript files is what I recommend. They perform better because you can cache those files. Using inline JavaScript can make programming easier because you can generate the scripts dynamically. I recommend that you use the ScriptLink control, mentioned in chapter 4, to add external JavaScript files to a Web Part. You can use ScriptLink as a control or add it to the control tree, or you can use one of its static methods to register a script. The following line shows how to register a custom script with a Web Part for the current page:
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ScriptLink.Register(this.Page, "Custom.js", false);
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The ScriptLink control ensures that the script element is used only once in the output. It also assumes that the JavaScript file is located in the Layouts folder. You can also tell the control to use a localized version of the JavaScript file by setting the third parameter to true. The ScriptLink control isn t available in sandboxed solutions. Instead you have to inject the <script> tag manually and point it to a location in SharePoint where the JavaScript file is deployed. You should do this in the RenderContents method of the Web Part, as shown in the following snippet:
protected override void RenderContents(HtmlTextWriter writer) { string url = SPContext.Current.Web.Url + "/SiteAssets/script.js"; writer.Write("<script type='text/javascript' src='" + url + "'></script>"); base.RenderContents(writer); }
The downside is that the script file will be loaded once for each Web Part instance. To add inline JavaScript code to a Web Part, you should use the ClientScriptManager object of the page. You can insert scripts declaratively in user controls or pages, but ClientScriptManager manages all scripts you add dynamically to the page. To add a script dynamically, you can choose among several methods depending on the situation. Each script has a key and a type that identifies the scripts and avoids duplicates. The ClientScriptManager is accessible from the sandbox, but any scripts registered won t be transferred from the sandbox. Use RegisterStartupScript to insert a script that should execute when the page load is complete. A Hello World example of a startup script might look like this:
this.Page.ClientScript.RegisterStartupScript( typeof(MyWebPart), "helloworld", "alert('Hello World');", true);
This code will display a JavaScript alert dialog box with the text Hello World once the page is loaded and rendered. The last parameter tells ClientScriptManager that it should enclose the script with script tags. The typeof keyword is used instead of the GetType method of the current object to get the type of the object. This is the preferred approach and avoids double registrations of the scripts when the Web Part is subclassed. You can avoid loading your JavaScript files multiple times in sandboxed Web Parts by loading them dynamically. One good way to achieve this is by using the SharePoint Script On Demand system, defined in the ECMAScript SP.SOD class. You can find more about it here: http://msdn.microsoft.com/en-us/ library/ff410742.aspx.
When working with partial postbacks and partial updates, you can t use ClientScriptManager. The ASP.NET AJAX Extensions have their own script manager, called ScriptManager, to handle the partial updates. ScriptManager is very similar in operation to ClientScriptManager. For instance, if you need to run a script on
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