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Before Ajax was used, you would pan around the map using North, South, East, and West buttons The Ajax implementation used XMLHttpRequest to download the surrounding areas of the map in the background while you were inspecting the map You would then drag the mouse around the map, and they would appear Of course, if you dragged quickly, the tiles may not have been in place yet, and you would get a blank map while they downloaded but the experience was still vastly superior to full page refreshes The improvement in the user experience and the reduced amount of bandwidth that needed to be served to provide it created an irresistible new design pattern for web sites and experiences, and it wasn t long before use of XMLHttpRequest became commonplace At the same time, the concept of Web 20 was emerging.

In the previous section, you learned to create XPathNavigator from XmlDocument and XPathDocument. In this section, you will see how to use XPathNavigator and access various attributes and elements. To work through this section, you need to create a Windows Forms application like the one shown in Figure 4-2.

PortletContext portletContext = getPortletContext(); PortletRequestDispatcher prd = portletContext.getRequestDispatcher("/patents"); prd.include(request,response);

This concept, invented by Tim O Reilly, described the next phase of the Web, built around services that allowed users, not just web developers, to be contributors of information Effective collaboration from novice users requires an enhanced experience, and Ajax was there to provide it As such, in developer terms, Ajax is pretty synonymous with Web 20 In the next section, we ll take a look at getting started with Ajax programming and some of the neat things that you can do with it!.

A named dispatcher is useful for loading servlets or JSP pages that have been given a name in the portlet application s web deployment descriptor. It also returns a PortletRequestDispatcher object. One important difference is that it is impossible to pass a query string to a servlet or JSP that is called through a named dispatcher. If we have a servlet named SingleSignOnServlet, we could include it when we render the portlet, using code like the following:

Figure 4-2. Navigating through XPathNavigator The application consists of a TreeView and a button. After you click the Load Tree button, the TreeView is populated with employee information from the Employees.xml file. The Click event handler of the Load Tree button contains the code shown in Listing 4-2.

PortletContext portletContext = getPortletContext(); PortletRequestDispatcher prd = PortletContext.getNamedDispatcher("SingleSignOnServlet"); prd.include(request,response);

As mentioned earlier, the heart of Ajax is the XMLHttpRequest object. For the rest of this chapter, you ll look at this object, building applications that use it to communicate with a back-end system. XMLHttpRequest is as useful as it is simple but as with anything that is powerful and simple, you should exercise caution in using it, and use it only when appropriate. It isn t, for example, appropriate to write a JavaScript function that uses XMLHttpRequest to script navigation between complete HTML pages, thus turning it into a browser within a browser! While XMLHttpRequest is supported in all major modern browsers, there are differences that need to be coped with in your scripts. Internet Explorer 7, Firefox, Safari, and Opera support it as a native JavaScript object, whereas Internet Explorer 6 and earlier implement it as an ActiveX object your code to implement the object needs to cater for all these browsers. Listing 11-1 shows simple JavaScript code that loads an instance of XMLHttpRequest into a standard var.

Listing 4-2. Navigating by Using XPathNavigator private void button1_Click(object sender, EventArgs e) { XPathDocument doc = new XPathDocument(Application.StartupPath + @"\employees.xml"); XPathNavigator navigator = doc.CreateNavigator(); navigator.MoveToRoot(); navigator.MoveToFirstChild(); TreeNode root = treeView1.Nodes.Add("Employees"); while (navigator.MoveToNext()) { if (navigator.HasChildren) { navigator.MoveToFirstChild(); do { string id = navigator.GetAttribute("employeeid", ""); TreeNode empnode = new TreeNode("Employee ID :" + id); root.Nodes.Add(empnode); navigator.MoveToFirstChild(); do { string name = navigator.Name; TreeNode node = new TreeNode(name + " : " + navigator.Value); empnode.Nodes.Add(node); } while (navigator.MoveToNext()); navigator.MoveToParent(); } while (navigator.MoveToNext()); } } }