Starbucks part 2 in Visual Studio .NET

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Starbucks part 2
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If you go back to the earlier Starbucks example, the open command is similar to placing the order, and the send command is like the order being placed in the queue. The callback function is the unique name associated with your order typically your name. Another interesting tidbit is that in IE, only two connections can be opened at a time, which is the equivalent of having two cashiers available to take the orders.
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Introducing ASP.NET AJAX
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When the status of the request changes and the callback function is invoked, the final step is to check the status and update the user interface with the contents returned from Welcome.htm (see listing 1.4).
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Listing 1.4 The callback function gets called every time the ready state changes for the asynchronous request.
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function onCallback() { if (xmlHttp.readyState == 4) {
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Look for completed ready state
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if (xmlHttp.status == 200){ var r = document.getElementById('results'); r.innerHTML = xmlHttp.responseText; } else { alert('Error: ' + xmlHttp.status); } } }
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Status 200 = successful request
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Display request results
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The status of the request is returned in the readyState B property of the XMLHttpRequest object. The value 4 indicates that the request has completed. Next, the response from the server C must be checked to confirm that everything was successful. Status code 200 is designated in the HTTP protocol to indicate that a request has succeeded. Finally, the innerHTML of a span element is updated to reflect the contents in the response D. Listing 1.5 shows the complete code for this example.
Listing 1.5 Using the XMLHttpRequest control to asynchronously retrieve data
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="XmlHttpRequest.aspx.cs" Inherits="CH_01_XmlHttpRequest" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head runat="server"> <title>ASP.NET AJAX In Action - XMLHttpRequest</title> </head> <body> <form id="form1" runat="server"> <div> <span id="results">Loading...</span>
What is Ajax
</div> </form> <script type="text/javascript"> var xmlHttp = null; window.onload = function() { loadXmlHttp(); sendRequest("Welcome.htm"); } function loadXmlHttp() { if (window.XMLHttpRequest) { // IE7, Mozilla, Safari, Opera xmlHttp = new XMLHttpRequest(); } else if (window.ActiveXObject) { try{ xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); //IE 5.x, 6 } catch(e) {} } } function sendRequest(url) { if (xmlHttp) { xmlHttp.open("GET", url, true); // true = async xmlHttp.onreadystatechange = onCallback; xmlHttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded'); xmlHttp.send(null); } } function onCallback() { if (xmlHttp.readyState == 4) { if (xmlHttp.status == 200){ var r = document.getElementById('results'); r.innerHTML = xmlHttp.responseText; } else { alert('Error: ' + xmlHttp.status); }
Introducing ASP.NET AJAX
} } </script> </body> </html>
A simple asynchronous request to another page
Figure 1.5 shows the output that results when you execute the example code. The example we just walked through demonstrates how to leverage the XMLHttpRequest object to make a simple asynchronous HTTP request to another page on the server. When the request is completed, you display the results on the page by dynamically updating the contents of one of its UI elements a span. There is a lot more to the XMLHttpRequest object that we didn t cover; we barely scratched the surface. The point of this exercise was to introduce you to the basics of the Ajax programming pattern. You should recognize some of the issues that arise with Ajax development, such as cross-browser compatibility and the need for a lot of plumbing code to execute requests to the server. This takes us into the next section, which discusses other issues and complexities in Ajax development.
Ajax development issues
Without a toolkit or framework to leverage, developing Ajax-enabled applications is no trivial task. Several development issues arise, the most obvious of which is browser compatibility. Aside from the different implementations of the XMLHttpRequest object, each browser also implements a slightly different version of the
ASP.NET AJAX architecture
DOM. Keeping up with changes between browsers and managing browser detection can be a tedious and error-prone process. One of the goals of a toolkit or framework is to abstract away the complexities and discrepancies between browsers so you can use a simple and consistent set of APIs to perform the same operations. Another challenge is the requirement for a strong grasp of the JavaScript language. JavaScript isn t inherently a complex language; however, many ASP.NET developers lack expertise in it. In addition, JavaScript doesn t offer the objectoriented, type-safe features that .NET developers have grown accustomed to with C#, VB.NET, and other .NET languages. Concepts such as inheritance, interfaces, and events can be simulated in JavaScript but are left to you to implement. Without a framework, this portion of JavaScript remains for you to master in order to make any progress. Debugging and the lack of support for client-scripting languages in integrated development environments (IDEs) adds to the complexity and challenges. By now, you probably see the direction we re headed: In almost every case, it s wiser to leverage a framework or toolkit when developing Ajax-enabled applications rather than deal with these complexities on your own. We re certain there are simple situations where coding something quickly with the XMLHttpRequest object can get the job done, but this book s aspirations are much greater. With that said, it s time to look at ASP.NET AJAX and what it has to offer as a framework and library.
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