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CHAPTER 2 THE NUTS AND BOLTS OF AJAX
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This could cause problems if the script attempts to read the request results before the request has been completed. Using the property readyState, it is possible to know the stage of the HTTP request. The property readyState can contain one of five values, each representing a request state: 0: The XMLHttpRequest instance is in an inconsistent state, and the result data should not be referencing. 1: A request is in progress, and the result data should not be retrieved. 2: The request has downloaded the result data and is preparing it for reference. 3: The script can interact with the XMLHttpRequest instance even though the data is not completely loaded. 4: The request and result data are completely downloaded and loaded as an object model. The request states seem to indicate that it is possible to manipulate various properties at different states. The problem is that not all browsers support the same property states at the same state codes. The only cross-platform solution is to reference the XMLHttpRequest result properties (status, statusText, responseText, and responseXML) when the request state is equal to 4. When the request state is 4, you can be sure that the result properties contain a valid value. Executing the asynchronous Ajax application results in a call being made, and the browser is not locked. You can click the button, open a new browser, and surf to another website. After the 10 seconds have expired, the generated HTML page should resemble Figure 2-6.
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Figure 2-6. Resulting HTML page using asynchronous XMLHttpRequest The asynchronous approach solves the problem of the hanging browser. The Ajax application could continue processing other data, and in fact multiple requests could be made. What is not optimal is that there is no busy indicator. You have no idea whether anything is working when you click the OK button. There should be some form of indicator that something is happening.
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CHAPTER 2 THE NUTS AND BOLTS OF AJAX
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Another problem is that some browsers will cache the results of the XMLHttpRequest. This is an age-old problem because caching can result in unpredictable behavior, and caching still happens even if the Ajax HTML page is reloaded.
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The Factory pattern implementation that was used to abstract the instantiation of XMLHttpRequest was a good first step. Using an asynchronous request is a good second step, as it improves the Ajax experience, but other problems remain, such as user feedback and how to use security that falls in the context of same origin policy.
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Implementing an Asynchronous Calling Mechanism
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When executed in the context of a web browser, JavaScript is not a multithreaded programming language, and therefore it is not possible to instantiate a thread that processes some data, while the main Ajax application is executing. Using an asynchronous XMLHttpRequest instance is sort of multithreading in that the application can continue execution while waiting for a response. Asynchronous programming means writing event-driven code, and that requires a different way of programming with JavaScript. Yet writing code with JavaScript is not like writing code in an object-oriented language. JavaScript is more or less a procedural language that has some hand-wired extensions that make it appear object oriented.
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The Modified Ajax Application
In this section, I m going to again modify the Ajax application that has been illustrated multiple times, except this time I ll add a button to make another request. To illustrate asynchronous programming, two requests will be made simultaneously. One request will return immediately with the data, and the second will call the 10-second delay page. Following is the modified HTML code: <html> <head> <title>Sample Page</title> </head> <script language="JavaScript" src="/lib/factory.js"></script> <script language="JavaScript" src="/lib/asynchronous.js"></script> <script language="JavaScript" type="text/javascript"> function AsyncUpdateEvent(status, statusText, responseText, responseXML) { document.getElementById('httpcode').innerHTML = status; document.getElementById('httpstatus').innerHTML = statusText; document.getElementById('result').innerHTML = responseText; document.getElementById('xmlresult').innerHTML = responseXML; }
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