print barcode labels using c# 4-1. IMPLEMENTING AN SOA ARCHITECTURE in Font

Painting Code 3/9 in Font 4-1. IMPLEMENTING AN SOA ARCHITECTURE

CHAPTER 4 4-1. IMPLEMENTING AN SOA ARCHITECTURE
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<body> <button onclick="GetIt('/chap02/serverhang.aspx')">Get a document</button> <p><table border="1"> <tr><td>Document</td><td><span id="status">No Result</span></td> <td><span id="result">No Result</span></td></tr> </table></p> </body> </html> There are several new additions to the rewritten Ajax application, and they deal with the technical issues of loading content asynchronously. Let s start by focusing on the GetIt function. The implementation of GetIt is similar to previous Ajax application examples, except that the third parameter of the open method is true to indicate that the request will be asynchronous. This means that when the send method is called, it will return immediately. Whenever XMLHttpRequest operates in asynchronous modes, feedback is given to the caller on the state of the request. The onreadystatechange property is a function that receives the feedback. It is important to note that the feedback function must be assigned before each send, because upon request completion the onreadystatechange property is reset. This is evident in the Mozilla and Firefox source. The onreadstatechange property is assigned the AsyncUpdateEvent function. In the implementation of AsyncUpdateEvent is a switch statement that tests the current state of the request. When an asynchronous request is made, the script is free to continue executing other code. This could cause problems if the script attempts to read the request results before the request has been completed. Using the readyState property, it is possible to know the stage of the HTTP request. The readyState property can contain one of five values, where each value represents 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 complete and have been finished. The request states would 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 Web site.
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CHAPTER 4 4-1. IMPLEMENTING AN SOA ARCHITECTURE
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Implementing the Asynchronous Class Having determined asynchronous XMLHttpRequests are the way to go, the Asynchronous encapsulation class is defined. An encapsulation class is not necessary, but it makes it simpler to work with the XMLHttpRequest object. The main advantage of using an encapsulation class is the association of the XMLHttpRequest object with a code block. For a refresher, let s look at the code to load an Atom feed again: function LoadAtomFeed() { var asynchronous = new Asynchronous(); asynchronous.settings = { onComplete : function(xmlhttp) { parseAtom( xmlhttp.responseXML); flexbox.update(); } } asynchronous.get("/services/blog/entries/current"); } To make the Asynchronous class work, the user needs to do two things: associate the settings data member with some information and call an appropriate method (e.g., get) to make an HTTP request. The purpose of the settings data member is to provide the callbacks and extra information associated with a request. In the example, the onComplete method is called once the HTTP request has completed. The code as it stands is fairly easy to understand, but remember that the code is asynchronous. As a result, when the asynchronous.get method is called, the LoadAtomFeed function will exit before calling onComplete and returning control to the user. In a worst-case scenario, if a user goes click-crazy, there could be dozens of requests being made. The alternative is to use synchronous calls, which lock the browser, and that itself is not an option. On the positive side, asynchronous behavior allows a user to launch tasks and wait for the results. The programmer has to remain vigilant and make sure that the user does not get ahead of him- or herself. Now that you have a basic understanding of how to use the Asynchronous class, let s look at the details of the Asynchronous implementation. The class exposes a method for each HTTP verb, and in the example the get method corresponds to the HTTP verb GET. The implementation of get is as follows: function HttpRequest_get(strurl) { this.call({ action : "GET", url: strurl}); } In the implementation of HttpRequest_get, which is mapped to Asynchronous.get, a reference to the function call is made. The get function is a convenience method to the call method. The call method accepts a single parameter: an object with specific data members. In the case of the get method, the action and url data members are defined. So why create an object, when you could have used two parameters, action and url, for the method call The answer is flexibility. JavaScript does not understand overloaded functions, and there can be only a single function with a single set of parameters. So that the developer does not have to play the permutations and combinations game with the parameters (e.g., which parameters, what order, etc.), an object with data members is created.
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