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You want to put XMLHttpRequest into a factory.
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When you are using XMLHttpRequest, the problem is how to instantiate XMLHttpRequest. Each browser has a different way of instantiating XMLHttpRequest. Here is what Mozilla and most other browsers use (this code will also work in Internet Explorer starting with version 7): var xmlhttp = new XMLHttpRequest(); You use the following source code for Internet Explorer versions before 7: var xmlhttp = new ActiveXObject('Microsoft.XMLHTTP'); The Microsoft.XMLHTTP buffer is a general identifier, as there are specializations available depending on the browser.
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The short answer to the problem is that to instantiate XMLHttpRequest you need to use different techniques on different browsers. To be able to instantiate the XMLHttpRequest object, you need to create an abstraction. The abstraction could be a class or a function, but which you choose does not matter. What does matter is that you create an abstraction or implement the Factory pattern.1 In the Factory pattern implementation, the main challenge is to identify which browser is executing the script and then use the appropriate XMLHttpRequest instantiation. The libraries jsolait, Prototype, Yahoo! patterns, and Dojo Toolkit all implement an XMLHttpRequest abstraction that is coded in the same manner. To illustrate the gist of how it is coded consider the following source code, which is from the Yahoo! Design Pattern Library (http://developer.yahoo.com/ypatterns). createXhrObject:function(transactionId) { var obj,http; try { // Instantiates XMLHttpRequest in non-IE browsers and assigns to http. http = new XMLHttpRequest(); // Object literal with http and id properties obj = { conn:http, tId:transactionId }; } catch(e) { for(var i=0; i<this._msxml_progid.length; ++i){ try { // Instantiates XMLHttpRequest for IE and assign to http. http = new ActiveXObject(this._msxml_progid[i]); // Object literal with http and id properties obj = { conn:http, tId:transactionId }; } catch(e){} } } finally { return obj; } }, The instantiations are wrapped in a series of try and catch blocks. A try and catch block in JavaScript is an exception block. The idea behind this form of abstraction is that you try executing some source code, and if it fails you do something else.
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1. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software (New York: Addison-Wesley, 1995), p. 107.
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The following steps are carried out in the source code: 1. Enter the first exception block. 2. Execute the XMLHttpRequest instantiation using the new keyword. 3. If the instantiation works, then the finally block is reached. 4. If the instantiation fails, an exception is generated and the catch block starts a loop. 5. The loop attempts to instantiate the Internet Explorer XMLHttpRequest using different identifiers referenced by the this._msxml_progid data member. 6. If the XMLHttpRequest object cannot be instantiated, an empty catch block captures the exception. 7. Regardless of what happens, the finally block is executed and returns the object instance stored in the obj variable. The main idea of the abstraction is to execute the code, and if the browser does not support the functionality, an exception is generated. The exception is caught and causes a different instantiation sequence to start. What troubles me about this code is that it does not attempt to figure out what is supported, but uses JavaScript exceptions. As the implementation stands, an exception is generated for a majority of cases. This is because at the time of this writing, Internet Explorer has a market share greater than 80%. This means that attempting to instantiate XMLHttpRequest using the new keyword will not work and will generate an exception. The alternate way of instantiating XMLHttpRequest is used by the library Jaxson, as outlined in the books Ajax in Action (Manning, 2005) and Professional Ajax (Wrox, 2006). The technique shown in Ajax in Action is perfect2 and is implemented by Jaxson as follows. Source: /website/ROOT/scripts/communications.js if( window.ActiveXObject) { FactoryXMLHttpRequest = function() { return new ActiveXObject( "Microsoft.XMLHTTP"); } } else if( window.XMLHttpRequest) { FactoryXMLHttpRequest = function() { return new XMLHttpRequest(); } } throw new Error( "Could not instantiate XMLHttpRequest"); The FactoryXMLHttpRequest function is different in that it is assigned as the page is being loaded by the browser. The advantage of assigning the FactoryXMLHttpRequest method is that you don t need to make yet another decision via a decision structure. You are defining the
2. A variation illustrated in Professional Ajax is the technique of checking if the type XMLHttpRequest exists.
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