java qr code generator example Designing the client/server interactions in Java

Making QR Code in Java Designing the client/server interactions

9.2.2 Designing the client/server interactions
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The FillTerritory() function s main purpose is to gather the information that is needed to send a request to the server. This information includes the selected option from the first list, the name of the form, and the name of the second selection list. With this information we can use the Ajax functions in our JavaScript library to send a request to the server. The first thing we need to do is add our Ajax functionality. The code needed to link to the external JavaScript file, net.js, which defines the ContentLoader object, is trivial. Just add this between the head tags of your HTML document:
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<script type="text/javascript" src="net.js"></script>
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The ContentLoader object does all of the work of determining how to send a request to the server, hiding any browser-specific code behind the easy-to-use wrapper object that we introduced in chapter 3. It allows us to send and retrieve the data from the server without refreshing the page. With the Ajax functionality added, we are able to build the function FillTerritory(), shown in listing 9.2, which we also add between the head tags of our document.
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Listing 9.2 The function FillTerritory() initializes the Ajax request.
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<script type="text/javascript"> function FillTerritory(oElem,oTarget){ Obtain value from var strValue = oElem.options[ selection list oElem.selectedIndex].value; var url = "DoubleComboXML.aspx"; Set the target URL var strParams = "q=" + strValue + Build the "&f=" + oTarget.form.name + parameter "&e=" + oTarget.name; string
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var loader1 = new net.ContentLoader(url,FillDropDown,null, "POST",strParams); }
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Initiate the content loader
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The FillTerritory() function accepts two parameters, passed in this case from the onchange event handler on the first selection list. These are references to the first and second select elements. b We access the value that the user selected in the first list. c We set the URL of our target server-side script. d We then build the parameters to be sent to the server by creating a string that has the same type of syntax as a querystring, using an ampersand to separate each name-value pair. For this example we are sending the value representing the selected region as q, the name of the form as f, and the name of the second select as e. The serverside code will use the selected region value to query the database, and it will send the names of the form and the select element back to the client in its XML response document. The client will use that information to determine which form and control to update. Once the parameter string is built, the only thing left is to initiate the Ajax process. e To start the process, we call the ContentLoader() constructor, and pass in the target URL, the function to be called when the server s response is received, the error-handler function, the HTTP method to use, and the parameters to be sent. In this case, the FillDropDown() function will be called when the data is returned from the server, we will rely on ContentLoader s default error-handler function, and we are using a POST request. At this point, the ContentLoader will wait for the server to return an XML document. The client-side code continues in section 9.4, but first, the server has some work to do.
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9.3 Implementing the server: VB .NET
The server-side code needs to retrieve the territories belonging to the user s selected region from the database, and return them to the client in an XML document. The result set from the SQL query is used to create an XML document that is returned to the client side. Figure 9.6 shows the flow of the serverside process. The server-side code is invoked by the request sent from the client-side ContentLoader object. The server-side code first retrieves the value of the request parameter q, representing the selected region. The value of q is used to create a
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