create and print barcode c# HTML and JavaScript Code on the Client in Visual Studio .NET

Paint QR in Visual Studio .NET HTML and JavaScript Code on the Client

HTML and JavaScript Code on the Client
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Because the real action occurs on the client in this case, let s see the HTML and script code that the MultiplierEx class has to generate first. Only when it s clear what we need
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User Controls and Custom Controls
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to produce will we have a look at the source code of the class. This is the text pro duced by an HTML form that contains an instance of the MultiplierEx control:
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<form name="Form1 method="post action="WebForm1.aspx id="Form1"> <input type="hidden name="__VIEWSTATE value="dDw2MzYwNDk3MTk7Oz4= /> <script language="javascript"><!- function MultiplierExecute(txt1, txt2, txt3) { var op1 = parseFloat( txt1.value ); var op2 = parseFloat( txt2.value ); txt3.value = (op1 * op2).toString(); } --></script> <input name="MultiplierEx1:_ctl0 type="text id="MultiplierEx1__ctl0" style="width:82px; /> <span> * </span> <input name="MultiplierEx1:_ctl2 type="text id="MultiplierEx1__ctl2 style="width:82px; /> <input type="button name="MultiplierEx1__ctl3 value= = onClick="javascript:MultiplierExecute(MultiplierEx1__ctl0, MultiplierEx1__ctl2, MultiplierEx1__ctl4); /> <input name="MultiplierEx1:_ctl4 type="text readonly="readonly id="MultiplierEx1__ctl4 style="width:82px; /></P> </form>
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As you see, the key to client-side functionality of the control is the onClick attribute of the button control, which cancels the default submit action and redirects the execution to the MultiplierExecute JavaScript procedure. This procedure receives a reference to the three text fields, so it can multiply the value of the first control by the value of the second con trol and store the result in the third control. Before you can correctly generate this HTML from inside the MultiplierEx class, you must solve the following problems:
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How do you generate the MultiplierExecute procedure and send it to the browser How do you ensure that this procedure appears only once in the page, even if the form contains multiple MultiplierEx controls How do you force ASP.NET to generate the id attribute for the three text fields
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The second point is important: without the id attribute, the JavaScript code wouldn t have a valid reference for accessing the controls values. Unfortunately, you can t use the name attribute that ASP.NET generates for this purpose because the default name attribute string contains a colon, which is an invalid character inside JavaScript variables.
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The MultiplierEx Custom Control
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Let s see how to address the first issue, the one related to the generation of the clientside code block. The Page class exposes several methods that help developers imple ment client-side functionality in custom controls. (See Table 28-1.) The Registerxxxx methods can add pieces of script code in specific positions on the page, and above all they ensure that the same piece of script is sent to the client only once, even if there
Part VI:
Internet Applications
are multiple instances of the same control on the page. Needless to say, you should select an unlikely name for the key value passed to the first argument, in the hope that no other custom control from a different author uses it. The best place to output pieces of client-side scripts is in the OnPreRender method. This method is defined in the base Control class and is called by ASP.NET immediately before it calls the Render method of all the controls on the form.
Table 28-1 Members of the Page Class That Are Related to Client-Side
Script Generation
Syntax RegisterClientScriptBlock(key,script) Description Emits a block of script code immediately after the <form> tag. It registers the code with a key so that subsequent requests with the same key are ignored. Returns True if a script block with a given key has been registered for this page. Emits a block of script code at the bottom of the page so that the elements it references are guaran teed to exist when the script runs. Returns True if a script block with a given key has been registered as a start-up script for this page. Creates and registers a hidden field on the page. The field is accessible to client script and to serverside code as postback data. (This method is useful when a server control must make a value visible to client scripts.) Associates a script code with the onSubmit attribute of the form so that the code runs when the form is submitted. The code can be an inline statement or a call to a script routine registered separately. Registers the name of a client-side array that will be declared in the page and adds the specified value to the array. (This method is useful when all the instances of a given custom control must be processed together, as is the case with validation controls.)
IsClientScriptBlockRegistered(key) RegisterStartupScript(key,script)
IsStartupScriptRegistered(key) RegisterHiddenField (fieldname,initialvalue)
RegisterOnSubmitStatement (key,script)
RegisterArrayDeclaration (arrayname,value)
The design of the MultiplierEx class poses an interesting design problem that comes up regularly when you re attaching client-side script functionality to composite controls. On the one hand, you must override the custom control s Render method to insert the correct value for the onClick attribute of the btnEval constituent control. On the other hand, overriding the Render method means that you give up the convenience offered by composite controls, revert to using the IPostBackDataHandler interface to trap postbacks, manage the state of individual constituent controls manually, and so on.
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