c# thoughtworks qrcode Working with Client-Side Scripting, AJAX, and jQuery in Visual C#

Creation QR-Code in Visual C# Working with Client-Side Scripting, AJAX, and jQuery

ChAPTER 9
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Working with Client-Side Scripting, AJAX, and jQuery
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Let s see how to implement this example. First, you would need to implement the ICallbackEventHandler interface for the code-behind page. This code might look as follows.
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Sample of Visual Basic Code Partial Class _Default Inherits System.Web.UI.Page Implements System.Web.UI.ICallbackEventHandler Sample of C# Code public partial class Default : System.Web.UI.Page, System.Web.UI.ICallbackEventHandler
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Next, you would implement the RaiseCallbackEvent. This event is called by the client during a callback. It receives any event arguments from the client as a string. For this example, these event arguments represent the selected choice in the DropDownList. Suppose you want to store these event arguments in a class-level variable. Your code should look as follows.
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Sample of Visual Basic Code Dim _callbackArgs As String Public Sub RaiseCallbackEvent(ByVal eventArgument As String) _ Implements System.Web.UI.ICallbackEventHandler.RaiseCallbackEvent _callbackArgs = eventArgument End Sub Sample of C# Code string _callbackArgs; public void RaiseCallbackEvent(string eventArgument) { _callbackArgs = eventArgument; }
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You also would have to implement the GetCallbackResult method. This method returns a result back to the client following the server-side processing. In our example, we simply return the user s selected item from the DropDownList. In a real-world scenario, you might do some processing, calling the database, and so forth. The code for this method looks as follows.
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Sample of Visual Basic Code Public Function GetCallbackResult() As String _ Implements System.Web.UI.ICallbackEventHandler.GetCallbackResult Return _callbackArgs End Function Sample of C# Code public string GetCallbackResult() { return _callbackArgs; }
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Lesson 2: Creating Client Scripts with the Microsoft AJAX Library
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The next step is to add code to the page load event that registers the various client scripts. First, you register the client script that will be called by the server following the server-side processing. In this example, this script is called ClientCallbackFunction. This function is defined in the markup for the page. Next, you create a JavaScript function that is used to call the serverside code. In the example, this function is called MyServerCall. It is referenced in the page markup to make the server-side call. You register this function with the ClientScriptManager by using the RegisterClientScriptBlock method, as shown here.
Sample of Visual Basic Code Protected Sub Page_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Me.Load 'register the name of the client-side function that will ' be called by the server Dim callbackRef As String = Page.ClientScript.GetCallbackEventReference( _ Me, "args", "ClientCallbackFunction", "") 'define a function used by the client to call the server Dim callbackScript As String = String.Format( _ "function MyServerCall(args) {{0};}, ", callbackRef) 'register the client function with the page Page.ClientScript.RegisterClientScriptBlock(Me.GetType(), _ "MyServerCall", callbackScript, True) End Sub Sample of C# Code protected void Page_Load(object sender, EventArgs e) { //register the name of the client-side function that will // be called by the server string callbackRef = Page.ClientScript.GetCallbackEventReference( this, "args", "ClientCallbackFunction", ""); //define a function used by the client to call the server string callbackScript = String.Format( "function MyServerCall(args) {{0};}, ", callbackRef); //register the client function with the page Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "MyServerCall", callbackScript, true); }
The last step in this example is to write the page markup. Here you define a client script that receives the callback from the server. Recall that this script is named ClientCallbackFunction and takes a string (args) as a parameter. In this example, this function writes out the args value to a Label control. You also must initiate the call to the server from the client. For this, recall that you need to call the MyServerCall method. In the example, this method is called as a result of
ChAPTER 9
Working with Client-Side Scripting, AJAX, and jQuery
the OnChange event for the DropDownList control. The example passes the selected item as a parameter. These page markup items look as follows.
<script type="text/javascript"> function ClientCallbackFunction(args) { LabelMessage.innerText = args; } </script> <asp:DropDownList ID="DropDownListChoice" runat="server" OnChange="MyServerCall(DropDownListChoice.value)"> <asp:ListItem>Choice 1</asp:ListItem> <asp:ListItem>Choice 2</asp:ListItem> <asp:ListItem>Choice 3</asp:ListItem> </asp:DropDownList> <br /><br /> <asp:Label ID="LabelMessage" runat="server"></asp:Label>
When the page is run, a user selects a choice from the drop-down list. This calls the MyServerCall client-side method, which initiates a call to the server. The RaiseCallbackEvent method is then called on the server; this accepts the event arguments (the user s choice). The server processes the request and calls the GetCallbackResult method. The results are then passed back to the client. When back on the client, the ClientCallbackFunction JavaScript method is executed, and the result (the user s choice) is shown to the user.
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