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The ICallbackEventHandler interface has only one method, RaiseCallbackEvent:
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public virtual string RaiseCallbackEvent(string eventArgument) { // eventArgument indicates the parameter(s) passed // TO DO: retrieve the server-side values based on the // parameter(s) and pack everything into a return string }
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The following page provides a full demonstration of script callbacks. The page shows a drop-down list populated with employee names taken from the Northwind database. The More Info button triggers a piece of JavaScript code that retrieves the currently selected employee and passes that ID to the serverside code.
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Creating an ASP.NET 2.0 Application
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<%@ page language="C# %> <%@ import namespace="System.Data %> <%@ implements interface="System.Web.UI.ICallbackEventHandler %> <script language="javascript"> function UpdateEmployeeViewHandler(result, context) { // The result is presented as a comma-separated string var obj = result.split( , ); e_ID.innerHTML = obj[0]; e_FName.innerHTML = obj[1]; e_LName.innerHTML = obj[2]; e_Title.innerHTML = obj[3]; e_Country.innerHTML = obj[4]; e_Notes.innerHTML = obj[5]; } </script> <script runat="server"> public virtual string RaiseCallbackEvent(string eventArgument) { // Get more info about the specified employee int empID = Convert.ToInt32 (eventArgument); EmployeesManager empMan = new EmployeesManager(); EmployeeInfo emp = empMan.GetEmployeeDetails(empID); // Pack the data into a string string[] buf = new string[6]; buf[0] = emp.ID.ToString (); buf[1] = emp.FirstName; buf[2] = emp.LastName; buf[3] = emp.Title; buf[4] = emp.Country; buf[5] = emp.Notes; return String.Join( ,", buf); } void Page_Load (Object sender, EventArgs e) { // Populate the drop-down list EmployeesManager empMan = new EmployeesManager(); DataTable dt = empMan.GetListOfNames(); cboEmployees.DataSource = dt; cboEmployees.DataTextField = lastname"; cboEmployees.DataValueField = employeeid"; cboEmployees.DataBind(); // Prepare the Javascript function to call string callbackRef = GetCallbackEventReference(this, document.all[ cboEmployees ].value", UpdateEmployeeViewHandler", null", null ); // Bind the callback to a client button buttonTrigger.Attributes["onclick"] =
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Part I ASP.NET Essentials
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String.Format( javascript:{0}", callbackRef); } </script> <html> <body> <form runat="server"> <asp:dropdownlist id="cboEmployees runat="server /> <button runat="server id="buttonTrigger">More Info</button><br> <table> <tr><td><b>ID</b></td><td><span id="e_ID /></td></tr> <tr><td><b>Name</b></td><td><span id="e_FName /></td></tr> <tr><td><b>Last Name</b></td><td><span id="e_LName /></td></tr> <tr><td><b>Title</b></td><td><span id="e_Title /></td></tr> <tr><td><b>Country</b></td><td><span id="e_Country /></td></tr> <tr><td><b>Notes</b></td><td><i><span id="e_Notes /></i></td></tr> </table> </form> </body> </html>
The page posts back to the server and gets initialized as usual. The Init and Load events are fired, and the IsCallback property is set to true. RaiseCallbackEvent is invoked when the load phase is completed. After that, the request ends without entering the rendering phase. A client-side system component the Callback Manager controls the interaction between the page and the server. The Callback Manager is a script library sent to the client by ASP.NET. It is responsible for creating a request to the server, and will fire the appropriate server-side event. It also parses the response and passes the appropriate data to the specified JavaScript return callback.
Should you write the call to WebForm_DoCallback yourself Should you know about its details Not necessarily. The GetCallbackEventReference method on the Page class returns the script string that starts the callback. You prefix this string with javascript: and attach it to the onclick attribute of a client button. Alternatively, you can wrap the string in another JavaScript function and inject it into the page. The whole process of generating the client script code for starting the callback is a bit more complicated if you have parameters to pass. In this case, it s easier if you use WebForm_DoCallback directly. However, a runtime error occurs if no call to GetCallbackEventReference is made. To ensure that all required references are inserted, you insert a fake call to the method for initialization purposes. GetCallbackEventReference(this, arg", callback", null", null );
Note
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1
Creating an ASP.NET 2.0 Application
Setting the Focus
A useful feature that ASP.NET 1.x lacks is the ability to assign the input focus to a particular control when the page is displayed. In ASP.NET 2.0, the Page class is purposely endowed with the SetFocus method. The following code shows how to set the focus to a TextBox control named txtLastName:
void Page_Load(object sender, EventArgs e) { if (!IsPostBack) SetFocus( txtLastName ); }
The SetFocus method caches the ID of the control and forces the Page class to generate ad hoc script code when the page is rendered. The following code shows an alternative approach to setting the input focus:
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