User Controls and Custom Controls in Visual Studio .NET

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User Controls and Custom Controls
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Fortunately, you can implement custom code generation for a single constituent con trol the Button, in the example while continuing to use the composition technique for all others. The trick is actually simple: instead of instantiating a regular Button object, you create an instance of a custom Button class that you have defined. This cus tom Button class derives its functionality from the real Button, but overrides two methods: in the Render method you output the custom <input> tag with a proper onClick attribute, while in the OnPreRender method you emit the code for the Multi plierExecute script routine. The custom Button class appears as a nested class inside the main MultiplierEx class so that the name Button now refers to the nested custom class instead of the standard Button class in the System.Web.UI.WebControls namespace. This trick enables you to reuse all the code in the main class because the nested Button class shadows the ASP.NET Button class. Not counting the implementation of the EnableClientScript property, which is just a wrapper around an element in the ViewState collection, the only addition to the main MultiplierEx class is a block of three lines in the CreateChildControls routine (empha sized in boldface in the following listing). These statements ensure that the name attribute that ASP.NET generates for the three text fields is a valid variable name in JavaScript. This can be achieved easily by assigning the ClientID property to the ID property.
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<ToolboxData( <{0}:MultiplierEx runat=server></{0}:MultiplierEx> )> _ Public Class MultiplierEx Inherits System.Web.UI.WebControls.WebControl Implements INamingContainer ...(All variable declarations and procedures as in Multiplier class except the ones that follow)... Protected Overrides Sub CreateChildControls() Create all child controls. txtFirst = New TextBox() txtSecond = New TextBox() btnEval = New Button() <= It refers to the nested class!! txtResult = New TextBox() Dim lblAsterisk As New Label() This is necessary to achieve syntactically correct ID properties.
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txtFirst.ID = txtFirst.ClientID
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txtSecond.ID = txtSecond.ClientID
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txtResult.ID = txtResult.ClientID
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...(The remainder of the procedure as in Multiplier control)... End Sub Property EnableClientScript() As Boolean Get Dim objValue As Object = Me.ViewState( EnableClientScript ) If objValue Is Nothing Then Return True The default value Else Return CBool(objValue)
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End If End Get Set(ByVal Value As Boolean) Me.ViewState( EnableClientScript ) = Value End Set End Property The custom Button control Friend Class ButtonEx Inherits System.Web.UI.WebControls.Button Protected Overrides Sub Render(ByVal writer As HtmlTextWriter) If client scripts are disabled, render as usual and exit. If Not IsClientScriptEnabled() Then MyBase.Render(writer) Exit Sub End If Get a reference to the parent control.
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Dim parCtrl As MultiplierEx = DirectCast(Me.Parent, MultiplierEx)
Prepare the code that invokes the script.
Dim scriptInvoke As String = _
String.Format( javascript:MultiplierExecute({0},{1},{2});", _ parCtrl.txtFirst.ClientID, parCtrl.txtSecond.ClientID, _ parCtrl.txtResult.ClientID) Output standard attributes plus the onClick attribute.
writer.AddAttribute( type", button )
writer.AddAttribute( name", Me.ClientID)
writer.AddAttribute( value", Me.Text)
writer.AddAttribute( onClick", scriptInvoke)
Enclose attributes in <input> tag.
writer.RenderBeginTag( input )
writer.RenderEndTag()
End Sub Output the client script code if requested. Protected Overrides Sub OnPreRender(ByVal e As EventArgs) Let the base class do what it needs to do. MyBase.OnPreRender(e) Nothing else to do if client scripts are disabled. If Not IsClientScriptEnabled() Then Exit Sub Prepare the script routine.
Dim s As String
s &= <script language=""javascript""><!-- & ControlChars.CrLf
s &= function MultiplierExecute(txt1, txt2, txt3) { & _
ControlChars.CrLf s &= var op1 = parseFloat( txt1.value ); & ControlChars.CrLf s &= var op2 = parseFloat( txt2.value ); & ControlChars.CrLf s &= txt3.value = (op1 * op2).toString(); & _ ControlChars.CrLf s &= } & ControlChars.CrLf s &= --></script> & ControlChars.CrLf Register the script on the page. Page.RegisterClientScriptBlock( MultiplierExecute", s) End Sub
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User Controls and Custom Controls
Return True if client script support is requested and possible. Private Function IsClientScriptEnabled() As Boolean We need a Try block because accessing the Browser property at design time may throw an exception. Try Return False if DOM version is too low. If Page.Request.Browser.W3CDomVersion.Major < 1 Then Return False End If Return False if EcmaScript version is too low. If Page.Request.Browser.EcmaScriptVersion.CompareTo( _ New Version(1, 2)) < 0 Then Return False End If If all tests passed, return the EnableClientScript property of the parent MultiplierEx control Return DirectCast(Me.Parent, MultiplierEx).EnableClientScript Catch Return False if any error occurs. Return False End Try End Function End Class ButtonEx class End Class MultiplierEx class
Because it s a nested class, the custom Button class is able to access Private variables in its container class, which it must do to read the ClientID property of the txtFirst, txtSecond, and txtResult controls. You can use the IsClientScriptEnabled auxiliary to decide whether the client-side script code should be emitted or not. For example, you must not output script code when the control is running inside the designer, when the request comes from a browser that doesn t support scripting, and of course when the developer using your control has set the EnableClientScript property to False. You can easily reuse this code in all your con trols that rely on client-side scripting. Client-side script programming is a complex topic, and I ve just scratched the surface here. For example, you can use the Page.Request.Browser object to test specific browser features and generate HTML and the client-side script code that take advan tage of these features. You can perform sophisticated validation chores right in the browser to increase your site s scalability even more. You can take advantage of Dynamic HTML to do eye-catching animations. You can open secondary windows for showing help messages or subforms, and so on. You now know enough to create valuable user and custom controls, and it s time to move to the chapter about Web Services, where we ll apply many of the concepts learned in this chapter and the previous chapter. We can do this because Web services are just a different kind of ASP.NET application, as you ll see in a moment.
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