ClickOnce Button Server Control

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Introduction

Back in the days of classic ASP, when we wanted to prevent the end user from repeatedly mashing the submit button, we would disable the button with JavaScript. But with ASP.NET, it's not as simple as that anymore. When you disable any form element client side, you essentially disable it server side as well since its name value pair will not be sent with the form post. The drawback from this is that the button_click event is now rendered useless. It will never be raised!

I have searched the ASP.NET forums over for an elegant solution for this problem and I have seen a couple of nice work arounds but nothing that gives the true effect of rendering the button disabled after being clicked. So I came up with my own. Enter the ClickOnceButton server control.

The server control behaves exactly like the intrinsic button control with the exception of two new properties: DisableAfterClick and DisabledText.

When DisableAfterClick is true, the button will be disabled client side after click. If DisabledText is set, the text of the button will be set to the DisabledText after the button is disabled. Useful if you want to get a li'l visual cue (such as changing the buttons text to 'Processing..').

The main work happens in the AddAttributesToRender event:

    Protected Overrides Sub AddAttributesToRender(ByVal writer As HtmlTextWriter)
        Dim strOnClick As String

        If IsNothing(MyBase.Page) Then
            MyBase.Page.VerifyRenderingInServerForm(Me)
        End If

        writer.AddAttribute(HtmlTextWriterAttribute.Type, "submit")
        writer.AddAttribute(HtmlTextWriterAttribute.Name, MyBase.UniqueID)
        writer.AddAttribute(HtmlTextWriterAttribute.Value, Me.Text)

        If Not IsNothing(MyBase.Page) And Me.CausesValidation_
                    And MyBase.Page.Validators.Count > 0 Then
            If Me.DisableAfterClick Then
                strOnClick = Me.GetClickOnceClientValidateEvent()
            Else
                strOnClick = Me.GetClientValidateEvent()
            End If

            If MyBase.Attributes.Count > 0 And Not _
                    IsNothing(MyBase.Attributes("onclick")) Then
              strOnClick = String.Concat(MyBase.Attributes("onclick"), strOnClick)
              MyBase.Attributes.Remove("onclick")
            End If

            writer.AddAttribute("language", "javascript")
            writer.AddAttribute(HtmlTextWriterAttribute.Onclick, strOnClick)
        ElseIf DisableAfterClick = True Then
            strOnClick = Me.GetOnceClickJavascript()

            If MyBase.Attributes.Count > 0 And Not _
                   IsNothing(MyBase.Attributes("onclick")) Then
              strOnClick = String.Concat(MyBase.Attributes("onclick"), strOnClick)
              MyBase.Attributes.Remove("onclick")
            End If

            writer.AddAttribute("language", "javascript")
            writer.AddAttribute(HtmlTextWriterAttribute.Onclick, strOnClick)
        End If

        MyBase.AddAttributesToRender(writer)
    End Sub

This creates the button programmatically as well as determines if the button causes validation and there are validators on the page or if the button just simply needs to be disabled. You'll notice it calls three distinct different properties. These are what makes the magic happen. They are as follows:

    Friend ReadOnly Property GetClientValidateEvent() As String
        Get
            Return "if (typeof(Page_ClientValidate) _
                      == 'function') Page_ClientValidate(); "
        End Get
    End Property

    Friend ReadOnly Property GetClickOnceClientValidateEvent() As String
        Get
            Return "if (typeof(Page_ClientValidate) == 'function') _
                    { if(Page_ClientValidate()) { " + _
                    GetOnceClickJavascript + " }} else { " + _
                    GetOnceClickJavascript + " }"
        End Get
    End Property

    Friend ReadOnly Property GetOnceClickJavascript() As String
        Get
            Return "document.getElementsByName('" + _
              Me.OnceClickBtnName + "').item(0).setAttribute('name'," + _
              "this.getAttribute('name')); this.disabled = true; " + _
              IIf(DisabledText = String.Empty, String.Empty, _
              "this.value = '" + DisabledText + "';") + _
              "this.form.submit();"
        End Get
    End Property

The property, GetClientValidateEvent(), returns the same JavaScript that the framework normally appends if your button control causes validation and there are validators on the page. This is to handle the scenario where you have the button on a page, set to not be disabled after click, and you have validators.

The property, GetClickOnceClientValidateEvent(), returns a slightly modified version of the above script. This handles the scenario when there are validators on the page and you want the button to be disabled after click. But you want to make sure it only gets disabled if the Page_ClientValidate() returns true or in the case of non IE browsers it'll just disable the button and submit.

The property, GetOnceClickJavascript(), returns the base set of JavaScript that disables handles disabling the button. It also makes sure the button gets sent over in the post so that the button click will be raised. It does this by exchanging the name attribute of a hidden field with the name of the button. The framework just needs to see the name/ID of the control sent over to raise its click event. The hidden field gets registered in the control's OnInit(EventArgs) event.

    Protected Overrides Sub OnInit(ByVal e As EventArgs)
        If Me.DisableAfterClick And Not Me.IsHiddenFieldRegistered Then
            MyBase.Page.RegisterHiddenField(Me.OnceClickBtnName, "")
        End If

        MyBase.OnInit(e)
    End Sub

    Private Function IsHiddenFieldRegistered() As Boolean
        For Each ctl As Control In MyBase.Page.Controls
            If TypeOf ctl Is HtmlControls.HtmlInputHidden Then
                If ctl.ID = Me.OnceClickBtnName Then
                    Return True
                End If
            End If
        Next

        Return False
    End Function

The default constant value of OnceClickBtnValue is __onceClickBtn. The IsHiddenFieldRegistered function just makes sure that the field is registered only once. This allows us to possibly have more than one button on the form that disables after click.

You may be wondering why I didn't just inherit from the System.Web.UI.WebControls.Button class. Well, the reason was the AddAttributesToRender event. I could have just hijacked that event and added the same code I have here and not have had to go through the entire process of creating the properties, functions, etc. The problem was I still needed to call MyBase.AddAttributesToRender which would then finally call the AddAttributesToRender in the System.Web.UI.WebControls.WebControl class. If I inherited from the Button class, I would end up with two onclicks and no control over the JavaScript outputted in the event of validation. This gives me complete control.

To use this in your own projects, simply add the clickoncebutton assembly to your toolbox and drag and drop it onto your webform and you're set! Enjoy!

Note: Tested on IE 6.0 and NS 7.0 with version 1.1 of the .NET framework.