Question

I’m a bit stuck with a problem. I feel like the solution should be fairly straight forward but I’m completely out of ideas for some reason.

Here’s the problem. I’ve got a user control with a couple of buttons. Think of them as ‘On’ and ‘Off’. When either button is clicked an async method is called. If the method is successful an event is fired. Within the event I want to update the enabled property of the two buttons so that only a single button is clickable at any one time. The problem is that any changes I make to the properties are not shown on screen because the postback is already complete. I tried wrapping the buttons in an UpdatePanel but I get an “Update method can only be called on UpdatePanel with ID ‘xxxx’ before Render’ error. I understand why the problem occurs but I can’t think of a solution. Help!

Ideally what I’d like to do is simply call a method within the event that will update the UI, but I don’t know if that’s possible.

EDIT

Here’s some example code. I can’t post the exact code, but this explains the problem:

Partial Class ExampleUserControl
    Inherits Inherits System.Web.UI.UserControl

    Private WithEvents obj As MyObj

    Private Sub btnOn_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnOn.Click
        obj = New MyObj()
        obj.On() ' Calls OnEvent on success
    End Sub

    Private Sub btnOff_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnOff.Click
        obj = New MyObj()
        obj.Off() ' Calls OffEvent on success
    End Sub

    Private Sub On() Handles obj.OnEvent
        btnOn.Enabled = False
        btnOff.Enabled = True
        'updatePanel.Update() This throws an exception
    End Sub

    Private Sub Off() Handles obj.OffEvent
        btnOn.Enabled = True
        btnOff.Enabled = False
        'updatePanel.Update() This throws an exception
    End Sub

End Class

Answer 1

You can’t just “push” html to the browser, updatepanel or not. You can only react to a “pull” from the browser.

So you need to have the update panel refresh itself every x seconds. Then you can check the server side status and update the panel accordingly.

See for instance: http://ajax.net-tutorials.com/controls/timer-control/

Don’t forget to switch off the timer when the action is done.