Question

I’m trying to write an extension method that will allow me to set focus on a Control. I’ve written the method below which works fine, however if the control is already loaded then obviously hooking up the Loaded event isn’t going to be any use – I also need a way to check if the control has been loaded so I can simply run the Focus() code without hooking up the event.

Is there any way to simulate an IsLoaded property on a control?

public static void SetFocus(this Control control)
{
    // return if the control is not visible
    if (control.Visibility == Visibility.Collapsed) { return; }

    control.Loaded += (sender, routedeventArgs) =>
        {
            // focus the Silverlight plugin first
            System.Windows.Browser.HtmlPage.Plugin.Focus();

            control.IsTabStop = true; // required to allow Focus
            control.Focus();

            if (control is TextBox)
            {
                ((TextBox)control).SelectAll();
            }
        };
}

EDIT: As per ColinE’s answer below, I implemented it like this:

public static void SetFocus(this Control control)
{
    // return if the control is not visible
    if (control.Visibility == Visibility.Collapsed) { return; }

    if (control.Descendants().Count() > 0)
    {
        // already loaded, just set focus and return
        SetFocusDelegate(control);
        return;
    }

    // not loaded, wait for load before setting focus
    control.Loaded += (sender, routedeventArgs) =>
    {
        SetFocusDelegate(control);
    };
}

public static void SetFocusDelegate(Control control)
{
    // focus the Silverlight plugin first
    System.Windows.Browser.HtmlPage.Plugin.Focus();

    control.IsTabStop = true; // required to allow Focus
    control.Focus();

    if (control is TextBox)
    {
        ((TextBox)control).SelectAll();
    }
}

Answer 1

If a control has not been loaded, then the various elements within its template will not have been constructed. Using Linq-to-VisualTree you can confirm this:

Debug.WriteLine(control.Descendants().Count());

control.Loaded += (s, e) =>
  {
    Debug.WriteLine(foo.Descendants().Count());
  };

The first debug output should show ‘0’, the second will be a number >0 which indicates the number of child elements of the control once the template has been applied.