TransformToAncestor doesn't always work

Question

I'm currently working on my first WPF app. I'm getting data from some XML file and based on that file I populate stackpanel with some dynamically created buttons.

later on I need to retrieve X coordinate from specific button for which i use method:

private double GetStationCoordinates(int stationIdx)
    {
        foreach (Button station in StationMap.Children)
        {
            if (stationIdx != Convert.ToInt16(station.Tag)) continue;
            var relativePoint = station.TransformToAncestor(Application.Current.MainWindow).Transform(new Point(0, 0));
            //Console.WriteLine(stationIdx + " " + relativePoint.ToString());
            return relativePoint.X;
        }
        return 0;
    }

And then I use this coordinates to paint some points in other panel under the buttons. It all worked fine until I set my main window property

SizeToContent="WidthAndHeight"

Now - when I paint my points for the first time after launching the app (and after I populate StackPanel with buttons) they all (points) receive the same X coordinate - 11. When i hit Refresh button everything is ok.But the first paint - just after adding buttons is problematic. Same problem occurs when I reload buttons configuration xml file. I'm starting with clearing all the Children in StackPanel and populating it with new ones. After that first painting of points is broken - GetStationCoordinates returns 11 for every button. Is it somehow connected to autosize property of main window?

Answer 1

You're probably trying to get the position of elements that haven't been correctly positioned yet. Defer that code until the view has loaded, using any of these two approaches:

a) Subscribe to the Loaded event of the view and do it there.

private void Window_Loaded(object sender, EventArgs e)
{
    this.Loaded -= Window_Loaded;  // Or else it could get called again later

    int id;
    // do stuff

    GetStationCoordinates(id);
}

b) Put it in an asynchronous call with Dispatcher.BeginInvoke

private void SomeCode()
{
    int id;
    // do other stuff

    Dispatcher.BeginInvoke(new Action(() =>
        {
            GetStationCoordinates(id);
        }),
        DispatcherPriority.Loaded);
}