c# - How to center a popup in window (Windows store apps)

Question

I have a custom popup (as a user control) which I load programatically. I'm not able to center it on x axis, only on vertical. The popup is not added onto an xaml file, but it is added on the root window.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;
using System.Windows;
using Windows.UI.Core;

// The User Control item template is documented at http://go.microsoft.com/fwlink/?LinkId=234236

namespace QSTLibrary.WIN8.Tools
{
    public sealed partial class CustomProgressRingPopup : UserControl
    {

        public CustomProgressRingPopup()
        {
            this.InitializeComponent();
        }

        public string Text
        {
            get { return (string)GetValue(TextProperty); }
            set { SetValue(TextProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Text.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty TextProperty = DependencyProperty.Register(
                        "Text", 
                        typeof(string), 
                        typeof(CustomProgressRingPopup),
                        new PropertyMetadata("", OnTextChanged));


        public void OpenPopup()
        {
            this.ParentPopup.IsOpen = true;
        }

        public void ClosePopup()
        {
            this.ParentPopup.IsOpen = false;
        }

        private static void OnTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var instance = d as CustomProgressRingPopup;
            var newValue = e.NewValue as string;
            if (instance != null && newValue != null)
            {
                instance.CustomTextBlock.Text = newValue;
            }
        }

        private void OnPopupLoaded(object sender, RoutedEventArgs e)
        {
            this.ParentPopup.HorizontalOffset = (Window.Current.Bounds.Width - gdChild.ActualWidth) / 2;
            this.ParentPopup.VerticalOffset = (Window.Current.Bounds.Height - gdChild.ActualHeight) / 2;
        }
    }
}

userControl xaml:

<UserControl
    x:Class="QSTLibrary.WIN8.Tools.CustomProgressRingPopup"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:QSTLibrary.WIN8.Tools"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300"
    d:DesignWidth="400">

    <Popup x:Name="ParentPopup" HorizontalAlignment="Center" VerticalAlignment="Center" Loaded="OnPopupLoaded">
        <Grid x:Name="gdChild" Height="auto" Width="auto" Background="Blue" Margin="20">
            <Grid.RowDefinitions>
                <RowDefinition/>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <ProgressRing x:Name="CustomProgressRing" Height="40" Width="40" IsActive="true" Grid.Column="0" Margin="20"/>
            <TextBlock x:Name="CustomTextBlock" Height="auto" Width="auto" FontSize="25" Grid.Column="1" Margin="20"/>
        </Grid>
    </Popup>    
</UserControl>

Here is how I use it external:

_loginProgressRingPopup.Text = "Logging in";
_loginProgressRingPopup.OpenPopup();

Answer 1

I've added the LayoutUpdate callback of the popup in the cs file and works now.

private void OnLayoutUpdated(object sender, object e)
        {
            if(gdChild.ActualWidth == 0 && gdChild.ActualHeight == 0)
            {
                return;
            }

            double ActualHorizontalOffset = this.ParentPopup.HorizontalOffset;
            double ActualVerticalOffset = this.ParentPopup.VerticalOffset;

            double NewHorizontalOffset = (Window.Current.Bounds.Width - gdChild.ActualWidth) / 2;
            double NewVerticalOffset = (Window.Current.Bounds.Height - gdChild.ActualHeight) / 2;

            if (ActualHorizontalOffset != NewHorizontalOffset || ActualVerticalOffset != NewVerticalOffset)
            {
                this.ParentPopup.HorizontalOffset = NewHorizontalOffset;
                this.ParentPopup.VerticalOffset = NewVerticalOffset;
            }
        }

and the popup from XAML:

<Popup x:Name="ParentPopup" LayoutUpdated="OnLayoutUpdated">