Popup.axaml

XAML

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="LosslessAdapterManager2.Views.Popup">
  <StackPanel x:Name="MainStackPanel" Orientation="Vertical" Margin="10 10 10 10">
    <SelectableTextBlock x:Name="Contents" TextWrapping="Wrap" HorizontalAlignment="Center" VerticalAlignment="Center"/>
    <StackPanel x:Name="Buttons" HorizontalAlignment="Right" Margin="0 10 0 0" Orientation="Horizontal"/>
  </StackPanel>
</UserControl>

Codebehind

using Avalonia;
using Avalonia.Controls;
using Avalonia.Input;
using Avalonia.Threading;
using LosslessAdapterManager2.Utils;
using Material.Icons;
using System.Linq;
using System.Threading.Tasks;

namespace LosslessAdapterManager2.Views
{
    public class PopupOptions
    {
        public bool topMost = false;
        public string? okButtonText = null;
        public MaterialIconKind? icon = null;
        public bool yesNo = false;
        public bool noButton = false;
    }

    public partial class Popup : Window
    {
        public bool resultIsYes = false;
        private Popup(string title, string contents, PopupOptions options)
        {
            InitializeComponent();
            Topmost = options.topMost;
            Title = title;
            Icon = WindowIconUtils.WindowIconFromMaterialIcon(options.icon != null ? options.icon.Value : MaterialIconKind.Warning);

            var _contents = this.FindControl<TextBlock>("Contents");
            if (_contents != null) {
                _contents.Text = contents;
            }

            var buttons = this.FindControl<StackPanel>("Buttons");
            if (buttons != null)
            {
                if (options.noButton)
                {
                    KeyDown += (sender, e) => {
                        if ((new[] { Key.Space, Key.Enter, Key.Escape }).Any(x => x == e.Key))
                            Close();
                    };
                }
                else if (options.yesNo)
                {
                    var noButton = new Button() { Content = "No", Margin = new Thickness(10, 0, 0, 0), MinWidth = 60, HorizontalContentAlignment = Avalonia.Layout.HorizontalAlignment.Center };
                    noButton.Click += (sender, e) =>
                    {
                        resultIsYes = false;
                        Close();
                    };
                    buttons.Children.Add(noButton);

                    var yesButton = new Button() { Content = "Yes", Margin = new Thickness(10, 0, 0, 0), MinWidth = 60, HorizontalContentAlignment = Avalonia.Layout.HorizontalAlignment.Center };
                    yesButton.Click += (sender, e) =>
                    {
                        resultIsYes = true;
                        Close();
                    };
                    buttons.Children.Add(yesButton);

                    KeyDown += (sender, e) => {
                        if ((new[] { Key.Space, Key.Enter }).Any(x => x == e.Key))
                        {
                            resultIsYes = true;
                            Close();
                        }
                        if ((new[] { Key.Escape }).Any(x => x == e.Key))
                        {
                            resultIsYes = false;
                            Close();
                        }
                    };
                }
                else
                {
                    var okButton = new Button() { Content = options.okButtonText == null ? "OK" : options.okButtonText, MinWidth = 60, HorizontalContentAlignment = Avalonia.Layout.HorizontalAlignment.Center };
                    okButton.Click += (sender, e) => Close();
                    buttons.Children.Add(okButton);

                    KeyDown += (sender, e) => {
                        if ((new[] { Key.Space, Key.Enter, Key.Escape, Key.Right, Key.Down }).Any(x => x == e.Key))
                            Close();
                    };
                }

                var mainStackPanel = this.FindControl<StackPanel>("MainStackPanel");
                if (mainStackPanel != null)
                {
                    //mainStackPanel.MaxWidth = 1000;
                    mainStackPanel.Measure(new Size(1000, double.PositiveInfinity));
                    Size desiredSize = mainStackPanel.DesiredSize;
                    Width = desiredSize.Width+20;
                    Height = desiredSize.Height;
                }
            }

            /*var okButton = this.FindControl<Button>("OKButton");
            if (okButton != null)
            {
                okButton.Click += (sender, e) => Close();
                if (options.okButtonText != null) okButton.Content = options.okButtonText;
            }*/
        }

        public static Task<bool> Show(string title, string contents)
        {
            return Show(title, contents, new PopupOptions());
        }

        public static Task<bool> Show(string title, string contents, PopupOptions options)
        {
            return Dispatcher.UIThread.InvokeAsync(() =>
            {
                var window = new Popup(title, contents, options);
                var tcs = new TaskCompletionSource<bool>();

                window.Closed += (sender, e) =>
                {
                    tcs.TrySetResult(window.resultIsYes);
                };

                window.Show();

                return tcs.Task;
            });
        }
    }
}