allow use of custom window for hosting file dialogs

src/Avalonia.Dialogs/Internal/ManagedFileChooser.xaml

@@ -4,7 +4,7 @@
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:dialogs="clr-namespace:Avalonia.Dialogs.Internal"
              xmlns:internal="clr-namespace:Avalonia.Dialogs.Internal"
-             x:Class="Avalonia.Dialogs.Internal.ManagedFileChooser">
+             x:Class="Avalonia.Dialogs.Internal.ManagedFileChooser" Margin="12 0 12 6">
     <UserControl.Resources>
         <internal:FileSizeStringConverter x:Key="FileSizeConverter" />
         <DrawingGroup x:Key="LevelUp">

src/Avalonia.Dialogs/Internal/ManagedFileDialog.xaml

@@ -1,10 +0,0 @@
-<Window xmlns="https://github.com/avaloniaui"
-        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-        xmlns:dialogs="clr-namespace:Avalonia.Dialogs.Internal"
-        xmlns:internal="clr-namespace:Avalonia.Dialogs.Internal"
-        x:Class="Avalonia.Dialogs.Internal.ManagedFileDialog"
-        Title="{Binding Title}" Width="1100" Height="500" MinWidth="1100" MinHeight="500"
-                      FontFamily="{DynamicResource UiFont}" FontSize="14"
-                      Foreground="{DynamicResource ThemeForegroundBrush}">
-  <internal:ManagedFileChooser Margin="12 0 12 6"/>
-</Window>

src/Avalonia.Dialogs/Internal/ManagedFileDialog.xaml.cs

@@ -1,20 +0,0 @@
-using System;
-using System.Runtime.InteropServices;
-using Avalonia;
-using Avalonia.Controls;
-using Avalonia.Markup.Xaml;
-using Avalonia.Media;
-
-namespace Avalonia.Dialogs.Internal
-{
-	class ManagedFileDialog : Window
-	{
-		public ManagedFileDialog()
-		{
-			AvaloniaXamlLoader.Load(this);
-#if DEBUG
-			this.AttachDevTools();
-#endif
-		}
-	}
-}

src/Avalonia.Dialogs/ManagedFileDialogExtensions.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Linq;
 using System.Threading.Tasks;
 using Avalonia;
@@ -10,46 +11,55 @@ namespace Avalonia.Dialogs
 {
 	public static class ManagedFileDialogExtensions
 	{
-		class ManagedSystemDialogImpl : ISystemDialogImpl
-		{
-			async Task<string[]> Show(SystemDialog d, IWindowImpl parent)
-			{
-				var model = new ManagedFileChooserViewModel((FileSystemDialog)d);
-
-				var dialog = new ManagedFileDialog
-				{
-					DataContext = model
-				};
-
-				string[] result = null;
-				model.CompleteRequested += items =>
-				{
-					result = items;
-					dialog.Close();
-				};
-				model.CancelRequested += dialog.Close;
-
-				await dialog.ShowDialog<object>(parent);
-				return result;
-			}
-
-			public async Task<string[]> ShowFileDialogAsync(FileDialog dialog, IWindowImpl parent)
-			{
-				return await Show(dialog, parent);
-			}
-
-			public async Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
-			{
-				return (await Show(dialog, parent))?.FirstOrDefault();
-			}
-		}
+        class CustomWindowManagedSystemDialogImpl<T> : ISystemDialogImpl where T : Window, new()
+        {
+            async Task<string[]> Show(SystemDialog d, IWindowImpl parent)
+            {
+                var model = new ManagedFileChooserViewModel((FileSystemDialog)d);
+
+                var dialog = new T
+                {
+                    Content = new ManagedFileChooser(),
+                    DataContext = model
+                };
+
+                string[] result = null;
+                model.CompleteRequested += items =>
+                {
+                    result = items;
+                    dialog.Close();
+                };
+                model.CancelRequested += dialog.Close;
 
-		public static TAppBuilder UseManagedSystemDialogs<TAppBuilder>(this TAppBuilder builder)
+                await dialog.ShowDialog<object>(parent);
+                return result;
+            }
+
+            public async Task<string[]> ShowFileDialogAsync(FileDialog dialog, IWindowImpl parent)
+            {
+                return await Show(dialog, parent);
+            }
+
+            public async Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
+            {
+                return (await Show(dialog, parent))?.FirstOrDefault();
+            }
+        }
+
+        public static TAppBuilder UseManagedSystemDialogs<TAppBuilder>(this TAppBuilder builder)
 			where TAppBuilder : AppBuilderBase<TAppBuilder>, new()
 		{
 			builder.AfterSetup(_ =>
-				AvaloniaLocator.CurrentMutable.Bind<ISystemDialogImpl>().ToSingleton<ManagedSystemDialogImpl>());
+				AvaloniaLocator.CurrentMutable.Bind<ISystemDialogImpl>().ToSingleton<CustomWindowManagedSystemDialogImpl<Window>>());
 			return builder;
 		}
-	}
+
+        public static TAppBuilder UseManagedSystemDialogs<TAppBuilder, TWindow>(this TAppBuilder builder)
+            where TAppBuilder : AppBuilderBase<TAppBuilder>, new() where TWindow : Window, new()
+        {
+            builder.AfterSetup(_ =>
+                AvaloniaLocator.CurrentMutable.Bind<ISystemDialogImpl>().ToSingleton<CustomWindowManagedSystemDialogImpl<TWindow>>());
+            return builder;
+        }
+    }
 }