Add project files.

Views/Windows/MainWindow.xaml.cs

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+// This Source Code Form is subject to the terms of the MIT License.
+// If a copy of the MIT was not distributed with this file, You can obtain one at https://opensource.org/licenses/MIT.
+// Copyright (C) Leszek Pomianowski and WPF UI Contributors.
+// All Rights Reserved.
+
+using DownloadManager.ViewModels.Windows;
+using Wpf.Ui;
+using Wpf.Ui.Appearance;
+using Wpf.Ui.Controls;
+
+namespace DownloadManager.Views.Windows;
+public partial class MainWindow : INavigationWindow
+{
+    public MainWindowViewModel ViewModel
+    {
+        get;
+    }
+
+    public MainWindow(
+        MainWindowViewModel viewModel,
+        IPageService pageService,
+        INavigationService navigationService
+    )
+    {
+        ViewModel = viewModel;
+        DataContext = this;
+
+        SystemThemeWatcher.Watch(this);
+
+        InitializeComponent();
+        SetPageService(pageService);
+
+        navigationService.SetNavigationControl(RootNavigation);
+    }
+
+    #region INavigationWindow methods
+
+    public INavigationView GetNavigation() => RootNavigation;
+
+    public bool Navigate(Type pageType) => RootNavigation.Navigate(pageType);
+
+    public void SetPageService(IPageService pageService) => RootNavigation.SetPageService(pageService);
+
+    public void ShowWindow() => Show();
+
+    public void CloseWindow() => Close();
+
+    #endregion INavigationWindow methods
+
+    /// <summary>
+    /// Raises the closed event.
+    /// </summary>
+    protected override void OnClosed(EventArgs e)
+    {
+        base.OnClosed(e);
+
+        // Make sure that closing this window will begin the process of closing the application.
+        Application.Current.Shutdown();
+    }
+
+    INavigationView INavigationWindow.GetNavigation()
+    {
+        throw new NotImplementedException();
+    }
+
+    public void SetServiceProvider(IServiceProvider serviceProvider)
+    {
+        throw new NotImplementedException();
+    }
+}