Began porting my MultiPageViewModel from the Windows Forms library to the Avalonia library; updated copyright statement for the year 2026

Nuclex.Avalonia (netstandard-2.0).csproj

@@ -23,8 +23,8 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Avalonia" Version="11.3.7" />
+    <PackageReference Include="Avalonia" Version="11.3.14" />
-    <PackageReference Include="MessageBox.Avalonia" Version="3.2.0" />
+    <PackageReference Include="MessageBox.Avalonia" Version="3.3.1.1" />
     <PackageReference Include="Nuclex.Foundation" Version="1.3.0" />
     <PackageReference Include="Nullable" Version="1.3.1">
       <PrivateAssets>all</PrivateAssets>

Properties/AssemblyInfo.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -18,7 +18,6 @@ limitations under the License.
 #endregion // Apache License 2.0
 
 using System.Reflection;
-using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
 // General Information about an assembly is controlled through the following 

Source/AutoBinding/ConventionBinder.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/AutoBinding/IAutoBinder.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Collections/AsyncVirtualObservableReadOnlyList.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Collections/LazyFetchEventArgs.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Collections/VirtualObservableReadOnlyList.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Commands/AsyncRelayCommand.cs

@@ -0,0 +1,153 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using System.Windows.Input;
+
+namespace Nuclex.Avalonia.Commands {
+
+  /// <summary>Asynchronous command that delegates work to an external method</summary>
+  /// <typeparam name="TArgument">Type of argument accepted by the command</typeparam>
+  internal sealed class AsyncRelayCommand<TArgument> : IAsyncCommand<TArgument> {
+
+    /// <summary>Raised when the command's executable state has changed</summary>
+    public event EventHandler? CanExecuteChanged;
+
+    /// <summary>Initializes a new asynchronous relay command</summary>
+    /// <param name="executeAsync">Action that is executed when the command runs</param>
+    /// <param name="canExecute">
+    ///   Optional predicate that decides whether execution is currently allowed
+    /// </param>
+    /// <param name="allowConcurrentExecutions">
+    ///   Whether the command may be executed while a previous execution is still running
+    /// </param>
+    public AsyncRelayCommand(
+      Func<TArgument, Task> executeAsync,
+      Predicate<TArgument>? canExecute = null,
+      bool allowConcurrentExecutions = false
+    ) {
+      this.executeAsync = executeAsync ?? throw new ArgumentNullException(nameof(executeAsync));
+      this.canExecute = canExecute;
+      this.allowConcurrentExecutions = allowConcurrentExecutions;
+    }
+
+    /// <summary>Whether the command is currently executing</summary>
+    public bool IsRunning {
+      get { return (Volatile.Read(ref this.executionCount) > 0); }
+    }
+
+    /// <summary>Checks whether the command may currently execute</summary>
+    /// <param name="parameter">Parameter to be passed to the command callback</param>
+    /// <returns>True if command execution is currently allowed</returns>
+    public bool CanExecute(object? parameter) {
+      if((!this.allowConcurrentExecutions) && IsRunning) {
+        return false;
+      }
+
+      if(this.canExecute == null) {
+        return true;
+      }
+
+      return this.canExecute(getParameter(parameter));
+    }
+
+    /// <summary>Executes the command callback</summary>
+    /// <param name="parameter">Parameter passed to the command callback</param>
+    public void Execute(object? parameter) {
+      _ = ExecuteAsync(getParameter(parameter));
+    }
+
+    /// <summary>Executes the command callback asynchronously</summary>
+    /// <param name="parameter">Parameter passed to the command callback</param>
+    /// <returns>A task that finishes when command execution has completed</returns>
+    public async Task ExecuteAsync(TArgument parameter) {
+      if(!CanExecute(parameter)) {
+        return;
+      }
+
+      if(this.allowConcurrentExecutions) {
+        Interlocked.Increment(ref this.executionCount);
+
+        try {
+          await this.executeAsync(parameter).ConfigureAwait(false);
+        }
+        finally {
+          Interlocked.Decrement(ref this.executionCount);
+        }
+      } else {
+        Interlocked.Increment(ref this.executionCount);
+        NotifyCanExecuteChanged();
+
+        try {
+          await this.executeAsync(parameter).ConfigureAwait(false);
+        }
+        finally {
+          Interlocked.Decrement(ref this.executionCount);
+          NotifyCanExecuteChanged();
+        }
+      } // if concurrent executions allowed / not allowed
+    }
+
+    /// <summary>
+    ///   Notifies listeners that <see cref="CanExecute(object?)" /> should be reevaluated
+    /// </summary>
+    public void NotifyCanExecuteChanged() {
+      CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+    }
+
+    /// <summary>Converts and validates the untyped command parameter</summary>
+    /// <param name="parameter">Untyped parameter passed through <see cref="ICommand" /></param>
+    /// <returns>The parameter cast to the command's parameter type</returns>
+    private static TArgument getParameter(object? parameter) {
+      if(parameter is TArgument typedParameter) {
+        return typedParameter;
+      }
+
+      if(parameter == null) {
+        if(default(TArgument) == null) {
+          return default!;
+        }
+
+        throw new ArgumentException(
+          "This command expects a non-null parameter of the configured type.",
+          nameof(parameter)
+        );
+      }
+
+      throw new ArgumentException(
+        $"This command expects a parameter of type {typeof(TArgument).FullName}.",
+        nameof(parameter)
+      );
+    }
+
+    /// <summary>Asynchronous callback invoked when command execution is requested</summary>
+    private readonly Func<TArgument, Task> executeAsync;
+    /// <summary>Optional callback deciding whether command execution is currently allowed</summary>
+    private readonly Predicate<TArgument>? canExecute;
+    /// <summary>Whether the command may run while a previous invocation is still active</summary>
+    private readonly bool allowConcurrentExecutions;
+
+    /// <summary>Number of currently active command executions</summary>
+    private int executionCount;
+
+  }
+
+} // namespace Nuclex.Avalonia.Commands

Source/Commands/IAsyncCommand.cs

@@ -0,0 +1,43 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+using System.Threading.Tasks;
+using System.Windows.Input;
+
+namespace Nuclex.Avalonia.Commands {
+
+  /// <summary>Command that executes asynchronously</summary>
+  /// <typeparam name="TArgument">Type of an optional argument for the command</typeparam>
+  public interface IAsyncCommand<TArgument> : ICommand {
+
+    /// <summary>Whether the command is currently executing</summary>
+    bool IsRunning { get; }
+
+    /// <summary>Runs the command asynchronously</summary>
+    /// <param name="argument">Optional argument for the command</param>
+    /// <returns>A task that completes when the command has executed</returns>
+    Task ExecuteAsync(TArgument argument);
+
+    /// <summary>Triggers reevaluation of <see cref="ICommand.CanExecute(object)" /></summary>
+    void NotifyCanExecuteChanged();
+
+  }
+
+} // namespace Nuclex.Avalonia.Commands

Source/CommonDialogs/AvaloniaFileSelector.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/CommonDialogs/IDirectoryPickerService.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/CommonDialogs/IFilePickerService.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/IActiveWindowTracker.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/IWindowManager.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/IWindowScope.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Messages/AvaloniaMessagePresenter.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Messages/IMessageService.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Messages/MessageEventArgs.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Messages/MessageServiceHelper.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Messages/MessageText.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/NullActiveWindowTracker.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/Properties.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/ViewModels/DialogResultEventArgs.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/ViewModels/IDialogViewModel.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

Source/ViewModels/IMultiPageViewModel.cs

@@ -0,0 +1,41 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+
+using Nuclex.Avalonia.Commands;
+
+namespace Nuclex.Avalonia.ViewModels {
+
+  /// <summary>Interface for view models that can switch between different pages</summary>
+  /// <typeparam name="TPageEnumeration">Enum type by which pages can be indicated</typeparam>
+  public interface IMultiPageViewModel<TPageEnumeration> {
+
+    /// <summary>Command to switch the active tool page</summary>
+    IAsyncCommand<TPageEnumeration> SwitchPageCommand { get; }
+
+    /// <summary>The currently displayed page</summary>
+    TPageEnumeration? ActivePage { get; }
+
+    /// <summary>View model for the page that is currently being shown</summary>
+    object? ActivePageViewModel { get; }
+
+  }
+
+} // namespace Nuclex.Avalonia.ViewModels

Source/ViewModels/IViewLoadListener.cs

@@ -0,0 +1,36 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+using System.Threading.Tasks;
+
+namespace Nuclex.Avalonia.ViewModels {
+
+  /// <summary>
+  ///   Can be implemented by view models that wish to know when their view is loaded
+  /// </summary>
+  public interface IViewLoadListener {
+
+    /// <summary>Called when the view has finished loading</summary>
+    /// <returns>A task what finishes when all view load processing is done</returns>
+    Task OnViewLoaded();
+
+  }
+
+} // namespace Nuclex.Avalonia.ViewModels

Source/ViewModels/IViewUnloadListener.cs

@@ -0,0 +1,36 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+using System.Threading.Tasks;
+
+namespace Nuclex.Avalonia.ViewModels {
+
+  /// <summary>
+  ///   Can be implemented by view models that wish to know when their view is unloading
+  /// </summary>
+  public interface IViewUnloadListener {
+
+    /// <summary>Called when the view is about to unload</summary>
+    /// <returns>A task what finishes when all view unload processing is done</returns>
+    Task OnViewUnloading();
+
+  }
+
+} // namespace Nuclex.Avalonia.ViewModels

Source/ViewModels/MultiPageViewModel.cs

@@ -0,0 +1,164 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+using System.Collections.Concurrent;
+using System.Threading.Tasks;
+
+using Nuclex.Avalonia.Commands;
+using Nuclex.Support;
+
+namespace Nuclex.Avalonia.ViewModels {
+
+  /// <summary>Base class for view models that have multiple child view models</summary>
+  /// <typeparam name="TPageEnumeration">Enum type by which pages can be indicated</typeparam>
+  public abstract class MultiPageViewModel<TPageEnumeration> :
+    Observable, IMultiPageViewModel<TPageEnumeration>, IDisposable {
+
+    /// <summary>Initializes a new multi-page view model</summary>
+    /// <param name="windowManager">
+    ///   Window manager the view model uses to create child views
+    /// </param>
+    /// <param name="cachePageViewModels">
+    ///   Whether child view models will be kept alive and reused
+    /// </param>
+    public MultiPageViewModel(IWindowManager windowManager, bool cachePageViewModels = false) {
+      this.windowManager = windowManager;
+      this.createViewModelForPageDelegate = (
+        new Func<TPageEnumeration, object>(CreateViewModelForPage)
+      );
+
+      if(cachePageViewModels) {
+        this.cachedViewModels = new ConcurrentDictionary<TPageEnumeration, object>();
+      }
+
+      SwitchPageCommand = new AsyncRelayCommand<TPageEnumeration>(switchPageAsync);
+    }
+
+    /// <summary>Command to switch the active view</summary>
+    public IAsyncCommand<TPageEnumeration> SwitchPageCommand { get; }
+
+    /// <summary>Immediately releases all resources owned by the instance</summary>
+    public virtual void Dispose() {
+
+      // If view models are being cached, simply dispose anything in the cache that
+      // implements IDisposable, the active view will be part of the cache.
+      if(this.cachedViewModels != null) {
+        foreach(object cacheViewModel in this.cachedViewModels.Values) {
+          disposeIfSupported(cacheViewModel);
+        }
+        this.activePageViewModel = null;
+
+        this.cachedViewModels.Clear();
+      } else if(this.activePageViewModel != null) { // No cache? Dispose active view.
+        disposeIfSupported(this.activePageViewModel);
+        this.activePageViewModel = null;
+      }
+
+    }
+
+    /// <summary>Child page that is currently being displayed by the view model</summary>
+    public TPageEnumeration? ActivePage {
+      get { return this.activePage; }
+    }
+
+    /// <summary>Retrieves (and, if needed, creates) the view model for the active page</summary>
+    /// <returns>A view model for the active page on the multi-page view model</returns>
+    public object? ActivePageViewModel {
+      get { return this.activePageViewModel; }
+    }
+
+
+    /// <summary>Windowmanager that can create view models and display other views</summary>
+    protected IWindowManager WindowManager {
+      get { return this.windowManager; }
+    }
+
+    /// <summary>Creates a view model for the specified page</summary>
+    /// <param name="page">Page for which a view model will be created</param>
+    /// <returns>The view model for the specified page</returns>
+    protected abstract object CreateViewModelForPage(TPageEnumeration page);
+
+    /// <summary>Switches to another page</summary>
+    /// <param name="newPage">New page to switch to</param>
+    /// <returns>A task that will finish when the new page has been switched to</returns>
+    private Task switchPageAsync(TPageEnumeration? newPage) {
+      if(newPage.Equals(this.activePage)) {
+        return Task.CompletedTask;
+      }
+
+      object? viewModelToDispose;
+      if(this.cachedViewModels == null) {
+        viewModelToDispose = this.activePageViewModel;
+
+        object? newPageViewModel = CreateViewModelForPage(newPage);
+
+        this.activePage = newPage;
+        this.activePageViewModel = newPageViewModel;
+      } else {
+        viewModelToDispose = null;
+
+        // Double-checked locking to avoid creating a view model for the same page
+        // multiple times if the construction takes time
+        object newPageViewModel;
+        if(!this.cachedViewModels.TryGetValue(newPage, out newPageViewModel)) {
+          lock(this.cachedViewModels) {
+            if(!this.cachedViewModels.TryGetValue(newPage, out newPageViewModel)) {
+              newPageViewModel = CreateViewModelForPage(newPage);
+              this.cachedViewModels.TryAdd(newPage, newPageViewModel);
+            }
+          }
+        }
+
+        this.activePage = newPage;
+        this.activePageViewModel = newPageViewModel;
+      }
+
+      OnPropertyChanged(nameof(ActivePage));
+      OnPropertyChanged(nameof(ActivePageViewModel));
+
+      disposeIfSupported(viewModelToDispose);
+
+      return Task.CompletedTask;
+    }
+
+    /// <summary>Disposes the specified object if it is disposable</summary>
+    /// <param name="potentiallyDisposable">Object that will be disposed if supported</param>
+    private static void disposeIfSupported(object? potentiallyDisposable) {
+      var disposable = potentiallyDisposable as IDisposable;
+      if(disposable != null) {
+        disposable.Dispose();
+      }
+    }
+
+    /// <summary>Window manager that can be used to display other views</summary>
+    private readonly IWindowManager windowManager;
+    /// <summary>Delegate for the CreateViewModelForPage() method</summary>
+    private readonly Func<TPageEnumeration, object> createViewModelForPageDelegate;
+    /// <summary>Cached page view models, if caching is enabled</summary>
+    private readonly ConcurrentDictionary<TPageEnumeration, object>? cachedViewModels;
+
+    /// <summary>Page that is currently active in the multi-page view model</summary>
+    private TPageEnumeration activePage;
+    /// <summary>View model for the active page</summary>
+    private object? activePageViewModel;
+
+  }
+
+} // namespace Nuclex.Avalonia.ViewModels

Source/WindowManager.cs

@@ -1,7 +1,7 @@
 #region Apache License 2.0
 /*
 Nuclex Foundation libraries for .NET
-Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+Copyright (C) 2002-2026 Markus Ewald / Nuclex Development Labs
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.