Add IAsyncRelayCommandExtensions.CreateCancelCommand

CommunityToolkit.Mvvm/Input/AsyncRelayCommand.cs

@@ -8,6 +8,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using CommunityToolkit.Mvvm.ComponentModel.__Internals;
+using CommunityToolkit.Mvvm.Input.Internals;
 
 #pragma warning disable CS0618
 
@@ -19,7 +20,7 @@ namespace CommunityToolkit.Mvvm.Input;
 /// action, and providing an <see cref="ExecutionTask"/> property that notifies changes when
 /// <see cref="ExecuteAsync"/> is invoked and when the returned <see cref="Task"/> completes.
 /// </summary>
-public sealed class AsyncRelayCommand : IAsyncRelayCommand
+public sealed class AsyncRelayCommand : IAsyncRelayCommand, ICancellationAwareCommand
 {
     /// <summary>
     /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="ExecutionTask"/>.
@@ -252,6 +253,9 @@ static async void MonitorTask(AsyncRelayCommand @this, Task task)
     /// <inheritdoc/>
     public bool IsRunning => ExecutionTask is { IsCompleted: false };
 
+    /// <inheritdoc/>
+    bool ICancellationAwareCommand.IsCancellationSupported => this.execute is null;
+
     /// <inheritdoc/>
     public void NotifyCanExecuteChanged()
     {

CommunityToolkit.Mvvm/Input/AsyncRelayCommand{T}.cs

@@ -8,6 +8,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using CommunityToolkit.Mvvm.ComponentModel.__Internals;
+using CommunityToolkit.Mvvm.Input.Internals;
 
 #pragma warning disable CS0618
 
@@ -17,7 +18,7 @@ namespace CommunityToolkit.Mvvm.Input;
 /// A generic command that provides a more specific version of <see cref="AsyncRelayCommand"/>.
 /// </summary>
 /// <typeparam name="T">The type of parameter being passed as input to the callbacks.</typeparam>
-public sealed class AsyncRelayCommand<T> : IAsyncRelayCommand<T>
+public sealed class AsyncRelayCommand<T> : IAsyncRelayCommand<T>, ICancellationAwareCommand
 {
     /// <summary>
     /// The <see cref="Func{TResult}"/> to invoke when <see cref="Execute(T)"/> is used.
@@ -234,6 +235,9 @@ static async void MonitorTask(AsyncRelayCommand<T> @this, Task task)
     /// <inheritdoc/>
     public bool IsRunning => ExecutionTask is { IsCompleted: false };
 
+    /// <inheritdoc/>
+    bool ICancellationAwareCommand.IsCancellationSupported => this.execute is null;
+
     /// <inheritdoc/>
     public void NotifyCanExecuteChanged()
     {

CommunityToolkit.Mvvm/Input/IAsyncRelayCommandExtensions.cs

@@ -0,0 +1,36 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Windows.Input;
+using CommunityToolkit.Mvvm.Input.Internals;
+
+namespace CommunityToolkit.Mvvm.Input;
+
+/// <summary>
+/// Extensions for the <see cref="IAsyncRelayCommand"/> type.
+/// </summary>
+public static class IAsyncRelayCommandExtensions
+{
+    /// <summary>
+    /// Creates an <see cref="ICommand"/> instance that can be used to cancel execution on the input command.
+    /// The returned command will also notify when it can be executed based on the state of the wrapped command.
+    /// </summary>
+    /// <param name="command">The input <see cref="IAsyncRelayCommand"/> instance to create a cancellation command for.</param>
+    /// <returns>An <see cref="ICommand"/> instance that can be used to monitor and signal cancellation for <paramref name="command"/>.</returns>
+    /// <remarks>The reeturned instance is not guaranteed to be unique across multiple invocations with the same arguments.</remarks>
+    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="command"/> is <see langword="null"/>.</exception>
+    public static ICommand CreateCancelCommand(this IAsyncRelayCommand command)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        // If the command is known not to ever allow cancellation, just reuse the same instance
+        if (command is ICancellationAwareCommand { IsCancellationSupported: false })
+        {
+            return DisabledCommand.Instance;
+        }
+
+        // Create a new cancel command wrapping the input one
+        return new CancelCommand(command);
+    }
+}

CommunityToolkit.Mvvm/Input/Internals/CancelCommand.cs

@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.ComponentModel;
+using System.Windows.Input;
+
+namespace CommunityToolkit.Mvvm.Input.Internals;
+
+/// <summary>
+/// A <see cref="ICommand"/> implementation wrapping <see cref="IAsyncRelayCommand"/> to support cancellation.
+/// </summary>
+internal sealed class CancelCommand : ICommand
+{
+    /// <summary>
+    /// The wrapped <see cref="IAsyncRelayCommand"/> instance.
+    /// </summary>
+    private readonly IAsyncRelayCommand command;
+
+    /// <summary>
+    /// Creates a new <see cref="CancelCommand"/> instance.
+    /// </summary>
+    /// <param name="command">The <see cref="IAsyncRelayCommand"/> instance to wrap.</param>
+    public CancelCommand(IAsyncRelayCommand command)
+    {
+        this.command = command;
+
+        this.command.PropertyChanged += OnPropertyChanged;
+    }
+
+    /// <inheritdoc/>
+    public event EventHandler? CanExecuteChanged;
+
+    /// <inheritdoc/>
+    public bool CanExecute(object? parameter)
+    {
+        return this.command.CanBeCanceled;
+    }
+
+    /// <inheritdoc/>
+    public void Execute(object? parameter)
+    {
+        this.command.Cancel();
+    }
+
+    /// <inheritdoc cref="PropertyChangedEventHandler"/>
+    private void OnPropertyChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        if (e.PropertyName is null or nameof(IAsyncRelayCommand.CanBeCanceled))
+        {
+            CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+        }
+    }
+}

CommunityToolkit.Mvvm/Input/Internals/DisabledCommand.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Windows.Input;
+
+namespace CommunityToolkit.Mvvm.Input.Internals;
+
+/// <summary>
+/// A reusable <see cref="ICommand"/> instance that is always disabled.
+/// </summary>
+internal sealed class DisabledCommand : ICommand
+{
+    /// <inheritdoc/>
+    public event EventHandler? CanExecuteChanged
+    {
+        add { }
+        remove { }
+    }
+
+    /// <summary>
+    /// Gets a shared, reusable <see cref="DisabledCommand"/> instance.
+    /// </summary>
+    /// <remarks>
+    /// This instance can safely be used across multiple objects without having
+    /// to worry about this static keeping others alive, as the event uses a
+    /// custom accessor that just discards items (as the event is known to never
+    /// be raised). As such, this instance will never act as root for other objects.
+    /// </remarks>
+    public static DisabledCommand Instance { get; } = new();
+
+    /// <inheritdoc/>
+    public bool CanExecute(object? parameter)
+    {
+        return false;
+    }
+
+    /// <inheritdoc/>
+    public void Execute(object? parameter)
+    {
+    }
+}

CommunityToolkit.Mvvm/Input/Internals/ICancellationAwareCommand.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace CommunityToolkit.Mvvm.Input.Internals;
+
+/// <summary>
+/// An interface for commands that know whether they support cancellation or not.
+/// </summary>
+internal interface ICancellationAwareCommand
+{
+    /// <summary>
+    /// Gets whether or not the current command supports cancellation.
+    /// </summary>
+    bool IsCancellationSupported { get; }
+}