Added ICommandAttribute type

Author: Sergio0694

Microsoft.Toolkit.Mvvm.SourceGenerators/Microsoft.Toolkit.Mvvm.SourceGenerators.csproj

@@ -19,6 +19,7 @@
     <Compile Include="..\Microsoft.Toolkit.Mvvm\ComponentModel\Attributes\ObservableObjectAttribute.cs" Link="Microsoft.Toolkit.Mvvm\ComponentModel\Attributes\ObservableObjectAttribute.cs" />
     <Compile Include="..\Microsoft.Toolkit.Mvvm\ComponentModel\Attributes\ObservablePropertyAttribute.cs" Link="Microsoft.Toolkit.Mvvm\ComponentModel\Attributes\ObservablePropertyAttribute.cs" />
     <Compile Include="..\Microsoft.Toolkit.Mvvm\ComponentModel\Attributes\ObservableRecipientAttribute.cs" Link="Microsoft.Toolkit.Mvvm\ComponentModel\Attributes\ObservableRecipientAttribute.cs" />
+    <Compile Include="..\Microsoft.Toolkit.Mvvm\Input\Attributes\ICommandAttribute.cs" Link="Microsoft.Toolkit.Mvvm\Input\Attributes\ICommandAttribute.cs" />
   </ItemGroup>
 
   <ItemGroup>

Microsoft.Toolkit.Mvvm/Input/Attributes/ICommandAttribute.cs

@@ -0,0 +1,68 @@
+// 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.
+
+#pragma warning disable CS1574
+
+using System;
+using System.Windows.Input;
+
+namespace Microsoft.Toolkit.Mvvm.Input
+{
+    /// <summary>
+    /// An attribute that can be used to automatically generate <see cref="ICommand"/> properties from declared methods. When this attribute
+    /// is used to decorate a method, a generator will create a command property with the corresponding <see cref="IRelayCommand"/> interface
+    /// depending on the signature of the method. If an invalid method signature is used, the generator will report an error.
+    /// <para>
+    /// In order to use this attribute, the containing type doesn't need to implement any interfaces. The generated properties will be lazily
+    /// assigned but their value will never change, so there is no need to support property change notifications or other additional functionality.
+    /// </para>
+    /// <para>
+    /// This attribute can be used as follows:
+    /// <code>
+    /// partial class MyViewModel
+    /// {
+    ///     [ICommand]
+    ///     private void GreetUser(User? user)
+    ///     {
+    ///         Console.WriteLine($"Hello {user.Name}!");
+    ///     }
+    /// }
+    /// </code>
+    /// And with this, code analogous to this will be generated:
+    /// <code>
+    /// partial class MyViewModel
+    /// {
+    ///     private IRelayCommand? greetUserCommand;
+    ///
+    ///     public IRelayCommand GreetUserCommand => greetUserCommand ??= new RelayCommand(GreetUser);
+    /// }
+    /// </code>
+    /// </para>
+    /// <para>
+    /// The following signatures are supported for annotated methods:
+    /// <code>
+    /// void Method();
+    /// </code>
+    /// Will generate an <see cref="IRelayCommand"/> property (using a <see cref="RelayCommand"/> instance).
+    /// <code>
+    /// void Method(T?);
+    /// </code>
+    /// Will generate an <see cref="IRelayCommand{T}"/> property (using a <see cref="RelayCommand{T}"/> instance).
+    /// <code>
+    /// Task Method();
+    /// Task Method(CancellationToken);
+    /// </code>
+    /// Will both generate an <see cref="IAsyncRelayCommand"/> property (using an <see cref="AsyncRelayCommand{T}"/> instance).
+    /// <code>
+    /// Task Method(T?);
+    /// Task Method(T?, CancellationToken);
+    /// </code>
+    /// Will both generate an <see cref="IAsyncRelayCommand{T}"/> property (using an <see cref="AsyncRelayCommand{T}"/> instance).
+    /// </para>
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+    public sealed class ICommandAttribute : Attribute
+    {
+    }
+}