feat: support ReactiveCommand background execution (#473)

feat: support background ReactiveCommands

Implementation:
- add the RunInBackground ReactiveCommand attribute option
- emit CreateRunInBackground for synchronous command methods
- preserve task and observable command factories
- handle output scheduler overloads across ReactiveUI 23 and 24

Tests:
- add TUnit generator and integration coverage
- verify ReactiveUI 23, v24 Primitives, and v24 System.Reactive compatibility
- update generated command attribute snapshots

Documentation:
- document background command generation and supported method behavior

Author: ChrisPulman

README.md

@@ -45,6 +45,7 @@ ReactiveUI Source Generators automatically generate ReactiveUI objects to stream
 - `[ObservableAsProperty(PropertyName = "ReadOnlyPropertyName")]`
 - `[ObservableAsProperty(InitialValue = "Default Value")]` Only valid for partial properties using (C# 13 Visual Studio Version 17.12.0)
 - `[ReactiveCommand]`
+- `[ReactiveCommand(RunInBackground = true)]` runs a synchronous command on ReactiveUI's background scheduler
 - `[ReactiveCommand(CanExecute = nameof(IObservableBoolName))]` with CanExecute
 - `[ReactiveCommand(OutputScheduler = "RxSchedulers.MainThreadScheduler")]` using a ReactiveUI Scheduler
 - `[ReactiveCommand(OutputScheduler = nameof(_isheduler))]` using a Scheduler defined in the class
@@ -458,6 +459,20 @@ public partial class MyReactiveClass
 }
 ```
 
+### Usage ReactiveCommand on the background scheduler
+
+Use `RunInBackground` for synchronous command methods that should be created with `ReactiveCommand.CreateRunInBackground`. Task- and observable-returning methods continue to use their asynchronous ReactiveCommand factories.
+
+```csharp
+using ReactiveUI.SourceGenerators;
+
+public partial class MyReactiveClass
+{
+    [ReactiveCommand(RunInBackground = true)]
+    private void ExecuteExpensiveWork() { }
+}
+```
+
 ### Usage ReactiveCommand with parameter and return value
 ```csharp
 using ReactiveUI.SourceGenerators;

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.Access#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     /// 

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.AsyncWithParam#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     /// 

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.Basic#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     /// 

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.FromReactiveAsyncCommand#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     /// 

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.Nested#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     /// 

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.NullableTypeReturn#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     /// 

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.RunInBackground#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -0,0 +1,52 @@
+//HintName: ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.cs
+// Copyright (c) 2026 .NET Foundation and Contributors. All rights reserved.
+// 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 full license information.
+
+// 
+#pragma warning disable
+#nullable enable
+namespace ReactiveUI.SourceGenerators;
+
+/// 
+/// ReactiveCommand Attribute.
+/// 
+/// 
+[global::System.AttributeUsage(global::System.AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+internal sealed class ReactiveCommandAttribute : global::System.Attribute
+{
+    /// 
+    /// Gets the can execute method or property.
+    /// 
+    /// 
+    /// The name of the CanExecute Observable of bool.
+    /// 
+    public string? CanExecute { get; init; }
+    
+    /// 
+    /// Gets the output scheduler.
+    /// 
+    /// 
+    /// The output scheduler.
+    /// 
+    public string? OutputScheduler { get; init; }
+
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
+    /// 
+    /// Gets the AccessModifier of the ReactiveCommand property.
+    /// 
+    /// 
+    /// The AccessModifier of the property.
+    /// 
+    public PropertyAccessModifier AccessModifier { get; init; }
+}
+#nullable restore
+#pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.RunInBackground#TestNs.TestVM.ReactiveCommands.g.verified.cs

@@ -0,0 +1,35 @@
+//HintName: TestNs.TestVM.ReactiveCommands.g.cs
+// 
+
+#pragma warning disable
+#nullable enable
+
+namespace TestNs
+{
+    
+    public partial class TestVM
+    {
+        private global::ReactiveUI.ReactiveCommand<global::ReactiveUI.Primitives.RxVoid, global::ReactiveUI.Primitives.RxVoid>? _saveCommand;
+
+
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public global::ReactiveUI.ReactiveCommand<global::ReactiveUI.Primitives.RxVoid, global::ReactiveUI.Primitives.RxVoid> SaveCommand { get => _saveCommand ??= global::ReactiveUI.ReactiveCommand.CreateRunInBackground(Save); }
+        private global::ReactiveUI.ReactiveCommand<int, int>? _calculateCommand;
+
+
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public global::ReactiveUI.ReactiveCommand<int, int> CalculateCommand { get => _calculateCommand ??= global::ReactiveUI.ReactiveCommand.CreateRunInBackground<int, int>(Calculate); }
+        private global::ReactiveUI.ReactiveCommand<int, string>? _formatCommand;
+
+
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public global::ReactiveUI.ReactiveCommand<int, string> FormatCommand { get => _formatCommand ??= global::ReactiveUI.ReactiveCommand.CreateRunInBackground<int, string>(Format, CanRun, backgroundScheduler: null, outputScheduler: global::ReactiveUI.RxSchedulers.MainThreadScheduler); }
+        private global::ReactiveUI.ReactiveCommand<global::ReactiveUI.Primitives.RxVoid, global::ReactiveUI.Primitives.RxVoid>? _foregroundCommand;
+
+
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public global::ReactiveUI.ReactiveCommand<global::ReactiveUI.Primitives.RxVoid, global::ReactiveUI.Primitives.RxVoid> ForegroundCommand { get => _foregroundCommand ??= global::ReactiveUI.ReactiveCommand.Create(Foreground); }
+    }
+}
+#nullable restore
+#pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVECMD/ReactiveCMDGeneratorTests.Scheduler#ReactiveUI.SourceGenerators.ReactiveCommandAttribute.g.verified.cs

@@ -32,6 +32,14 @@ internal sealed class ReactiveCommandAttribute : global::System.Attribute
     /// 
     public string? OutputScheduler { get; init; }
 
+    /// 
+    /// Gets a value indicating whether a synchronous method runs on ReactiveUI's background scheduler.
+    /// 
+    /// 
+    ///  to create the command with ReactiveCommand.CreateRunInBackground.
+    /// 
+    public bool RunInBackground { get; init; }
+
     /// 
     /// Gets the AccessModifier of the ReactiveCommand property.
     ///