Ensure Splat initialization in builder and tests (#4110)

(<!-- Please be sure to read the
[Contribute]https://github.com/reactiveui/reactiveui#contribute) section
of the README -->

**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

update

**What is the new behavior?**
<!-- If this is a feature change -->

This pull request refactors platform-specific builder extension methods
for Blazor and AndroidX, consolidating them under the
`ReactiveUI.Builder` namespace and updating their APIs for consistency
and improved scheduler configuration. It also updates related tests to
use the new builder creation method, streamlining service registration
and test setup. Minor dependency and warning suppression updates are
included.

**Platform Extension Refactor and Consolidation:**

* Added new extension classes `BlazorReactiveUIBuilderExtensions` and
`AndroidXReactiveUIBuilderExtensions` in the `ReactiveUI.Builder`
namespace, providing consistent APIs for configuring platform schedulers
and modules. These replace the older extension files and introduce
properties for main thread schedulers.
* Removed legacy extension files `ReactiveUIBuilderBlazorExtensions.cs`
and `ReactiveUIBuilderAndroidXExtensions.cs` from platform-specific
namespaces, migrating their functionality to the new consolidated
location.

**Test Suite Modernization:**

* Updated all builder-related tests to use
`locator.CreateReactiveUIBuilder()` instead of the previous
`locator.CreateBuilder()` method, and replaced direct calls to
`AppBuilder.ResetBuilderStateForTests()` with
`Splat.Builder.AppBuilder.ResetBuilderStateForTests()`. This change
ensures tests use the new builder API and maintain proper isolation.

**Dependency and Warning Updates:**

* Bumped the `SplatVersion` property in `Directory.Packages.props` from
`15.5.3` to `16.0.1` to use the latest version.
* Added `CA1510` to the `NoWarn` property in `Directory.build.props` to
suppress a new code analysis warning.

**AOT Test Initialization:**

* Ensured that `RxApp.EnsureInitialized()` is called in the constructor
of `ViewLocatorAOTMappingTests` to guarantee ReactiveUI is initialized
for AOT mapping tests.

**What might this PR break?**

N/A new unreleased feature

**Please check if the PR fulfills these requirements**
- [x] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: ChrisPulman

src/Directory.Packages.props

@@ -18,7 +18,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="6.0.1" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
-    <PackageVersion Include="Microsoft.Windows.SDK.BuildTools" Version="10.0.26100.4654" />
+    <PackageVersion Include="Microsoft.Windows.SDK.BuildTools" Version="10.0.26100.4948" />
     <PackageVersion Include="Microsoft.Xaml.Behaviors.Wpf" Version="1.1.135" />
     <PackageVersion Include="Mocks.Maui" Version="1.2.5" />
     <PackageVersion Include="Nerdbank.GitVersioning" Version="3.7.115" />

src/Directory.build.props

@@ -17,7 +17,7 @@
     <PackageReleaseNotes>https://github.com/reactiveui/ReactiveUI/releases</PackageReleaseNotes>
     <RepositoryUrl>https://github.com/reactiveui/reactiveui</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
-    <NoWarn>$(NoWarn);IDE0060;IDE1006;IDE0130;VSSpell001</NoWarn>
+    <NoWarn>$(NoWarn);IDE0060;IDE1006;IDE0130;VSSpell001;CA1510</NoWarn>
     <!-- Publish the repository URL in the built .nupkg (in the NuSpec <Repository> element) -->
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <!-- Embed source files that are not tracked by the source control manager in the PDB -->
@@ -35,6 +35,8 @@
     <WindowsTargetFrameworks>net462;net472;net8.0-windows10.0.17763.0;net9.0-windows10.0.17763.0;net8.0-windows10.0.19041.0;net9.0-windows10.0.19041.0</WindowsTargetFrameworks>
     <MobileTargetFrameworks>net8.0-android;net8.0-ios;net8.0-tvos;net8.0-macos;net8.0-maccatalyst;net9.0-android;net9.0-ios;net9.0-tvos;net9.0-macos;net9.0-maccatalyst</MobileTargetFrameworks>
     <BaseTargetFrameworks>netstandard2.0;net8.0;net9.0</BaseTargetFrameworks>
+    <!-- Ensure all test runs use our runsettings to control cross-assembly parallelism -->
+    <RunSettingsFilePath>$(MSBuildThisFileDirectory)tests.runsettings</RunSettingsFilePath>
   </PropertyGroup>  
   <PropertyGroup Condition="'$(IsTestProject)' != 'true' and ($(TargetFramework.StartsWith('net8.0')) or $(TargetFramework.StartsWith('net9.0')))">
     <IsAotCompatible>true</IsAotCompatible>

src/ReactiveUI.AOTTests/ComprehensiveAOTTests.cs

@@ -161,6 +161,8 @@ public void ObservableAsPropertyHelper_StringBased_WorksInAOT()
     [Fact]
     public void DependencyInjection_BasicUsage_WorksInAOT()
     {
+        Splat.Builder.AppBuilder.ResetBuilderStateForTests();
+
         // Basic DI operations that work in AOT
         var resolver = Locator.CurrentMutable;
 

src/ReactiveUI.AOTTests/TestConfiguration.cs

@@ -0,0 +1,6 @@
+// Copyright (c) 2025 .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.
+
+[assembly: CollectionBehavior(DisableTestParallelization = true, MaxParallelThreads = 1)]

src/ReactiveUI.AndroidX/Builder/AndroidXReactiveUIBuilderExtensions.cs

@@ -0,0 +1,54 @@
+// Copyright (c) 2025 .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.
+
+using System.Reactive.Concurrency;
+
+namespace ReactiveUI.Builder;
+
+/// <summary>
+/// AndroidX-specific extensions for the ReactiveUI builder.
+/// </summary>
+public static class AndroidXReactiveUIBuilderExtensions
+{
+    /// <summary>
+    /// Gets the android x main thread scheduler.
+    /// </summary>
+    /// <value>
+    /// The android x main thread scheduler.
+    /// </value>
+    public static IScheduler AndroidXMainThreadScheduler { get; } = HandlerScheduler.MainThreadScheduler;
+
+    /// <summary>
+    /// Configures ReactiveUI for AndroidX platform with appropriate schedulers.
+    /// </summary>
+    /// <param name="builder">The builder instance.</param>
+    /// <returns>The builder instance for chaining.</returns>
+    public static ReactiveUIBuilder WithAndroidX(this ReactiveUIBuilder builder)
+    {
+        if (builder is null)
+        {
+            throw new ArgumentNullException(nameof(builder));
+        }
+
+        return builder
+            .WithMainThreadScheduler(HandlerScheduler.MainThreadScheduler)
+            .WithPlatformModule<AndroidX.Registrations>();
+    }
+
+    /// <summary>
+    /// Withes the android x scheduler.
+    /// </summary>
+    /// <param name="builder">The builder.</param>
+    /// <returns>The builder instance for chaining.</returns>
+    public static ReactiveUIBuilder WithAndroidXScheduler(this ReactiveUIBuilder builder)
+    {
+        if (builder is null)
+        {
+            throw new ArgumentNullException(nameof(builder));
+        }
+
+        return builder.WithMainThreadScheduler(AndroidXMainThreadScheduler);
+    }
+}

src/ReactiveUI.AndroidX/ReactiveUIBuilderAndroidXExtensions.cs

@@ -0,0 +1,52 @@
+// Copyright (c) 2025 .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.
+
+namespace ReactiveUI.Builder;
+
+/// <summary>
+/// Blazor-specific extensions for the ReactiveUI builder.
+/// </summary>
+public static class BlazorReactiveUIBuilderExtensions
+{
+    /// <summary>
+    /// Gets the blazor main thread scheduler.
+    /// </summary>
+    /// <value>
+    /// The blazor main thread scheduler.
+    /// </value>
+    public static IScheduler BlazorMainThreadScheduler { get; } = CurrentThreadScheduler.Instance;
+
+    /// <summary>
+    /// Configures ReactiveUI for Blazor platform with appropriate schedulers.
+    /// </summary>
+    /// <param name="builder">The builder instance.</param>
+    /// <returns>The builder instance for chaining.</returns>
+    public static ReactiveUIBuilder WithBlazor(this ReactiveUIBuilder builder)
+    {
+        if (builder is null)
+        {
+            throw new ArgumentNullException(nameof(builder));
+        }
+
+        return builder
+            .WithBlazorScheduler()
+            .WithPlatformModule<Blazor.Registrations>();
+    }
+
+    /// <summary>
+    /// Withes the blazor scheduler.
+    /// </summary>
+    /// <param name="builder">The builder.</param>
+    /// <returns>The builder instance for chaining.</returns>
+    public static ReactiveUIBuilder WithBlazorScheduler(this ReactiveUIBuilder builder)
+    {
+        if (builder is null)
+        {
+            throw new ArgumentNullException(nameof(builder));
+        }
+
+        return builder.WithMainThreadScheduler(BlazorMainThreadScheduler);
+    }
+}

src/ReactiveUI.Blazor/Builder/BlazorReactiveUIBuilderExtensions.cs

@@ -3,7 +3,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using ReactiveUI.Maui;
 using Splat.Builder;
 
 namespace ReactiveUI.Builder.Maui.Tests;
@@ -16,21 +15,7 @@ public void WithMaui_Should_Register_Services()
         AppBuilder.ResetBuilderStateForTests();
         using var locator = new ModernDependencyResolver();
 
-        locator.CreateBuilder()
-               .WithMaui()
-               .Build();
-
-        var typeConverters = locator.GetServices<IBindingTypeConverter>();
-        Assert.NotNull(typeConverters);
-    }
-
-    [Fact]
-    public void WithCoreServices_AndMaui_Should_Register_All_Services()
-    {
-        AppBuilder.ResetBuilderStateForTests();
-        using var locator = new ModernDependencyResolver();
-
-        locator.CreateBuilder()
+        locator.CreateReactiveUIBuilder()
                .WithMaui()
                .Build();
 

src/ReactiveUI.Blazor/ReactiveUIBuilderBlazorExtensions.cs

@@ -0,0 +1,6 @@
+// Copyright (c) 2025 .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.
+
+[assembly: CollectionBehavior(DisableTestParallelization = true, MaxParallelThreads = 1)]