Refactor switches into 'FeatureSwitches' type

Author: Sergio0694

src/CommunityToolkit.Mvvm/ComponentModel/ObservableObject.cs

@@ -57,6 +57,12 @@ protected virtual void OnPropertyChanging(PropertyChangingEventArgs e)
     {
         ArgumentNullException.ThrowIfNull(e);
 
+        // When support is disabled, just do nothing
+        if (!FeatureSwitches.EnableINotifyPropertyChangingSupport)
+        {
+            return;
+        }
+
         PropertyChanging?.Invoke(this, e);
     }
 
@@ -75,7 +81,8 @@ protected void OnPropertyChanged([CallerMemberName] string? propertyName = null)
     /// <param name="propertyName">(optional) The name of the property that changed.</param>
     protected void OnPropertyChanging([CallerMemberName] string? propertyName = null)
     {
-        if (Configuration.IsINotifyPropertyChangingDisabled)
+        // When support is disabled, avoid instantiating the event args entirely
+        if (!FeatureSwitches.EnableINotifyPropertyChangingSupport)
         {
             return;
         }

src/CommunityToolkit.Mvvm/Properties/Configuration.cs

@@ -0,0 +1,81 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+using System.Runtime.CompilerServices;
+
+namespace CommunityToolkit.Mvvm;
+
+/// <summary>
+/// A container for all shared <see cref="AppContext"/> configuration switches for the MVVM Toolkit.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This type uses a very specific setup for configuration switches to ensure ILLink can work the best.
+/// This mirrors the architecture of feature switches in the runtime as well, and it's needed so that
+/// no static constructor is generated for the type.
+/// </para>
+/// <para>
+/// For more info, see <see href="https://github.com/dotnet/runtime/blob/main/docs/workflow/trimming/feature-switches.md#adding-new-feature-switch"/>.
+/// </para>
+/// </remarks>
+internal static class FeatureSwitches
+{
+    /// <summary>
+    /// The configuration property name for <see cref="EnableINotifyPropertyChangingSupport"/>.
+    /// </summary>
+    private const string EnableINotifyPropertyChangingSupportPropertyName = "MVVMTOOLKIT_ENABLE_INOTIFYPROPERTYCHANGING_SUPPORT";
+
+    /// <summary>
+    /// The backing field for <see cref="EnableINotifyPropertyChangingSupport"/>.
+    /// </summary>
+    private static int enableINotifyPropertyChangingSupport;
+
+    /// <summary>
+    /// Gets a value indicating whether or not support for <see cref="System.ComponentModel.INotifyPropertyChanging"/> should be enabled (defaults to <see langword="true"/>).
+    /// </summary>
+    public static bool EnableINotifyPropertyChangingSupport
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => GetConfigurationValue(EnableINotifyPropertyChangingSupportPropertyName, ref enableINotifyPropertyChangingSupport, true);
+    }
+
+    /// <summary>
+    /// Gets a configuration value for a specified property.
+    /// </summary>
+    /// <param name="propertyName">The property name to retrieve the value for.</param>
+    /// <param name="cachedResult">The cached result for the target configuration value.</param>
+    /// <param name="defaultValue">The default value for the feature switch, if not set.</param>
+    /// <returns>The value of the specified configuration setting.</returns>
+    private static bool GetConfigurationValue(string propertyName, ref int cachedResult, bool defaultValue)
+    {
+        // The cached switch value has 3 states:
+        //   0: unknown.
+        //   1: true
+        //   -1: false
+        //
+        // This method doesn't need to worry about concurrent accesses to the cached result,
+        // as even if the configuration value is retrieved twice, that'll always be the same.
+        if (cachedResult < 0)
+        {
+            return false;
+        }
+
+        if (cachedResult > 0)
+        {
+            return true;
+        }
+
+        // Get the configuration switch value, or its default.
+        // All feature switches have a default set in the .targets file.
+        if (!AppContext.TryGetSwitch(propertyName, out bool isEnabled))
+        {
+            isEnabled = defaultValue;
+        }
+
+        // Update the cached result
+        cachedResult = isEnabled ? 1 : -1;
+
+        return isEnabled;
+    }
+}