Make sure default avatar factory can be replaced for current call con…

…text

Just like the BehaviorPipelineFactory, this will make it easier for scenarios where you want to replace the factory for an entire call chain without affecting other concurrently executing tests, for example.

src/Avatar.UnitTests/AvatarFactoryTests.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Reflection;
+using System.Threading;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Avatars.UnitTests
@@ -27,6 +29,33 @@ public void ReplaceDefaultFactory()
             Assert.Same(instance, actual);
         }
 
+        [Fact]
+        public async Task ReplaceFactoryLocally()
+        {
+            var factory1 = new TestFactory(new object());
+            var factory2 = new TestFactory(new object());
+
+            await Task.WhenAll(
+                Task.Run(() =>
+                {
+                    AvatarFactory.LocalDefault = factory1;
+                    Thread.Sleep(50);
+                    Assert.Same(factory1, AvatarFactory.Default);
+                }),
+                Task.Run(() =>
+                {
+                    AvatarFactory.LocalDefault = factory2;
+                    Thread.Sleep(50);
+                    Assert.Same(factory2, AvatarFactory.Default);
+                })
+            );
+
+            Assert.NotSame(factory1, AvatarFactory.Default);
+            Assert.NotSame(factory2, AvatarFactory.Default);
+        }
+
+
+
         public class TestFactory : IAvatarFactory
         {
             object instance;

src/Avatar/AvatarFactory.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Reflection;
+using System.Threading;
 
 namespace Avatars
 {
@@ -9,13 +10,33 @@ namespace Avatars
     /// </summary>
     public class AvatarFactory : IAvatarFactory
     {
+        static readonly AsyncLocal<IAvatarFactory?> localFactory = new();
         static readonly IAvatarFactory nullFactory = new AvatarFactory();
+        static IAvatarFactory defaultFactory = nullFactory;
 
         /// <summary>
         /// Gets or sets the default <see cref="IAvatarFactory"/> to use 
         /// to create avatars. Defaults to the <see cref="NotImplemented"/> factory.
         /// </summary>
-        public static IAvatarFactory Default { get; set; } = nullFactory;
+        /// <remarks>
+        /// A <see cref="LocalDefault"/> can override the value of this global 
+        /// default, if assigned to a non-null value.
+        /// </remarks>
+        public static IAvatarFactory Default 
+        { 
+            get => localFactory.Value ?? defaultFactory; 
+            set => defaultFactory = value; 
+        }
+
+        /// <summary>
+        /// Gets or sets the <see cref="IAvatarFactory"/> to use 
+        /// in the current (async) flow, so it does not affect other threads/flows.
+        /// </summary>
+        public static IAvatarFactory? LocalDefault
+        {
+            get => localFactory.Value;
+            set => localFactory.Value = value;
+        }
 
         /// <summary>
         /// A factory that throws <see cref="NotImplementedException"/>.