Added internal Gen2GcCallback type

Author: Sergio0694

Microsoft.Toolkit.Mvvm/Messaging/Internals/System/Gen2GcCallback.cs

@@ -0,0 +1,73 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.ConstrainedExecution;
+using System.Runtime.InteropServices;
+
+namespace System
+{
+    /// <summary>
+    /// Schedules a callback roughly every gen 2 GC (you may see a Gen 0 an Gen 1 but only once).
+    /// Ported from https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Gen2GcCallback.cs.
+    /// </summary>
+    internal sealed class Gen2GcCallback : CriticalFinalizerObject
+    {
+        /// <summary>
+        /// The callback to invoke at each GC.
+        /// </summary>
+        private readonly Action<object> callback;
+
+        /// <summary>
+        /// A weak <see cref="GCHandle"/> to the target object to pass to <see cref="callback"/>.
+        /// </summary>
+        private GCHandle handle;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Gen2GcCallback"/> class.
+        /// </summary>
+        /// <param name="callback">The callback to invoke at each GC.</param>
+        /// <param name="target">The target object to pass as argument to <paramref name="callback"/>.</param>
+        private Gen2GcCallback(Action<object> callback, object target)
+        {
+            this.callback = callback;
+            this.handle = GCHandle.Alloc(target, GCHandleType.Weak);
+        }
+
+        /// <summary>
+        /// Schedules a callback to be called on each GC until the target is collected.
+        /// </summary>
+        /// <param name="callback">The callback to invoke at each GC.</param>
+        /// <param name="target">The target object to pass as argument to <paramref name="callback"/>.</param>
+        public static void Register(Action<object> callback, object target)
+        {
+            _ = new Gen2GcCallback(callback, target);
+        }
+
+        /// <summary>
+        /// Finalizes an instance of the <see cref="Gen2GcCallback"/> class.
+        /// This finalizer is re-registered with <see cref="GC.ReRegisterForFinalize(object)"/> as long as
+        /// the target object is alive, which means it will be executed again every time a generation 2
+        /// collection is triggered (as the <see cref="Gen2GcCallback"/> instance itself would be moved to
+        /// that generation after surviving the generation 0 and 1 collections the first time).
+        /// </summary>
+        ~Gen2GcCallback()
+        {
+            if (this.handle.Target is object target)
+            {
+                try
+                {
+                    this.callback(target);
+                }
+                catch
+                {
+                }
+
+                GC.ReRegisterForFinalize(this);
+            }
+            else
+            {
+                handle.Free();
+            }
+        }
+    }
+}