Added IMessenger.Cleanup API (to help with extensibility)

Author: Sergio0694

Microsoft.Toolkit.Mvvm/Messaging/IMessenger.cs

@@ -94,6 +94,14 @@ TMessage Send<TMessage, TToken>(TMessage message, TToken token)
             where TMessage : class
             where TToken : IEquatable<TToken>;
 
+        /// <summary>
+        /// Performs a cleanup on the current messenger.
+        /// Invoking this method does not unregister any of the currently registered
+        /// recipient, and it can be used to perform cleanup operations such as
+        /// trimming the internal data structures of a messenger implementation.
+        /// </summary>
+        void Cleanup();
+
         /// <summary>
         /// Resets the <see cref="IMessenger"/> instance and unregisters all the existing recipients.
         /// </summary>

Microsoft.Toolkit.Mvvm/Messaging/Messenger.cs

@@ -480,6 +480,16 @@ public unsafe TMessage Send<TMessage, TToken>(TMessage message, TToken token)
             return message;
         }
 
+        /// <inheritdoc/>
+        void IMessenger.Cleanup()
+        {
+            // The current implementation doesn't require any kind of cleanup operation, as
+            // all the internal data structures are already kept in sync whenever a recipient
+            // is added or removed. This method is implemented through an explicit interface
+            // implementation so that developers using this type directly will not see it in
+            // the API surface (as it wouldn't be useful anyway, since it's a no-op here).
+        }
+
         /// <inheritdoc/>
         public void Reset()
         {