Added SetAndNotifyOnCompletion overload with callback

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableObject.cs

@@ -350,6 +350,36 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
             return SetAndNotifyOnCompletion(ref field, fieldExpression, newValue, out _);
         }
 
+        /// <summary>
+        /// Compares the current and new values for a given field (which should be the backing
+        /// field for a property). If the value has changed, raises the <see cref="PropertyChanging"/>
+        /// event, updates the field and then raises the <see cref="PropertyChanged"/> event.
+        /// This method is just like <see cref="SetAndNotifyOnCompletion{TTask}(ref TTask,Expression{Func{TTask}},TTask,string)"/>,
+        /// with the difference being an extra <see cref="Action{T}"/> parameter with a callback being invoked
+        /// either immediately, if the new task has already completed or is <see langword="null"/>, or upon completion.
+        /// </summary>
+        /// <typeparam name="TTask">The type of <see cref="Task"/> to set and monitor.</typeparam>
+        /// <param name="field">The field storing the property's value.</param>
+        /// <param name="fieldExpression">
+        /// An <see cref="Expression{TDelegate}"/> returning the field to update.</param>
+        /// <param name="newValue">The property's value after the change occurred.</param>
+        /// <param name="callback">A callback to invoke to update the property value.</param>
+        /// <param name="propertyName">(optional) The name of the property that changed.</param>
+        /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
+        /// <remarks>
+        /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
+        /// if the current and new value for the target property are the same.
+        /// </remarks>
+        protected bool SetAndNotifyOnCompletion<TTask>(ref TTask? field, Expression<Func<TTask?>> fieldExpression, TTask? newValue, Action<TTask?> callback, [CallerMemberName] string propertyName = null!)
+            where TTask : Task
+        {
+            bool result = SetAndNotifyOnCompletion(ref field, fieldExpression, newValue, out Task task);
+
+            task.ContinueWith(t => callback((TTask?)t));
+
+            return result;
+        }
+
         /// <summary>
         /// Throws an <see cref="ArgumentException"/> when a given <see cref="Expression{TDelegate}"/> is invalid for a property.
         /// </summary>