docs mvux messaging observe overloads

doc/Learn/Mvux/Advanced/Messaging.md

@@ -302,6 +302,80 @@ The `SelectedPersonPhone` state will only be refreshed if it meets the predicate
 
 This overload is the same as the previous one, except it watches a single-item state rather than a `ListState`, as in the last example.
 
+#### Fluent API Observe overloads
+
+These extension methods are available for both `IState<TEntity>` and `IListState<TEntity>` objects.
+
+- `IState<TEntity> Observe<TEntity, TKey>(this IState<TEntity> state, IMessenger messenger, Func<TEntity, TKey> keySelector)`
+
+    This overload is a fluent API extension method that can be used to observe entity-change messages from the messenger for a specific entity type and apply them to the state. It returns the state itself, so it can be chained with other methods.
+
+    ```csharp
+    public partial record MyModel
+    {
+        protected IUserService UserService { get; }
+
+        public MyModel(IUserService userService, IMessenger messenger)
+        {
+            UserService = userService;
+
+            CurrentUser
+                .Observe(messenger, user => user.Id)
+                .Observe(messenger, user => user.Name);
+        }
+
+        public IState<User> CurrentUser => State.Async(this, UserService.GetCurrentUser);
+    }
+    ```
+
+    or in a more Fluent API way:
+
+    ```csharp
+    public partial record MyModel(IUserService UserService, IMessenger Messenger)
+    {
+        public IState<User> CurrentUser => State.Async(this, UserService.GetCurrentUser)
+            .Observe(Messenger, user => user.Id)
+            .Observe(Messenger, user => user.Name);
+    }
+    ```
+
+    > [!NOTE]
+    > Please note that in this example we are using C# Primary Constructors, which is a feature available in C# 9.0.
+
+- `IState<TEntity> Observe<TEntity, TKey>(this IState<TEntity> state, IMessenger messenger, Func<TEntity, TKey> keySelector, out IDisposable disposable)`
+
+    This overload is the same as the previous one, except it returns an `IDisposable`, which can be used to dispose of the subscription. When disposed, the state will stop observing further entity-change messages from the messenger.
+
+    ```csharp
+    public partial record MyModel
+    {
+        protected IUserService UserService { get; }
+        private IDisposable subscriptions;
+
+        public MyModel(IUserService userService, IMessenger messenger)
+        {
+            UserService = userService;
+
+            CurrentUser
+                .Observe(messenger, user => user.Id, out subscriptions);
+        }
+
+        public IState<User> CurrentUser => State.Async(this, UserService.GetCurrentUser);
+
+        // Call this method to cancel the subscription
+        private void CancelSubscriptions()
+        {
+            subscriptions.Dispose();
+        }
+    }
+    ```
+
+Two more overload extensions are available for `IListState<TEntity>`, and they behave the same as the `IState<TEntity>` overloads.
+
+- `IListState<TEntity> Observe<TEntity, TKey>(this IListState<TEntity> listState, IMessenger messenger, Func<TEntity, TKey> keySelector, out IDisposable disposable)`
+
+- `IListState<TEntity> Observe<TEntity, TKey>(this IListState<TEntity> listState, IMessenger messenger, Func<TEntity, TKey> keySelector)`
+
 ### Update
 
 The MVUX messaging package also includes a pair of `Update` methods that enable updating an `IState<T>` or an `IListState<T>` from an `EntityMessage<T>`.