Added back the MessageRouterJsonExtension.cs file and the original Su…

…bscribeAsync method.

src/messaging/dotnet/src/Client/Client/MessageRouterClient.cs

@@ -71,6 +71,17 @@ public ValueTask<IAsyncDisposable> SubscribeAsync(
         return SubscribeAsyncCore(GetTopic(topic), subscriber, cancellationToken);
     }
 
+    public ValueTask<IAsyncDisposable> SubscribeAsync(
+    string topic,
+    IAsyncObserver<TopicMessage> subscriber,
+    CancellationToken cancellationToken = default)
+    {
+        Protocol.Topic.Validate(topic);
+        CheckState();
+
+        return SubscribeAsyncCore(GetTopic(topic), subscriber, cancellationToken);
+    }
+
     public async ValueTask PublishAsync(
         string topic,
         IMessageBuffer? payload = null,

src/messaging/dotnet/src/Core/IMessageRouter.cs

@@ -47,6 +47,18 @@ ValueTask RegisterEndpointAsync(
         EndpointDescriptor? descriptor = null,
         CancellationToken cancellationToken = default);
 
+    /// <summary>
+    ///     Gets an observable that represents a topic.
+    /// </summary>
+    /// <param name="topic"></param>
+    /// <param name="subscriber"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    ValueTask<IAsyncDisposable> SubscribeAsync(
+        string topic,
+        IAsyncObserver<TopicMessage> subscriber,
+        CancellationToken cancellationToken = default);
+
     /// <summary>
     ///     Removes an endpoint registration.
     /// </summary>

src/messaging/dotnet/src/Core/MessageRouterJsonExtension.cs

@@ -0,0 +1,47 @@
+// Morgan Stanley makes this available to you under the Apache License,
+// Version 2.0 (the "License"). You may obtain a copy of the License at
+// 
+//      http://www.apache.org/licenses/LICENSE-2.0.
+// 
+// See the NOTICE file distributed with this work for additional information
+// regarding copyright ownership. Unless required by applicable law or agreed
+// to in writing, software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+// or implied. See the License for the specific language governing permissions
+// and limitations under the License.
+
+using System.Text.Json;
+
+namespace MorganStanley.ComposeUI.Messaging;
+
+/// <summary>
+/// Contains extension methods for working with JSON payloads.
+/// </summary>
+public static class MessageRouterJsonExtensions
+{
+    /// <summary>
+    /// Publishes a message to a topic, with the payload serialized to JSON.
+    /// </summary>
+    /// <param name="messageRouter"></param>
+    /// <param name="topic"></param>
+    /// <param name="payload"></param>
+    /// <param name="publishOptions"></param>
+    /// <param name="cancellationToken"></param>
+    /// <param name="jsonSerializerOptions"></param>
+    /// <typeparam name="TPayload"></typeparam>
+    /// <returns></returns>
+    public static ValueTask PublishJsonAsync<TPayload>(
+        this IMessageRouter messageRouter,
+        string topic,
+        TPayload payload,
+        JsonSerializerOptions? jsonSerializerOptions = null,
+        PublishOptions publishOptions = default,
+        CancellationToken cancellationToken = default)
+    {
+        return messageRouter.PublishAsync(
+            topic,
+            MessageBuffer.CreateJson(payload, jsonSerializerOptions),
+            publishOptions,
+            cancellationToken);
+    }
+}