Add new method for reading a single retained message.

.github/workflows/ReleaseNotes.md

@@ -8,3 +8,4 @@
 * [Server] Exposed server session items at server level and allow custom session items for injected application messages (#1638).
 * [Server] Improved performance for retained message handling when subscribing using "DoNotSendOnSubscribe" or "SendAtSubscribeIfNewSubscriptionOnly" (#1661, thanks to @Int32Overflow).
 * [Server] Added support for changing the used TLS certificate while the server is running (#1652, thanks to @YAJeff). The certificate provider will now be invoked for every new connection!
+* [Server] Added a new API method which allows reading a single retained message without the need to processing the entire set of retained messages (#1659).

Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs

@@ -22,18 +22,19 @@ namespace MQTTnet.Server
         public MqttRetainedMessagesManager(MqttServerEventContainer eventContainer, IMqttNetLogger logger)
         {
             _eventContainer = eventContainer ?? throw new ArgumentNullException(nameof(eventContainer));
-            
-            if (logger == null) throw new ArgumentNullException(nameof(logger));
+
+            if (logger == null)
+                throw new ArgumentNullException(nameof(logger));
             _logger = logger.WithSource(nameof(MqttRetainedMessagesManager));
         }
-        
+
         public async Task Start()
         {
             try
             {
                 var eventArgs = new LoadingRetainedMessagesEventArgs();
                 await _eventContainer.LoadingRetainedMessagesEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
-                
+
                 lock (_messages)
                 {
                     _messages.Clear();
@@ -64,7 +65,7 @@ namespace MQTTnet.Server
             {
                 List<MqttApplicationMessage> messagesForSave = null;
                 var saveIsRequired = false;
-                
+
                 lock (_messages)
                 {
                     var hasPayload = applicationMessage.Payload != null && applicationMessage.Payload.Length > 0;
@@ -83,7 +84,8 @@ namespace MQTTnet.Server
                         }
                         else
                         {
-                            if (existingMessage.QualityOfServiceLevel != applicationMessage.QualityOfServiceLevel || !existingMessage.Payload.SequenceEqual(applicationMessage.Payload ?? EmptyBuffer.Array))
+                            if (existingMessage.QualityOfServiceLevel != applicationMessage.QualityOfServiceLevel ||
+                                !existingMessage.Payload.SequenceEqual(applicationMessage.Payload ?? EmptyBuffer.Array))
                             {
                                 _messages[applicationMessage.Topic] = applicationMessage;
                                 saveIsRequired = true;
@@ -113,7 +115,7 @@ namespace MQTTnet.Server
                 _logger.Error(exception, "Unhandled exception while handling retained messages.");
             }
         }
-        
+
         public Task<IList<MqttApplicationMessage>> GetMessages()
         {
             lock (_messages)
@@ -123,6 +125,19 @@ namespace MQTTnet.Server
             }
         }
 
+        public Task<MqttApplicationMessage> GetMessage(string topic)
+        {
+            lock (_messages)
+            {
+                if (_messages.TryGetValue(topic, out var message))
+                {
+                    return Task.FromResult(message);
+                }
+
+                return null;
+            }
+        }
+
         public async Task ClearMessages()
         {
             lock (_messages)
@@ -136,4 +151,4 @@ namespace MQTTnet.Server
             }
         }
     }
-}
+}
\ No newline at end of file

Source/MQTTnet/Server/MqttServer.cs

@@ -204,6 +204,18 @@ namespace MQTTnet.Server
             return _retainedMessagesManager.GetMessages();
         }
 
+        public Task<MqttApplicationMessage> GetRetainedMessageAsync(string topic)
+        {
+            if (topic == null)
+            {
+                throw new ArgumentNullException(nameof(topic));
+            }
+
+            ThrowIfNotStarted();
+
+            return _retainedMessagesManager.GetMessage(topic);
+        }
+
         public Task<IList<MqttSessionStatus>> GetSessionsAsync()
         {
             ThrowIfNotStarted();
@@ -233,7 +245,7 @@ namespace MQTTnet.Server
             }
 
             var sessionItems = injectedApplicationMessage.CustomSessionItems ?? ServerSessionItems;
-            
+
             return _clientSessionsManager.DispatchApplicationMessage(
                 injectedApplicationMessage.SenderClientId,
                 sessionItems,