Add an event to receive all application messages at server level.

MQTTnet.Core/Server/MqttApplicationMessageReceivedEventArgs.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace MQTTnet.Core.Server
+{
+    public sealed class MqttApplicationMessageReceivedEventArgs : EventArgs
+    {
+        public MqttApplicationMessageReceivedEventArgs(string clientId, MqttApplicationMessage applicationMessage)
+        {
+            ClientId = clientId;
+            ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
+        }
+
+        public string ClientId { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; }
+    }
+}

MQTTnet.Core/Server/MqttClientSession.cs

@@ -19,20 +19,23 @@ namespace MQTTnet.Core.Server
         private readonly MqttClientMessageQueue _messageQueue;
         private readonly Action<MqttClientSession, MqttPublishPacket> _publishPacketReceivedCallback;
         private readonly MqttServerOptions _options;
-
+        
         private CancellationTokenSource _cancellationTokenSource;
         private IMqttCommunicationAdapter _adapter;
         private string _identifier;
         private MqttApplicationMessage _willApplicationMessage;
 
-        public MqttClientSession(MqttServerOptions options, Action<MqttClientSession, MqttPublishPacket> publishPacketReceivedCallback)
+        public MqttClientSession(string clientId, MqttServerOptions options, Action<MqttClientSession, MqttPublishPacket> publishPacketReceivedCallback)
         {
+            ClientId = clientId;
             _options = options ?? throw new ArgumentNullException(nameof(options));
             _publishPacketReceivedCallback = publishPacketReceivedCallback ?? throw new ArgumentNullException(nameof(publishPacketReceivedCallback));
 
             _messageQueue = new MqttClientMessageQueue(options);
         }
 
+        public string ClientId { get; }
+
         public bool IsConnected => _adapter != null;
 
         public async Task RunAsync(string identifier, MqttApplicationMessage willApplicationMessage, IMqttCommunicationAdapter adapter)

MQTTnet.Core/Server/MqttClientSessionsManager.cs

@@ -5,6 +5,7 @@ using System.Threading.Tasks;
 using MQTTnet.Core.Adapter;
 using MQTTnet.Core.Diagnostics;
 using MQTTnet.Core.Exceptions;
+using MQTTnet.Core.Internal;
 using MQTTnet.Core.Packets;
 using MQTTnet.Core.Protocol;
 
@@ -21,6 +22,8 @@ namespace MQTTnet.Core.Server
             _options = options ?? throw new ArgumentNullException(nameof(options));
         }
 
+        public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived; 
+
         public async Task RunClientSessionAsync(MqttClientConnectedEventArgs eventArgs)
         {
             try
@@ -114,7 +117,7 @@ namespace MQTTnet.Core.Server
                 {
                     isExistingSession = false;
 
-                    clientSession = new MqttClientSession(_options, DispatchPublishPacket);
+                    clientSession = new MqttClientSession(connectPacket.ClientId, _options, DispatchPublishPacket);
                     _clientSessions[connectPacket.ClientId] = clientSession;
 
                     MqttTrace.Verbose(nameof(MqttClientSessionsManager), $"Created a new session for client '{connectPacket.ClientId}'.");
@@ -126,6 +129,9 @@ namespace MQTTnet.Core.Server
 
         private void DispatchPublishPacket(MqttClientSession senderClientSession, MqttPublishPacket publishPacket)
         {
+            var eventArgs = new MqttApplicationMessageReceivedEventArgs(senderClientSession.ClientId, publishPacket.ToApplicationMessage());
+            ApplicationMessageReceived?.Invoke(this, eventArgs);
+
             foreach (var clientSession in _clientSessions.Values.ToList())
             {
                 clientSession.EnqueuePublishPacket(senderClientSession, publishPacket);

MQTTnet.Core/Server/MqttServer.cs

@@ -21,6 +21,7 @@ namespace MQTTnet.Core.Server
             _adapters = adapters ?? throw new ArgumentNullException(nameof(adapters));
             
             _clientSessionsManager = new MqttClientSessionsManager(options);
+            _clientSessionsManager.ApplicationMessageReceived += (s, e) => ApplicationMessageReceived?.Invoke(s, e);
         }
 
         public IList<string> GetConnectedClients()
@@ -30,6 +31,8 @@ namespace MQTTnet.Core.Server
 
         public event EventHandler<MqttClientConnectedEventArgs> ClientConnected;
 
+        public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
+
         public void InjectClient(string identifier, IMqttCommunicationAdapter adapter)
         {
             if (adapter == null) throw new ArgumentNullException(nameof(adapter));