Update docs.

Source/MQTTnet/Client/MqttClient.cs

@@ -637,25 +637,27 @@ namespace MQTTnet.Client
                     }
                     else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
                     {
-                        var reasonCode = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
-                        if (reasonCode != null && Enum.IsDefined(typeof(MqttPubAckReasonCode), reasonCode.Value))
+                        var eventArgs = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
+
+                        if (!eventArgs.ProcessingFailed)
                         {
                             await SendAsync(new MqttPubAckPacket
                             {
                                 PacketIdentifier = publishPacket.PacketIdentifier,
-                                ReasonCode = (MqttPubAckReasonCode)reasonCode.Value
+                                ReasonCode = (MqttPubAckReasonCode)eventArgs.ReasonCode
                             }, cancellationToken).ConfigureAwait(false);
                         }
                     }
                     else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
                     {
-                        var reasonCode = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
-                        if (reasonCode != null && Enum.IsDefined(typeof(MqttPubRecReasonCode), reasonCode.Value))
+                        var eventArgs = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
+
+                        if (!eventArgs.ProcessingFailed)
                         {
                             await SendAsync(new MqttPubRecPacket
                             {
                                 PacketIdentifier = publishPacket.PacketIdentifier,
-                                ReasonCode = (MqttPubRecReasonCode)reasonCode.Value
+                                ReasonCode = (MqttPubRecReasonCode)eventArgs.ReasonCode
                             }, cancellationToken).ConfigureAwait(false);
                         }
                     }
@@ -678,7 +680,7 @@ namespace MQTTnet.Client
         {
             if (!_packetDispatcher.TryDispatch(pubRecPacket))
             {
-                // The packet is unknown. Probably due to a restart of the client. 
+                // The packet is unknown. Probably due to a restart of the client.
                 // So wen send this to the server to trigger a full resend of the message.
                 return SendAsync(new MqttPubRelPacket
                 {
@@ -756,22 +758,18 @@ namespace MQTTnet.Client
             return _adapter.PacketFormatterAdapter.DataConverter.CreatePublishResult(pubRecPacket, pubCompPacket);
         }
 
-        async Task<int?> HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
+        async Task<MqttApplicationMessageReceivedEventArgs> HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
         {
-            int? reasonCode = 0;
             var applicationMessage = _adapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket);
+            var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage);
 
             var handler = ApplicationMessageReceivedHandler;
             if (handler != null)
             {
-                var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage);
                 await handler.HandleApplicationMessageReceivedAsync(eventArgs).ConfigureAwait(false);
-                if (eventArgs.ProcessingFailed)
-                {
-                    reasonCode = eventArgs.ReasonCode;
-                }
             }
-            return reasonCode;
+
+            return eventArgs;
         }
 
         async Task WaitForTaskAsync(Task task, Task sender)

Source/MQTTnet/Formatter/MqttPacketWriter.cs

@@ -97,7 +97,7 @@ namespace MQTTnet.Formatter
 
                 _buffer[_offset] = (byte)(bufferSize >> 8);
                 _buffer[_offset + 1] = (byte)bufferSize;
-                
+
                 Encoding.UTF8.GetBytes(value, 0, value.Length, _buffer, _offset + 2);
 
                 IncreasePosition(bufferSize + 2);
@@ -121,7 +121,7 @@ namespace MQTTnet.Formatter
 
                 _buffer[_offset] = (byte)(value.Length >> 8);
                 _buffer[_offset + 1] = (byte)value.Length;
-                
+
                 Array.Copy(value, 0, _buffer, _offset + 2, value.Length);
                 IncreasePosition(value.Length + 2);
             }
@@ -201,8 +201,8 @@ namespace MQTTnet.Formatter
         {
             // This method frees the used memory by shrinking the buffer. This is required because the buffer
             // is used across several messages. In general this is not a big issue because subsequent Ping packages
-            // have the same size but a very big publish package with 100 MB of payload will increase the buffer 
-            // a lot and the size will never reduced. So this method tries to find a size which can be held in 
+            // have the same size but a very big publish package with 100 MB of payload will increase the buffer
+            // a lot and the size will never reduced. So this method tries to find a size which can be held in
             // memory for a long time without causing troubles.
 
             if (_buffer.Length < MaxBufferSize)
@@ -212,7 +212,7 @@ namespace MQTTnet.Formatter
 
             Array.Resize(ref _buffer, MaxBufferSize);
         }
-        
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         void EnsureAdditionalCapacity(int additionalCapacity)
         {

Source/MQTTnet/MqttApplicationMessageReceivedEventArgs.cs

@@ -1,5 +1,4 @@
-using MQTTnet.Protocol;
-using System;
+using System;
 
 namespace MQTTnet
 {
@@ -17,6 +16,6 @@ namespace MQTTnet
 
         public bool ProcessingFailed { get; set; }
 
-        public int? ReasonCode { get; set; }
+        public MqttApplicationMessageReceivedReasonCode ReasonCode { get; set; } = MqttApplicationMessageReceivedReasonCode.Success;
     }
 }

Source/MQTTnet/MqttApplicationMessageReceivedReasonCode.cs

@@ -0,0 +1,15 @@
+namespace MQTTnet
+{
+    public enum MqttApplicationMessageReceivedReasonCode
+    {
+        Success = 0,
+        NoMatchingSubscribers = 16,
+        UnspecifiedError = 128,
+        ImplementationSpecificError = 131,
+        NotAuthorized = 135,
+        TopicNameInvalid = 144,
+        PacketIdentifierInUse = 145,
+        QuotaExceeded = 151,
+        PayloadFormatInvalid = 153
+    }
+}

Source/MQTTnet/MqttTopicFilterBuilder.cs

@@ -64,7 +64,7 @@ namespace MQTTnet
             _retainHandling = value;
             return this;
         }
-        
+
         public MqttTopicFilter Build()
         {
             if (string.IsNullOrEmpty(_topic))
@@ -74,7 +74,7 @@ namespace MQTTnet
 
             return new MqttTopicFilter
             {
-                Topic = _topic, 
+                Topic = _topic,
                 QualityOfServiceLevel = _qualityOfServiceLevel,
                 NoLocal = _noLocal,
                 RetainAsPublished = _retainAsPublished,