Add topic filter builder

MQTTnet.Core/Server/MqttServerOptions.cs

@@ -16,7 +16,7 @@ namespace MQTTnet.Core.Server
 
         public Func<MqttConnectPacket, MqttConnectReturnCode> ConnectionValidator { get; set; }
 
-        public Action<MqttApplicationMessage> ApplicationMessageInterceptor { get; set; }
+        public Func<MqttApplicationMessage, MqttApplicationMessage> ApplicationMessageInterceptor { get; set; }
 
         public IMqttServerStorage Storage { get; set; }
     }

MQTTnet.Core/TopicFilter.cs

@@ -1,6 +1,6 @@
 using MQTTnet.Core.Protocol;
 
-namespace MQTTnet.Core.Packets
+namespace MQTTnet.Core
 {
     public sealed class TopicFilter
     {

MQTTnet.Core/TopicFilterBuilder.cs

@@ -0,0 +1,51 @@
+using MQTTnet.Core.Protocol;
+using MQTTnet.Core.Exceptions;
+
+namespace MQTTnet.Core
+{
+    public class TopicFilterBuilder
+    {
+        private string _topic;
+        private MqttQualityOfServiceLevel _qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce;
+
+        public TopicFilterBuilder WithTopic(string topic)
+        {
+            _topic = topic;
+            return this;
+        }
+
+        public TopicFilterBuilder WithQualityOfServiceLevel(MqttQualityOfServiceLevel qualityOfServiceLevel)
+        {
+            _qualityOfServiceLevel = qualityOfServiceLevel;
+            return this;
+        }
+
+        public TopicFilterBuilder WithAtLeastOnceQoS()
+        {
+            _qualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce;
+            return this;
+        }
+
+        public TopicFilterBuilder WithAtMostOnceQoS()
+        {
+            _qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce;
+            return this;
+        }
+
+        public TopicFilterBuilder WithExactlyOnceQoS()
+        {
+            _qualityOfServiceLevel = MqttQualityOfServiceLevel.ExactlyOnce;
+            return this;
+        }
+
+        public TopicFilter Build()
+        {
+            if (string.IsNullOrEmpty(_topic))
+            {
+                throw new MqttProtocolViolationException("Topic is not set.");
+            }
+
+            return new TopicFilter(_topic, _qualityOfServiceLevel);
+        }
+    }
+}