Add interceptor for unsubscriptions.

Source/MQTTnet/Server/IMqttServerOptions.cs

@@ -15,6 +15,7 @@ namespace MQTTnet.Server
 
         IMqttServerConnectionValidator ConnectionValidator { get; }
         IMqttServerSubscriptionInterceptor SubscriptionInterceptor { get; }
+        IMqttServerUnsubscriptionInterceptor UnsubscriptionInterceptor { get; }
         IMqttServerApplicationMessageInterceptor ApplicationMessageInterceptor { get; }
         IMqttServerClientMessageQueueInterceptor ClientMessageQueueInterceptor { get; }
 

Source/MQTTnet/Server/IMqttServerUnsubscriptionInterceptor.cs

@@ -0,0 +1,9 @@
+using System.Threading.Tasks;
+
+namespace MQTTnet.Server
+{
+    public interface IMqttServerUnsubscriptionInterceptor
+    {
+        Task InterceptUnsubscriptionAsync(MqttUnsubscriptionInterceptorContext context);
+    }
+}

Source/MQTTnet/Server/MqttClientSubscriptionsManager.cs

@@ -107,9 +107,16 @@ namespace MQTTnet.Server
                 PacketIdentifier = unsubscribePacket.PacketIdentifier
             };
 
-            lock (_subscriptions)
+            foreach (var topicFilter in unsubscribePacket.TopicFilters)
             {
-                foreach (var topicFilter in unsubscribePacket.TopicFilters)
+                var interceptorContext = await InterceptUnsubscribeAsync(topicFilter).ConfigureAwait(false);
+                if (!interceptorContext.AcceptUnsubscription)
+                {
+                    unsubAckPacket.ReasonCodes.Add(MqttUnsubscribeReasonCode.ImplementationSpecificError);
+                    continue;
+                }
+
+                lock (_subscriptions)
                 {
                     if (_subscriptions.Remove(topicFilter))
                     {
@@ -130,19 +137,23 @@ namespace MQTTnet.Server
             return unsubAckPacket;
         }
 
-        public Task UnsubscribeAsync(IEnumerable<string> topicFilters)
+        public async Task UnsubscribeAsync(IEnumerable<string> topicFilters)
         {
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            lock (_subscriptions)
+            foreach (var topicFilter in topicFilters)
             {
-                foreach (var topicFilter in topicFilters)
+                var interceptorContext = await InterceptUnsubscribeAsync(topicFilter).ConfigureAwait(false);
+                if (!interceptorContext.AcceptUnsubscription)
+                {
+                    continue;
+                }
+
+                lock (_subscriptions)
                 {
                     _subscriptions.Remove(topicFilter);
-                }
+                }    
             }
-
-            return Task.FromResult(0);
         }
 
         public CheckSubscriptionsResult CheckSubscriptions(string topic, MqttQualityOfServiceLevel qosLevel)
@@ -206,6 +217,17 @@ namespace MQTTnet.Server
             return context;
         }
 
+        private async Task<MqttUnsubscriptionInterceptorContext> InterceptUnsubscribeAsync(string topicFilter)
+        {
+            var context = new MqttUnsubscriptionInterceptorContext(_clientSession.ClientId, topicFilter, _clientSession.Items);
+            if (_serverOptions.UnsubscriptionInterceptor != null)
+            {
+                await _serverOptions.UnsubscriptionInterceptor.InterceptUnsubscriptionAsync(context).ConfigureAwait(false);
+            }
+
+            return context;
+        }
+
         private static CheckSubscriptionsResult CreateSubscriptionResult(MqttQualityOfServiceLevel qosLevel, HashSet<MqttQualityOfServiceLevel> subscribedQoSLevels)
         {
             MqttQualityOfServiceLevel effectiveQoS;

Source/MQTTnet/Server/MqttServerOptions.cs

@@ -26,6 +26,8 @@ namespace MQTTnet.Server
 
         public IMqttServerSubscriptionInterceptor SubscriptionInterceptor { get; set; }
 
+        public IMqttServerUnsubscriptionInterceptor UnsubscriptionInterceptor { get; set; }
+
         public IMqttServerStorage Storage { get; set; }
     }
 }

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -155,6 +155,12 @@ namespace MQTTnet.Server
             return this;
         }
 
+        public MqttServerOptionsBuilder WithUnsubscriptionInterceptor(IMqttServerUnsubscriptionInterceptor value)
+        {
+            _options.UnsubscriptionInterceptor = value;
+            return this;
+        }
+
         public MqttServerOptionsBuilder WithSubscriptionInterceptor(Action<MqttSubscriptionInterceptorContext> value)
         {
             _options.SubscriptionInterceptor = new MqttServerSubscriptionInterceptorDelegate(value);

Source/MQTTnet/Server/MqttUnsubscriptionInterceptorContext.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+
+namespace MQTTnet.Server
+{
+    public class MqttUnsubscriptionInterceptorContext
+    {
+        public MqttUnsubscriptionInterceptorContext(string clientId, string topic, IDictionary<object, object> sessionItems)
+        {
+            ClientId = clientId;
+            Topic = topic;
+            SessionItems = sessionItems;
+        }
+
+        public string ClientId { get; }
+
+        public string Topic { get; set; }
+
+        /// <summary>
+        /// Gets or sets a key/value collection that can be used to share data within the scope of this session.
+        /// </summary>
+        public IDictionary<object, object> SessionItems { get; }
+
+        public bool AcceptUnsubscription { get; set; } = true;
+
+        public bool CloseConnection { get; set; }
+    }
+}