SubscriptionClient carried over.

code/MessengerApi.SubscriptionClient/SubscriptionClient.cs

@@ -0,0 +1,61 @@
+using MessengerApi.Model;
+using portaloggy;
+
+namespace MessengerApi
+{
+    /// <summary>
+    /// This exists so you can mock it.
+    /// </summary>
+    public interface ISubscriptionClient : IDisposable
+    {
+        /// <summary>
+        /// Subscribes to given message type mask. Thread-safe.
+        /// </summary>
+        /// <remarks>Expected format of mask: "MY-MESSAGE-TYPE". No wildcards or placeholders, we only compare MessageType.StartsWith using this value.</remarks>
+        Subscription Subscribe(string payloadTypeMask);
+
+        /// <summary>
+        /// Unsubscribes from given subscriptions. Thread-safe.
+        /// </summary>
+        void Unsubscribe(Subscription subscription);
+    }
+
+    /// <summary>
+    /// This is where you begin. Instantiate one of these and start subscribing with <see cref="Subscribe(string)"/>.
+    /// </summary>
+    public class SubscriptionClient : Client, ISubscriptionClient
+    {
+        private readonly SubscriptionPollingEngine _engine;
+
+        private bool _isDisposed;
+
+        public SubscriptionClient(Credentials credentials, HttpClient client = null, ILogger logger = null) : base(credentials, client, logger)
+        {
+            this._engine = new SubscriptionPollingEngine(this._logger, this);
+        }
+
+        public void Dispose()
+        {
+            if (this._isDisposed)
+            {
+                return;
+            }
+
+            this._engine.Dispose();
+            this._isDisposed = true;
+        }
+
+        public Subscription Subscribe(string messageTypeMask)
+        {
+            var sub = new Subscription(messageTypeMask, this);
+            this._engine.AddSubscription(sub);
+
+            return sub;
+        }
+
+        public void Unsubscribe(Subscription subscription)
+        {
+            this._engine.RemoveSubscription(subscription);
+        }
+    }
+}