Create an abstraction for APIMessagesRequest

osu.Game/Online/API/APIMessagesRequest.cs

@@ -0,0 +1,28 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Collections.Generic;
+using osu.Framework.IO.Network;
+using osu.Game.Online.Chat;
+
+namespace osu.Game.Online.API
+{
+    public abstract class APIMessagesRequest : APIRequest<List<Message>>
+    {
+        private long? sinceId;
+
+        protected APIMessagesRequest(long? sinceId)
+        {
+            this.sinceId = sinceId;
+        }
+
+        protected override WebRequest CreateWebRequest()
+        {
+            var req = base.CreateWebRequest();
+
+            if (sinceId.HasValue) req.AddParameter(@"since", sinceId.Value.ToString());
+
+            return req;
+        }
+    }
+}