GUI -- Further work on refactoring Topology View server side code. Still WIP...
- added topology client heartbeat.
- modified AbstractListenerRegistry to allow for extension.

Change-Id: Ib8ea6ad4ba34f5732d062f1c9ef545f105eb167b
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java b/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java
index 478bfe7..211058a 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/TopologyViewMessageHandler.java
@@ -99,6 +99,7 @@
     private static final String SPRITE_LIST_REQ = "spriteListRequest";
     private static final String SPRITE_DATA_REQ = "spriteDataRequest";
     private static final String TOPO_START = "topoStart";
+    private static final String TOPO_HEARTBEAT = "topoHeartbeat";
     private static final String TOPO_STOP = "topoStop";
 
 
@@ -170,6 +171,7 @@
     protected Collection<RequestHandler> createRequestHandlers() {
         return ImmutableSet.of(
                 new TopoStart(),
+                new TopoHeartbeat(),
                 new TopoStop(),
                 new ReqSummary(),
                 new CancelSummary(),
@@ -211,6 +213,18 @@
     }
 
     @Deprecated
+    private final class TopoHeartbeat extends RequestHandler {
+        private TopoHeartbeat() {
+            super(TOPO_HEARTBEAT);
+        }
+
+        @Override
+        public void process(long sid, ObjectNode payload) {
+            // place holder for now
+        }
+    }
+
+    @Deprecated
     private final class TopoStop extends RequestHandler {
         private TopoStop() {
             super(TOPO_STOP);