Adding model abstractions to help flesh-out the concepts.
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java
new file mode 100644
index 0000000..fcbe858
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyEvent.java
@@ -0,0 +1,43 @@
+package org.onlab.onos.net.topology;
+
+import org.onlab.onos.event.AbstractEvent;
+import org.onlab.onos.net.Topology;
+
+/**
+ * Describes network topology event.
+ */
+public class TopologyEvent extends AbstractEvent<TopologyEvent.Type, Topology> {
+
+ /**
+ * Type of topology events.
+ */
+ public enum Type {
+ /**
+ * Signifies that topology has changed.
+ */
+ TOPOLOGY_CHANGED
+ }
+
+ /**
+ * Creates an event of a given type and for the specified topology and the
+ * current time.
+ *
+ * @param type topology event type
+ * @param topology event topology subject
+ */
+ public TopologyEvent(Type type, Topology topology) {
+ super(type, topology);
+ }
+
+ /**
+ * Creates an event of a given type and for the specified topology and time.
+ *
+ * @param type link event type
+ * @param topology event topology subject
+ * @param time occurrence time
+ */
+ public TopologyEvent(Type type, Topology topology, long time) {
+ super(type, topology, time);
+ }
+
+}