Initial implementation of XMPP Publish/Subscribe

Change-Id: I9930056b83004fa7a085f72f63ba973f9ec2d95b
diff --git a/protocols/xmpp/pubsub/api/src/main/java/org/onosproject/xmpp/pubsub/XmppPubSubEvent.java b/protocols/xmpp/pubsub/api/src/main/java/org/onosproject/xmpp/pubsub/XmppPubSubEvent.java
new file mode 100644
index 0000000..d4c3b76
--- /dev/null
+++ b/protocols/xmpp/pubsub/api/src/main/java/org/onosproject/xmpp/pubsub/XmppPubSubEvent.java
@@ -0,0 +1,68 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.onosproject.xmpp.pubsub;
+
+import static com.google.common.base.MoreObjects.toStringHelper;
+
+/**
+ * Abstracts event of XMPP Publish/Subscribe.
+ */
+public final class XmppPubSubEvent<S> {
+
+    /**
+     * Types of XMPP Publish/Subscribe messages.
+     */
+    public enum Type {
+        SUBSCRIBE,
+        UNSUBSCRIBE,
+        PUBLISH,
+        RETRACT
+    }
+
+    private final Type type;
+    private final S subject;
+
+    public XmppPubSubEvent(Type type, S subject) {
+        this.type = type;
+        this.subject = subject;
+    }
+
+    /**
+     * Returns the type of event.
+     *
+     * @return event type
+     */
+    public Type type() {
+        return type;
+    }
+
+    /**
+     * Returns the subject of event.
+     *
+     * @return subject to which this event pertains
+     */
+    public S subject() {
+        return subject;
+    }
+
+    @Override
+    public String toString() {
+        return toStringHelper(this).add("type", type())
+                .add("subject", subject()).toString();
+    }
+
+}