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Change-Id: I0425764b2f3b07bc224a387ab7d544c6b360c691
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/topo/UiTopoSession.java b/web/gui/src/main/java/org/onosproject/ui/impl/topo/UiTopoSession.java
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+++ b/web/gui/src/main/java/org/onosproject/ui/impl/topo/UiTopoSession.java
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+/*
+ *  Copyright 2016 Open Networking Laboratory
+ *
+ *  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.ui.impl.topo;
+
+import org.onosproject.ui.UiTopoLayoutService;
+import org.onosproject.ui.impl.UiWebSocket;
+import org.onosproject.ui.impl.topo.model.UiSharedTopologyModel;
+import org.onosproject.ui.model.topo.UiTopoLayout;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Coordinates with the {@link UiTopoLayoutService} to access
+ * {@link UiTopoLayout}s, and with the {@link UiSharedTopologyModel} which
+ * maintains a local model of the network entities,
+ * tailored specifically for displaying on the UI.
+ * <p>
+ * Note that an instance of this class will be created for each
+ * {@link UiWebSocket} connection, and will contain
+ * the state of how the topology is laid out for the logged-in user.
+ */
+public class UiTopoSession {
+    private final Logger log = LoggerFactory.getLogger(getClass());
+
+    private final String username;
+    private final UiWebSocket webSocket;
+    private final UiSharedTopologyModel sharedModel;
+
+    private boolean registered = false;
+
+    private UiTopoLayoutService service;
+    private UiTopoLayout layout;
+
+    /**
+     * Creates a new topology layout.
+     */
+    public UiTopoSession(String username, UiWebSocket webSocket) {
+        this.username = username;
+        this.webSocket = webSocket;
+        this.sharedModel = UiSharedTopologyModel.instance();
+    }
+
+    /**
+     * Initializes the layout; registering with the shared model.
+     */
+    public void init() {
+        if (!registered) {
+            sharedModel.register(this);
+            registered = true;
+        } else {
+            log.warn("already registered");
+        }
+    }
+
+    /**
+     * Destroys the layout; unregistering from the shared model.
+     */
+    public void destroy() {
+        if (!registered) {
+            sharedModel.unregister(this);
+            registered = false;
+        } else {
+            log.warn("already unregistered");
+        }
+    }
+
+    @Override
+    public String toString() {
+        return String.format("{UiTopoSession for user <%s>}", username);
+    }
+}