Support for listening to DocumentTree modifications.

Change-Id: Ibe7c67e6615f5a19fe4c4c7dea182e1a59dc5eea

@@ -102,4 +102,29 @@ public interface DocumentTree<V> {
      * @throws IllegalDocumentModificationException if the remove to be removed
      */
     V removeNode(DocumentPath key);
+
+    /**
+     * Registers a listener to be notified when a subtree rooted at the specified path
+     * is modified.
+     *
+     * @param path path to root of subtree to monitor for updates
+     * @param listener listener to be notified
+     */
+    void addListener(DocumentPath path, DocumentTreeListener<V> listener);
+
+    /**
+     * Unregisters a previously added listener.
+     *
+     * @param listener listener to unregister
+     */
+    void removeListener(DocumentTreeListener<V> listener);
+
+    /**
+     * Registers a listener to be notified when the tree is modified.
+     *
+     * @param listener listener to be notified
+     */
+    default void addListener(DocumentTreeListener<V> listener) {
+        addListener(root(), listener);
+    }
 }

+/*
+ * Copyright 2016-present 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.store.service;
+
+import java.util.Optional;
+
+import com.google.common.base.MoreObjects;
+
+/**
+ * A document tree modification event.
+ *
+ * @param <V> tree node value type
+ */
+public class DocumentTreeEvent<V> {
+
+    /**
+     * Nature of document tree node change.
+     */
+    public enum Type {
+
+        /**
+         * Signifies node being created.
+         */
+        CREATED,
+
+        /**
+         * Signifies the value of an existing node being updated.
+         */
+        UPDATED,
+
+        /**
+         * Signifies an existing node being deleted.
+         */
+        DELETED
+    }
+
+    private final DocumentPath path;
+    private final Type type;
+    private final Optional<Versioned<V>> newValue;
+    private final Optional<Versioned<V>> oldValue;
+
+    /**
+     * Constructs a new {@code DocumentTreeEvent}.
+     *
+     * @param path path to the node
+     * @param type type of change
+     * @param newValue optional new value; will be empty if node was deleted
+     * @param oldValue optional old value; will be empty if node was created
+     */
+    public DocumentTreeEvent(DocumentPath path,
+                             Type type,
+                             Optional<Versioned<V>> newValue,
+                             Optional<Versioned<V>> oldValue) {
+        this.path = path;
+        this.type = type;
+        this.newValue = newValue;
+        this.oldValue = oldValue;
+    }
+
+    /**
+     * Returns the path to the changed node.
+     *
+     * @return node path
+     */
+    public DocumentPath path() {
+        return path;
+    }
+
+    /**
+     * Returns the change type.
+     * @return change type
+     */
+    public Type type() {
+        return type;
+    }
+
+    /**
+     * Returns the new value.
+     *
+     * @return optional new value; will be empty if node was deleted
+     */
+    public Optional<Versioned<V>> newValue() {
+        return newValue;
+    }
+
+    /**
+     * Returns the old value.
+     *
+     * @return optional old value; will be empty if node was created
+     */
+    public Optional<Versioned<V>> oldValue() {
+        return oldValue;
+    }
+
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(getClass())
+                .add("path", path)
+                .add("type", type)
+                .add("newValue", newValue)
+                .add("oldValue", oldValue)
+                .toString();
+    }
+}

+/*
+ * Copyright 2016-present 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.store.service;
+
+/**
+ * A listener for {@link DocumentTreeEvent}.
+ *
+ * @param <V> document tree node value type
+ */
+public interface DocumentTreeListener<V> {
+
+    /**
+     * Callback notifying about change to document tree node.
+     *
+     * @param event event
+     */
+    void event(DocumentTreeEvent<V> event);
+}