Implement support for roster versioning

Roster versioning is defined in RFC 6121, section 2.2.6; the protocol
was originally described in XEP-0237.

Fixes SMACK-399

source/org/jivesoftware/smack/RosterStore.java

@@ -0,0 +1,67 @@
+/**
+ * All rights reserved. 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.jivesoftware.smack;
+
+import java.util.Collection;
+
+import org.jivesoftware.smack.packet.RosterPacket;
+
+/**
+ * This is an interface for persistent roster store needed to implement
+ * roster versioning as per RFC 6121.
+ */
+
+public interface RosterStore {
+
+    /**
+     * This method returns a collection of all roster items contained in this store.
+     * @return List of {@link RosterEntry}
+     */
+    public Collection<RosterPacket.Item> getEntries();
+    /**
+     * This method returns the roster item in this store for the given JID.
+     * @param bareJid The bare JID of the RosterEntry
+     * @return The {@link RosterEntry} which belongs to that user
+     */
+    public RosterPacket.Item getEntry(String bareJid);
+    /**
+     * This method returns the version number as specified by the "ver" attribute
+     * of the local store. For a fresh store, this MUST be the empty string.
+     * @return local roster version
+     */
+    public String getRosterVersion();
+    /**
+     * This method stores a new roster entry in this store or updates an existing one.
+     * @param item the entry to store
+     * @param version the new roster version
+     * @return True if successful
+     */
+    public boolean addEntry(RosterPacket.Item item, String version);
+    /**
+     * This method updates the store so that it contains only the given entries.
+     * @param items the entries to store
+     * @param version the new roster version
+     * @return True if successful
+     */
+    public boolean resetEntries(Collection<RosterPacket.Item> items, String version);
+    /**
+     * Removes an entry from the store
+     * @param bareJid The bare JID of the entry to be removed
+     * @param version the new roster version
+     * @return True if successful
+     */
+    public boolean removeEntry(String bareJid, String version);
+
+}