@Override
 public void clear() {
   super.clear();
   manager.mSelectedStatusIds.clear();
   manager.mSelectedUserIds.clear();
   manager.onItemsCleared();
 }
 public void unregisterCallback(final Callback callback) {
   mCallbacks.remove(callback);
 }
 public void registerCallback(final Callback callback) {
   if (callback == null) return;
   mCallbacks.add(callback);
 }
 public boolean isUserSelected(final UserKey userKey) {
   return mSelectedUserIds.contains(userKey);
 }
 public boolean isStatusSelected(final String statusId) {
   return mSelectedStatusIds.contains(statusId);
 }