route: communicate routing changes using signals

The RoutingTable now has a signal that it emits upon finishing
calculation, which other modules can subscribe to. The NamePrefixTable
now references this emitted container to update its local copies of
routing information.

Change-Id: Id601776258ad0367cc47461ea8f86a710eada132
refs: #4249
diff --git a/src/signals.hpp b/src/signals.hpp
new file mode 100644
index 0000000..b8518b0
--- /dev/null
+++ b/src/signals.hpp
@@ -0,0 +1,36 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2014-2017,  The University of Memphis,
+ *                           Regents of the University of California
+ *
+ * This file is part of NLSR (Named-data Link State Routing).
+ * See AUTHORS.md for complete list of NLSR authors and contributors.
+ *
+ * NLSR is free software: you can redistribute it and/or modify it under the terms
+ * of the GNU General Public License as published by the Free Software Foundation,
+ * either version 3 of the License, or (at your option) any later version.
+ *
+ * NLSR is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+ * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+ * PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * NLSR, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ **/
+
+#ifndef NLSR_SIGNALS_HPP
+#define NLSR_SIGNALS_HPP
+
+#include <ndn-cxx/util/signal.hpp>
+
+namespace nlsr {
+
+class RoutingTable;
+class RoutingTableEntry;
+
+using AfterRoutingChange = ndn::util::Signal<RoutingTable, const std::list<RoutingTableEntry>&>;
+
+} // namespace nlsr
+
+#endif // NLSR_SIGNALS_HPP