Initial Contribution
diff --git a/froofle/protobuf/internal/message_listener.py b/froofle/protobuf/internal/message_listener.py
new file mode 100644
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+++ b/froofle/protobuf/internal/message_listener.py
@@ -0,0 +1,69 @@
+# Protocol Buffers - Google's data interchange format
+# Copyright 2008 Google Inc. All rights reserved.
+# http://code.google.com/p/protobuf/
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+# * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+# * Neither the name of Google Inc. nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+"""Defines a listener interface for observing certain
+state transitions on Message objects.
+
+Also defines a null implementation of this interface.
+"""
+
+__author__ = 'robinson@google.com (Will Robinson)'
+
+
+class MessageListener(object):
+
+ """Listens for transitions to nonempty and for invalidations of cached
+ byte sizes. Meant to be registered via Message._SetListener().
+ """
+
+ def TransitionToNonempty(self):
+ """Called the *first* time that this message becomes nonempty.
+ Implementations are free (but not required) to call this method multiple
+ times after the message has become nonempty.
+ """
+ raise NotImplementedError
+
+ def ByteSizeDirty(self):
+ """Called *every* time the cached byte size value
+ for this object is invalidated (transitions from being
+ "clean" to "dirty").
+ """
+ raise NotImplementedError
+
+
+class NullMessageListener(object):
+
+ """No-op MessageListener implementation."""
+
+ def TransitionToNonempty(self):
+ pass
+
+ def ByteSizeDirty(self):
+ pass