1 files changed, 55 insertions, 0 deletions
diff --git a/python/google/protobuf/internal/message_listener.py b/python/google/protobuf/internal/message_listener.py
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+++ b/python/google/protobuf/internal/message_listener.py
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+# Protocol Buffers - Google's data interchange format
+# Copyright 2008 Google Inc.
+# http://code.google.com/p/protobuf/
+#
+# 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.
+
+"""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