#!/usr/bin/env python
#
# Copyright 2007 Google Inc.
#
# 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.
"""



class MessageListener(object):

  """Listens for modifications made to a message.  Meant to be registered via
  Message._SetListener().

  Attributes:
    dirty:  If True, then calling Modified() would be a no-op.  This can be
            used to avoid these calls entirely in the common case.
  """

  def Modified(self):
    """Called every time the message is modified in such a way that the parent
    message may need to be updated.  This currently means either:
    (a) The message was modified for the first time, so the parent message
        should henceforth mark the message as present.
    (b) The message's cached byte size became dirty -- i.e. the message was
        modified for the first time after a previous call to ByteSize().
        Therefore the parent should also mark its byte size as dirty.
    Note that (a) implies (b), since new objects start out with a client cached
    size (zero).  However, we document (a) explicitly because it is important.

    Modified() will *only* be called in response to one of these two events --
    not every time the sub-message is modified.

    Note that if the listener's |dirty| attribute is true, then calling
    Modified at the moment would be a no-op, so it can be skipped.  Performance-
    sensitive callers should check this attribute directly before calling since
    it will be true most of the time.
    """

    raise NotImplementedError


class NullMessageListener(object):

  """No-op MessageListener implementation."""

  def Modified(self):
    pass
