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"""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 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
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