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31 """Defines a listener interface for observing certain
32 state transitions on Message objects.
34 Also defines a null implementation of this interface.
37 __author__ = 'robinson@google.com (Will Robinson)'
40 class MessageListener(object):
42 """Listens for modifications made to a message. Meant to be registered via
43 Message._SetListener().
46 dirty: If True, then calling Modified() would be a no-op. This can be
47 used to avoid these calls entirely in the common case.
51 """Called every time the message is modified in such a way that the parent
52 message may need to be updated. This currently means either:
53 (a) The message was modified for the first time, so the parent message
54 should henceforth mark the message as present.
55 (b) The message's cached byte size became dirty -- i.e. the message was
56 modified for the first time after a previous call to ByteSize().
57 Therefore the parent should also mark its byte size as dirty.
58 Note that (a) implies (b), since new objects start out with a client cached
59 size (zero). However, we document (a) explicitly because it is important.
61 Modified() will *only* be called in response to one of these two events --
62 not every time the sub-message is modified.
64 Note that if the listener's |dirty| attribute is true, then calling
65 Modified at the moment would be a no-op, so it can be skipped. Performance-
66 sensitive callers should check this attribute directly before calling since
67 it will be true most of the time.
70 raise NotImplementedError
73 class NullMessageListener(object):
75 """No-op MessageListener implementation."""