The article "Observing a Subject's Attributes" has been posted to the Ada95
patterns list.  It appears under "September 1999" in the patterns archive.

<http://www.acm.org/archives/patterns.html>

All the Ada95 source code is included in the message.

Here's the introduction:

In previous implementations of the Observer pattern, the observer would
query the entire state of the subject during a notification, because it
had no way of knowing which specific attribute had changed.

Here, we vary the pattern so that the observer can observe each subject
attribute individually, so it only has to display the attribute that has
changed.


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Matt