Observers register a callback on an event. Two specialised forms of callback allow an observer to be notified immediately before the main list so they can prepare, or after the main list so they can tidy up.
An observable raises an event notification by calling tikiinternaleventlib->raise_event(...). The library then calls all the functions in the pre_event, event and post_event lists in turn:
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IRC log of July 16th 2005 that started the whole thing