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The idea is generally good and something I have implemented in some modules already (article module for cw.dk and OBS_Classroom, the latter is available at
http://xoops.web-udvikling.dk).
The main challenge when making a generic system-wide solution is that there is practically no way that the core can know which pages are influenced by a change.
I mean - say we write a reply to a forum thead. Then the thread needs updating, the forum page needs updating and perhaps the top-level forum listing page needs updating.
Not to mention that any blocks showing the latest threads, latest posts or most active threads will also need updating.
The only one, who can actually know this, is the module developer - and even then, the blocks updating will be quite difficult.
So I'd say that module content can be cached and updated with this method, but blocks are more of a challenge, and it will not be easy to implement with core "magic" but must be implemented in the individual modules.
"When you can flatten entire cities at a whim, a tendency towards quiet reflection and seeing-things-from-the-other-fellow's-point-of-view is seldom necessary."
Cusix Software