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## Some context
An aggregation is an event that relates to another event: for instance,
a
reaction, a poll response, and so on and so forth.
## Some requirements
Because of the sync mechanisms and federation, it can happen that a
related
event is received *before* receiving the event it relates to. Those
events
must be accounted for, stashed somewhere, and reapplied later, if/when
the
related-to event shows up.
In addition to that, a room's event cache can also decide to move events
around, in its own internal representation (likely because it ran into
some
duplicate events, or it managed to decrypt a previously UTD event).
When that happens, a timeline opened on the given room
will see a removal then re-insertion of the given event. If that event
was
the target of aggregations, then those aggregations must be re-applied
when
the given event is reinserted.
## Some solution
To satisfy both requirements, the [`Aggregations`] "manager" object
provided
by this PR will take care of memoizing aggregations, **for the entire
lifetime of the timeline** (or until it's clear'd by some
caller). Aggregations are saved in memory, and have the same lifetime as
that of a timeline. This makes it possible to apply pending aggregations
to cater for the first use case, and to never lose any aggregations in
the
second use case.
## Some points for the reviewer
- I think the most controversial point is that all aggregations are
memoized for the entire lifetime of the timeline. Would that become an
issue, we can get back to some incremental scheme, in the future:
instead of memoizing aggregations for the entire lifetime of the
timeline, we'd attach them to a single timeline item. When that item is
removed, we'd put the aggregations back into a "pending" stash of
aggregations. If the item is reinserted later, we could peek at the
pending stash of aggregations, remove any that's in there, and reapply
them to the reinserted event. This is what the [first version of this
patch](ec64b9e0bc)
did, in a much more adhoc way, for reactions only; based on the current
PR, we could do the same in a simpler manner
- while the PR has small commits, they don't quite make sense to review
individually, I'm afraid, as I was trying to find a way to make a
general system that would work not only for reactions, poll responses
and ends. As a matter of fact, the first commits may have introduced
code that is changed in subsequent commits, making the review a bit
hazardous. Happy to have a live reviewing party over Element Call, if
that helps, considering the size of the patch.
- future work may include using the aggregations manager for edits too,
leading to more code removal.