This variant will be used to cause side-effects to existing timeline
items, because the event they relate to is for an aggregation
(edit/reaction/etc.).
This is used here for reactions.
This patch updates `EventHandler` to use the correct regions where
appropriate, thus reducing the complexity of the code, and removing
classes of bugs.
In the case of `Flow::Remote { position: TimelineItemPosition::At { …
}}`, we no longer need to skip the local timeline items, and to handle
the presence of the `TimelineStart` timeline item. The code is less
complex.
In the case of `Flow::Remote { position: TimelineItemPosition::End { …
}}`, that's exactly the same at the previous case.
In the case of `recycle_local_or_create_item`, the `try_fold` approach
is replaced entirely with a simple `iter_locals_region`, reducing the
size of the comments explaining the code, reducing the complexity of the
code, and reducing the surface of bugs.
This patch updates `TimelineMetadata` to work on the _remotes_ region
only, excluding the _start_ and the _locals_ regions. It helps to reduce
the risk of inserting items in an incorrect regions.
This patch also removes on more `rfind_event_by_id` usage, which is
nice.
This patch updates `ReadReceiptTimelineUpdate` to work on the _remotes_
region only, excluding the _start_ and the _lcoals_ regions. It helps
to reduce the risk of inserting a `ReadMarker` inside the _start_ or the
_locals_ regions.
This patch updates `DateDividerAdjuster` to work on _remotes_ and
_locals_ regions only, excluding the _start_ region. It helps to reduce
the risk of inserting a `DateDivider` inside the _start_ region.
This patch also uses the new `push_date_divider` method, which provides
a couple of invariants.
This patch defines a new concept in the `Timeline`: Regions.
The `ObservableItems` holds all the invariants about the _position_ of the
items. It defines three regions where items can live:
1. the _start_ region, which can only contain a single `TimelineStart`,
2. the _remotes_ region, which can only contain many `Remote` timeline
items with their decorations (only `DateDivider`s and `ReadMarker`s),
3. the _locals_ region, which can only contain many `Local` timeline items
with their decorations (only `DateDivider`s).
The `iter_all_regions` method allows to iterate over all regions.
`iter_remotes_region` will restrict the iterator over the _remotes_
region, and so on. These iterators provide the absolute indices of the
items, so that it's harder to make mistakes when manipulating the indices of
items with operations like `insert`, `remove`, `replace` etc.
Other methods like `push_local` or `push_date_divider` insert the items
in the correct region, and check a couple of invariants.
This patch implements the `has_local` method on
`ObservableItemsTransaction`, which is way faster than the previous the
previous solution which was to iterate over all items to find at least
one local timeline item.
This patch adds the `push_local` method on `ObservableItemsTransaction`
to add semantics and hardcode the invariant in a single place for the
different timeline items.
This patch adds the `push_timeline_start_if_missing` method on
`ObservableItemsTransaction` to add semantics and hardcode the
invariant in a single place for the different timeline items.
