refactor(ui): Create ObservableItemsTransactionEntry.

This patch creates `ObservableItemsTransactionEntry` that mimics
`ObservableVectorTransactionEntry`. The differences are `set` is
renamed `replace`, and `remove` is unsafe (because I failed to update
`AllRemoteEvents` in this method due to the borrow checker).

crates/matrix-sdk-ui/src/timeline/controller/mod.rs

@@ -54,7 +54,10 @@ use tracing::{
 #[cfg(test)]
 pub(super) use self::observable_items::ObservableItems;
 pub(super) use self::{
-    observable_items::{AllRemoteEvents, ObservableItemsEntry, ObservableItemsTransaction},
+    observable_items::{
+        AllRemoteEvents, ObservableItemsEntry, ObservableItemsTransaction,
+        ObservableItemsTransactionEntry,
+    },
     state::{
         FullEventMeta, PendingEdit, PendingEditKind, TimelineMetadata, TimelineNewItemPosition,
         TimelineState, TimelineStateTransaction,

crates/matrix-sdk-ui/src/timeline/controller/observable_items.rs

@@ -279,11 +279,11 @@ impl<'observable_items> ObservableItemsTransaction<'observable_items> {
 
     /// Call the given closure for every element in this `ObservableItems`,
     /// with an entry struct that allows updating that element.
-    pub fn for_each<F>(&mut self, f: F)
+    pub fn for_each<F>(&mut self, mut f: F)
     where
-        F: FnMut(ObservableVectorTransactionEntry<'_, 'observable_items, Arc<TimelineItem>>),
+        F: FnMut(ObservableItemsTransactionEntry<'_, 'observable_items>),
     {
-        self.items.for_each(f)
+        self.items.for_each(|entry| f(ObservableItemsTransactionEntry(entry)))
     }
 
     /// Commit this transaction, persisting the changes and notifying
@@ -302,6 +302,37 @@ impl Deref for ObservableItemsTransaction<'_> {
     }
 }
 
+/// A handle to a single timeline item in an `ObservableItemsTransaction`.
+pub struct ObservableItemsTransactionEntry<'a, 'observable_items>(
+    ObservableVectorTransactionEntry<'a, 'observable_items, Arc<TimelineItem>>,
+);
+
+impl<'a, 'o> ObservableItemsTransactionEntry<'a, 'o> {
+    /// Replace the timeline item by `timeline_item`.
+    pub fn replace(this: &mut Self, timeline_item: Arc<TimelineItem>) -> Arc<TimelineItem> {
+        ObservableVectorTransactionEntry::set(&mut this.0, timeline_item)
+    }
+
+    /// Remove this timeline item.
+    ///
+    /// # Safety
+    ///
+    /// This method doesn't update `AllRemoteEvents`. Be sure that the caller
+    /// doesn't break the mapping between remote events and timeline items. See
+    /// [`EventMeta::timeline_item_index`] to learn more.
+    pub unsafe fn remove(this: Self) {
+        ObservableVectorTransactionEntry::remove(this.0);
+    }
+}
+
+impl Deref for ObservableItemsTransactionEntry<'_, '_> {
+    type Target = Arc<TimelineItem>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
 /// A type for all remote events.
 ///
 /// Having this type helps to know exactly which parts of the code and how they

crates/matrix-sdk-ui/src/timeline/controller/state.rs

@@ -19,7 +19,6 @@ use std::{
     sync::{Arc, RwLock},
 };
 
-use eyeball_im::ObservableVectorTransactionEntry;
 use itertools::Itertools as _;
 use matrix_sdk::{
     deserialized_responses::SyncTimelineEvent, ring_buffer::RingBuffer, send_queue::SendHandle,
@@ -45,7 +44,10 @@ use ruma::{
 use tracing::{debug, instrument, trace, warn};
 
 use super::{
-    observable_items::{AllRemoteEvents, ObservableItems, ObservableItemsTransaction},
+    observable_items::{
+        AllRemoteEvents, ObservableItems, ObservableItemsTransaction,
+        ObservableItemsTransactionEntry,
+    },
     HandleManyEventsResult, TimelineFocusKind, TimelineSettings,
 };
 use crate::{
@@ -655,7 +657,12 @@ impl TimelineStateTransaction<'_> {
             // Remove all remote events and the read marker
             self.items.for_each(|entry| {
                 if entry.is_remote_event() || entry.is_read_marker() {
-                    ObservableVectorTransactionEntry::remove(entry);
+                    // SAFETY: this method removes all events except local events. Local events
+                    // don't have a mapping from remote events to timeline items because… well… they
+                    // are local events, not remove events.
+                    unsafe {
+                        ObservableItemsTransactionEntry::remove(entry);
+                    }
                 }
             });
 

crates/matrix-sdk-ui/src/timeline/event_handler.rs

@@ -15,7 +15,6 @@
 use std::sync::Arc;
 
 use as_variant::as_variant;
-use eyeball_im::ObservableVectorTransactionEntry;
 use indexmap::IndexMap;
 use matrix_sdk::{
     crypto::types::events::UtdCause,
@@ -52,30 +51,23 @@ use tracing::{debug, error, field::debug, info, instrument, trace, warn};
 
 use super::{
     controller::{
-        ObservableItemsTransaction, PendingEditKind, TimelineMetadata, TimelineStateTransaction,
+        ObservableItemsTransaction, ObservableItemsTransactionEntry, PendingEdit, PendingEditKind,
+        TimelineMetadata, TimelineStateTransaction,
     },
     day_dividers::DayDividerAdjuster,
     event_item::{
         extract_bundled_edit_event_json, extract_poll_edit_content, extract_room_msg_edit_content,
         AnyOtherFullStateEventContent, EventSendState, EventTimelineItemKind,
-        LocalEventTimelineItem, PollState, Profile, ReactionsByKeyBySender, RemoteEventOrigin,
-        RemoteEventTimelineItem, TimelineEventItemId,
+        LocalEventTimelineItem, PollState, Profile, ReactionInfo, ReactionStatus,
+        ReactionsByKeyBySender, RemoteEventOrigin, RemoteEventTimelineItem, TimelineEventItemId,
     },
-    reactions::FullReactionKey,
+    reactions::{FullReactionKey, PendingReaction},
+    traits::RoomDataProvider,
     util::{rfind_event_by_id, rfind_event_item},
-    EventTimelineItem, InReplyToDetails, OtherState, Sticker, TimelineDetails, TimelineItem,
-    TimelineItemContent,
-};
-use crate::{
-    events::SyncTimelineEventWithoutContent,
-    timeline::{
-        controller::PendingEdit,
-        event_item::{ReactionInfo, ReactionStatus},
-        reactions::PendingReaction,
-        traits::RoomDataProvider,
-        RepliedToEvent,
-    },
+    EventTimelineItem, InReplyToDetails, OtherState, RepliedToEvent, Sticker, TimelineDetails,
+    TimelineItem, TimelineItemContent,
 };
+use crate::events::SyncTimelineEventWithoutContent;
 
 /// When adding an event, useful information related to the source of the event.
 pub(super) enum Flow {
@@ -1262,7 +1254,7 @@ impl<'a, 'o> TimelineEventHandler<'a, 'o> {
                 TimelineItemContent::Message(message.with_in_reply_to(in_reply_to));
                 let new_reply_item =
                 entry.with_kind(event_item.with_content(new_reply_content, None));
-                ObservableVectorTransactionEntry::set(&mut entry, new_reply_item);
+                ObservableItemsTransactionEntry::replace(&mut entry, new_reply_item);
             }
         });
     }