feat(sdk): Emit a RoomEventCacheGenericUpdate when the ThreadEventCache is built.

This patch updates `ThreadEventCache::new` to emit a
`RoomEventCacheGenericUpdate` when it's built, similarly to
`RoomEventCache`. This is important for external API, like Latest Event.

Author: Hywan

crates/matrix-sdk/src/event_cache/caches/mod.rs

@@ -23,10 +23,8 @@ use matrix_sdk_base::{
 use ruma::{OwnedRoomId, RoomId};
 use tokio::sync::{broadcast::Sender, mpsc};
 
-use super::{EventCacheError, EventsOrigin, Result};
-use crate::{
-    client::WeakClient, event_cache::automatic_pagination::AutomaticPagination, room::WeakRoom,
-};
+use super::{EventCacheError, EventsOrigin, Result, automatic_pagination::AutomaticPagination};
+use crate::{client::WeakClient, room::WeakRoom};
 
 pub mod event_focused;
 pub mod event_linked_chunk;

crates/matrix-sdk/src/event_cache/caches/thread/mod.rs

@@ -80,26 +80,40 @@ impl ThreadEventCache {
         generic_update_sender: Sender<RoomEventCacheGenericUpdate>,
         linked_chunk_update_sender: Sender<RoomEventCacheLinkedChunkUpdate>,
     ) -> Result<Self> {
-        let update_sender = ThreadEventCacheUpdateSender::new(generic_update_sender);
+        let update_sender = ThreadEventCacheUpdateSender::new(generic_update_sender.clone());
+
+        let state = LockedThreadEventCacheState::new(
+            room_id.clone(),
+            thread_id.clone(),
+            own_user_id,
+            store,
+            update_sender.clone(),
+            linked_chunk_update_sender,
+        )
+        .await?;
+
+        let timeline_is_not_empty =
+            state.read().await?.thread_linked_chunk().revents().next().is_some();
 
-        Ok(Self {
+        let cache = Self {
             inner: Arc::new(ThreadEventCacheInner {
                 room_id: room_id.clone(),
-                thread_id: thread_id.clone(),
+                thread_id,
                 weak_room,
-                state: LockedThreadEventCacheState::new(
-                    room_id,
-                    thread_id,
-                    own_user_id,
-                    store,
-                    update_sender.clone(),
-                    linked_chunk_update_sender,
-                )
-                .await?,
+                state,
                 pagination_batch_token_notifier: Notify::new(),
                 update_sender,
             }),
-        })
+        };
+
+        // If at least one event has been loaded, it means there is a timeline. Let's
+        // emit a generic update.
+        if timeline_is_not_empty {
+            let _ = generic_update_sender
+                .send(RoomEventCacheGenericUpdate { room_id: room_id.to_owned() });
+        }
+
+        Ok(cache)
     }
 
     /// Subscribe to live events from this thread.