feat(room list service): skip the initial state when an initial pos is set

crates/matrix-sdk-ui/src/room_list_service/mod.rs

@@ -177,7 +177,17 @@ impl RoomListService {
         // Eagerly subscribe the event cache to sync responses.
         client.event_cache().subscribe()?;
 
-        Ok(Self { client, sliding_sync, state_machine: StateMachine::new() })
+        let state_machine = StateMachine::new();
+
+        // If the sliding sync has successfully restored a sync position, skip the
+        // waiting for the initial sync, and set the state to `SettingUp`; this
+        // way, the first sync will move us to the steady state, and update the
+        // sliding sync list to use the growing sync mode.
+        if sliding_sync.has_pos().await {
+            state_machine.set(State::SettingUp);
+        }
+
+        Ok(Self { client, sliding_sync, state_machine })
     }
 
     /// Start to sync the room list.

crates/matrix-sdk-ui/src/room_list_service/state.rs

@@ -107,8 +107,8 @@ impl StateMachine {
         self.state.subscribe()
     }
 
-    /// Transition to the next state, and execute the associated transition's
-    /// [`Actions`].
+    /// Transition to the next state, and execute the necessary transition on
+    /// the sliding sync list.
     pub(super) async fn next(&self, sliding_sync: &SlidingSync) -> Result<State, Error> {
         use State::*;
 

crates/matrix-sdk-ui/tests/integration/room_list_service.rs

@@ -1292,19 +1292,21 @@ async fn test_loading_states() -> Result<(), Error> {
             RoomListLoadingState::Loaded { maximum_number_of_rooms: Some(12) }
         );
 
+        // The sync skips the `Init` state, and immediately starts in the `SettingUp`
+        // state, as the sync `pos`ition marker was set.
         sync_then_assert_request_and_fake_response! {
             [server, room_list, sync]
-            states = Init => SettingUp,
+            states = SettingUp => Running,
             assert request >= {
                 "lists": {
                     ALL_ROOMS: {
-                        "ranges": [[0, 19]],
+                        "ranges": [[0, 11]],
                         "timeline_limit": 1,
                     },
                 },
             },
             respond with = {
-                "pos": "0",
+                "pos": "3",
                 "lists": {
                     ALL_ROOMS: {
                         "count": 13, // 1 more room

crates/matrix-sdk/src/sliding_sync/mod.rs

@@ -124,6 +124,12 @@ impl SlidingSync {
         Self { inner: Arc::new(inner) }
     }
 
+    /// Whether the current sliding sync instance has set a sync position
+    /// marker.
+    pub async fn has_pos(&self) -> bool {
+        self.inner.position.lock().await.pos.is_some()
+    }
+
     async fn cache_to_storage(&self, position: &SlidingSyncPositionMarkers) -> Result<()> {
         cache::store_sliding_sync_state(self, position).await
     }