Fetch and map the thread list items off the main thread

The thread list subscription was launched in the room coroutine scope, which runs on the main thread, so the initial sync FFI call and the mapping of all the items and diffs were done on the main thread every time a room was opened.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/JoinedRustRoom.kt

@@ -158,6 +158,7 @@ class JoinedRustRoom(
         inner = innerRoom.threadListService(),
         contentMapper = TimelineEventContentMapper(),
         roomCoroutineScope = roomCoroutineScope,
+        roomDispatcher = roomDispatcher,
     )
 
     override val syncUpdateFlow = flow {

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListService.kt

@@ -17,10 +17,12 @@ import io.element.android.libraries.matrix.api.room.threads.ThreadsListService
 import io.element.android.libraries.matrix.impl.timeline.item.event.TimelineEventContentMapper
 import io.element.android.libraries.matrix.impl.timeline.item.event.map
 import io.element.android.libraries.matrix.impl.util.mxCallbackFlow
+import kotlinx.coroutines.CoroutineDispatcher
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.flowOn
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
 import kotlinx.coroutines.flow.onStart
@@ -33,6 +35,7 @@ import org.matrix.rustcomponents.sdk.ThreadListService as InnerThreadListService
 class RustThreadsListService(
     private val inner: InnerThreadListService,
     private val roomCoroutineScope: CoroutineScope,
+    private val roomDispatcher: CoroutineDispatcher,
     private val contentMapper: TimelineEventContentMapper = TimelineEventContentMapper(),
 ) : ThreadsListService {
     private var itemSubscriptionJob: Job? = null
@@ -63,6 +66,8 @@ class RustThreadsListService(
                 updated.apply(diff, contentMapper)
                 items.value = updated
             }
+            // Fetching and mapping the items can be costly, do not do it on the main thread
+            .flowOn(roomDispatcher)
             .launchIn(roomCoroutineScope)
     }
 

libraries/matrix/impl/src/test/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListServiceTest.kt

@@ -18,6 +18,7 @@ import io.element.android.libraries.matrix.test.A_TIMESTAMP
 import io.element.android.libraries.matrix.test.A_USER_ID
 import io.element.android.tests.testutils.lambda.lambdaRecorder
 import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.test.StandardTestDispatcher
 import kotlinx.coroutines.test.TestScope
 import kotlinx.coroutines.test.runCurrent
 import kotlinx.coroutines.test.runTest
@@ -122,6 +123,7 @@ class RustThreadsListServiceTest {
     ) = RustThreadsListService(
         inner = inner,
         roomCoroutineScope = backgroundScope,
+        roomDispatcher = StandardTestDispatcher(testScheduler),
     )
 
     private fun aRustThreadListUpdate() = ThreadListUpdate.Append(