From 404c26c62ea99eb6047a19d3af1db499455fb3e7 Mon Sep 17 00:00:00 2001
From: Benoit Marty <benoit@matrix.org>
Date: Mon, 12 Oct 2026 14:33:15 +0200
Subject: [PATCH] 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>
---
 .../android/libraries/matrix/impl/room/JoinedRustRoom.kt     | 1 +
 .../matrix/impl/room/threads/RustThreadsListService.kt       | 5 +++++
 .../matrix/impl/room/threads/RustThreadsListServiceTest.kt   | 2 ++
 3 files changed, 8 insertions(+)

diff --git a/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/JoinedRustRoom.kt b/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/JoinedRustRoom.kt
index 17a3910c98..3be63f4f97 100644
--- a/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/JoinedRustRoom.kt
+++ b/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 {
diff --git a/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListService.kt b/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListService.kt
index a74c5bc378..98248b183b 100644
--- a/libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListService.kt
+++ b/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)
     }
 
diff --git a/libraries/matrix/impl/src/test/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListServiceTest.kt b/libraries/matrix/impl/src/test/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListServiceTest.kt
index 0fc9ad0603..fd9f311b95 100644
--- a/libraries/matrix/impl/src/test/kotlin/io/element/android/libraries/matrix/impl/room/threads/RustThreadsListServiceTest.kt
+++ b/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(