From 98bb5720d1b548ed76f371c75309bcfe8d7c17d7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jorge=20Mart=C3=ADn?= <jorgem@element.io>
Date: Wed, 12 Mar 2025 16:19:36 +0100
Subject: [PATCH] Add timeline prefetching

This should trigger when getting close the start of the loaded timeline, making scrolling back smoother, specially when combined with the persistent event cache.
---
 .../features/messages/impl/timeline/TimelineView.kt | 13 +++++++++++++
 .../timeline/components/TimelineItemVirtualRow.kt   |  2 ++
 2 files changed, 15 insertions(+)

diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/TimelineView.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/TimelineView.kt
index 115039a284..d11d655ec5 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/TimelineView.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/TimelineView.kt
@@ -70,8 +70,10 @@ import io.element.android.libraries.designsystem.theme.components.Icon
 import io.element.android.libraries.designsystem.utils.animateScrollToItemCenter
 import io.element.android.libraries.matrix.api.core.EventId
 import io.element.android.libraries.matrix.api.core.UserId
+import io.element.android.libraries.matrix.api.timeline.Timeline
 import io.element.android.libraries.ui.strings.CommonStrings
 import kotlinx.coroutines.launch
+import timber.log.Timber
 
 @Composable
 fun TimelineView(
@@ -175,6 +177,17 @@ fun TimelineView(
                 onClearFocusRequestState = ::clearFocusRequestState
             )
 
+            val isCloseToStartOfLoadedTimeline by remember { derivedStateOf {
+                lazyListState.firstVisibleItemIndex + lazyListState.layoutInfo.visibleItemsInfo.size >= lazyListState.layoutInfo.totalItemsCount - 10
+            } }
+            LaunchedEffect(isCloseToStartOfLoadedTimeline) {
+                // Only back paginate when we're close to the start of the loaded timeline items and the user is actively scrolling
+                if (lazyListState.isScrollInProgress && isCloseToStartOfLoadedTimeline) {
+                    Timber.d("Prefetching pagination with ${lazyListState.layoutInfo.totalItemsCount} items")
+                    state.eventSink(TimelineEvents.LoadMore(Timeline.PaginationDirection.BACKWARDS))
+                }
+            }
+
             TimelineScrollHelper(
                 hasAnyEvent = state.hasAnyEvent,
                 lazyListState = lazyListState,
diff --git a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/TimelineItemVirtualRow.kt b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/TimelineItemVirtualRow.kt
index aa4a006249..2507fb9f01 100644
--- a/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/TimelineItemVirtualRow.kt
+++ b/features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/components/TimelineItemVirtualRow.kt
@@ -28,6 +28,7 @@ import io.element.android.features.messages.impl.timeline.model.virtual.Timeline
 import io.element.android.features.messages.impl.timeline.model.virtual.TimelineItemRoomBeginningModel
 import io.element.android.features.messages.impl.timeline.model.virtual.TimelineItemTypingNotificationModel
 import io.element.android.features.messages.impl.typing.TypingNotificationView
+import timber.log.Timber
 
 @Composable
 fun TimelineItemVirtualRow(
@@ -45,6 +46,7 @@ fun TimelineItemVirtualRow(
                 TimelineLoadingMoreIndicator(virtual.model.direction)
                 val latestEventSink by rememberUpdatedState(eventSink)
                 LaunchedEffect(virtual.model.timestamp) {
+                    Timber.d("Pagination triggered by load more indicator")
                     latestEventSink(TimelineEvents.LoadMore(virtual.model.direction))
                 }
             }