fix: Restore the user's reading position under all circumstances (#133)

The previous code did not always work when the user returned to the app
after a lengthy absence (e.g., overnight).

Instead of restoring by scrolling in `TimelineFragment`, restore by
working with the platform.

Determine the initial page to fetch by looking half a page ahead of the
saved saved status ID, and fetch that status and the page immediately
prior. This seems to match the view's expectations about what will be
immediately available.

Set `jumpThreshold` and `enablePlaceholders` in the `PagingConfig` so
the paging system will jump to the saved status.

Remove the restoration code in `TimelineFragment`.

Fixes #53

app/lint-baseline.xml

@@ -3688,7 +3688,7 @@
         errorLine2="                                              ~~~~~~~~~">
         <location
             file="src/main/java/app/pachli/components/timeline/TimelineFragment.kt"
-            line="185"
+            line="180"
             column="47"/>
     </issue>
 

app/src/main/java/app/pachli/components/timeline/CachedTimelineRepository.kt

@@ -85,7 +85,7 @@ class CachedTimelineRepository @Inject constructor(
         Log.d(TAG, "initialKey: $initialKey is row: $row")
 
         return Pager(
-            config = PagingConfig(pageSize = pageSize),
+            config = PagingConfig(pageSize = pageSize, jumpThreshold = PAGE_SIZE * 3, enablePlaceholders = true),
             initialKey = row,
             remoteMediator = CachedTimelineRemoteMediator(
                 initialKey,

app/src/main/java/app/pachli/components/timeline/TimelineFragment.kt

@@ -131,9 +131,6 @@ class TimelineFragment :
 
     private var isSwipeToRefreshEnabled = true
 
-    /** True if the reading position should be restored when new data is submitted to the adapter */
-    private var shouldRestoreReadingPosition = false
-
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
 
@@ -141,8 +138,6 @@ class TimelineFragment :
 
         timelineKind = arguments.getParcelable(KIND_ARG)!!
 
-        shouldRestoreReadingPosition = timelineKind == TimelineKind.Home
-
         viewModel.init(timelineKind)
 
         isSwipeToRefreshEnabled = arguments.getBoolean(ARG_ENABLE_SWIPE_TO_REFRESH, true)
@@ -359,28 +354,6 @@ class TimelineFragment :
                         if (userRefreshState == UserRefreshState.COMPLETE) {
                             // Refresh has finished, pages are being prepended.
 
-                            // Restore the user's reading position, if appropriate.
-                            if (shouldRestoreReadingPosition) {
-                                Log.d(
-                                    TAG,
-                                    "Page updated, should restore reading position to ${viewModel.readingPositionId}",
-                                )
-                                adapter.snapshot()
-                                    .indexOfFirst { it?.id == viewModel.readingPositionId }
-                                    .takeIf { it != -1 }
-                                    ?.let { pos ->
-                                        Log.d(TAG, "restored reading position")
-                                        binding.recyclerView.post {
-                                            getView() ?: return@post
-                                            (binding.recyclerView.layoutManager as? LinearLayoutManager)?.scrollToPositionWithOffset(
-                                                pos,
-                                                0,
-                                            )
-                                        }
-                                        shouldRestoreReadingPosition = false
-                                    }
-                            }
-
                             // There might be multiple prepends after a refresh, only continue
                             // if one them has not already caused a peek.
                             if (peeked) return@collect
@@ -528,10 +501,11 @@ class TimelineFragment :
      * previous first status always remains visible.
      */
     fun saveVisibleId(statusId: String? = null) {
-        statusId ?: layoutManager.findFirstCompletelyVisibleItemPosition()
+        val id = statusId ?: layoutManager.findFirstCompletelyVisibleItemPosition()
             .takeIf { it != RecyclerView.NO_POSITION }
             ?.let { adapter.snapshot().getOrNull(it)?.id }
-            ?.let {
+
+        id?.let {
             Log.d(TAG, "Saving ID: $it")
             viewModel.accept(InfallibleUiAction.SaveVisibleId(visibleId = it))
         }
@@ -569,7 +543,6 @@ class TimelineFragment :
     }
 
     override fun onRefresh() {
-        shouldRestoreReadingPosition = timelineKind == TimelineKind.Home
         binding.statusView.hide()
         snackbar?.dismiss()
 

app/src/main/java/app/pachli/components/timeline/viewmodel/CachedTimelineRemoteMediator.kt

@@ -71,10 +71,12 @@ class CachedTimelineRemoteMediator(
         return try {
             val response = when (loadType) {
                 LoadType.REFRESH -> {
-                    val closestItem = state.anchorPosition?.let { state.closestItemToPosition(it) }?.status?.serverId
+                    val closestItem = state.anchorPosition?.let {
-                    val key = closestItem ?: initialKey
+                        state.closestItemToPosition(maxOf(0, it - (state.config.pageSize / 2)))
-                    Log.d(TAG, "Loading from item: $key")
+                    }?.status?.serverId
-                    getInitialPage(key, state.config.pageSize)
+                    val statusId = closestItem ?: initialKey
+                    Log.d(TAG, "Loading from item: $statusId")
+                    getInitialPage(statusId, state.config.pageSize)
                 }
                 LoadType.APPEND -> {
                     val rke = db.withTransaction {
@@ -200,18 +202,14 @@ class CachedTimelineRemoteMediator(
         // You can fetch the page immediately before the key, or the page immediately after, but
         // you can not fetch the page itself.
 
-        // Fetch the requested status, and the pages immediately before (prev) and after (next)
+        // Fetch the requested status, and the page immediately after (next)
         val deferredStatus = async { api.status(statusId = statusId) }
         val deferredNextPage = async {
             api.homeTimeline(maxId = statusId, limit = pageSize)
         }
-        val deferredPrevPage = async {
-            api.homeTimeline(minId = statusId, limit = pageSize)
-        }
 
         deferredStatus.await().getOrNull()?.let { status ->
             val statuses = buildList {
-                deferredPrevPage.await().body()?.let { this.addAll(it) }
                 this.add(status)
                 deferredNextPage.await().body()?.let { this.addAll(it) }
             }
@@ -243,7 +241,7 @@ class CachedTimelineRemoteMediator(
         // There were no statuses older than the user's desired status. Return the page
         // of statuses immediately newer than their desired status. This page must
         // *not* be empty (as noted earlier, if it is, paging stops).
-        deferredPrevPage.await().let { response ->
+        api.homeTimeline(minId = statusId, limit = pageSize).let { response ->
             if (response.isSuccessful) {
                 if (!response.body().isNullOrEmpty()) return@coroutineScope response
             }

app/src/main/java/app/pachli/db/AppDatabase.kt

@@ -24,6 +24,7 @@ import androidx.room.RoomDatabase
 import androidx.room.migration.AutoMigrationSpec
 import app.pachli.components.conversation.ConversationEntity
 
+@Suppress("ClassName")
 @Database(
     entities = [
         DraftEntity::class,