fix: prevent unwanted list jumping during paging refresh

Fixes #347

Fixes SDP-34

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 -   Fix a case where collapsing the parent of an already collapsed comment caused it to become expanded
 -   Tweak progress bar UI to use a linear indicator
 -   Allow app bar to hide when scrolling down
+-   Fix bug where refreshing a list of posts caused it to jump around multiple times
 
 ## [1.14.0] - 2022-12-12
 

android/src/main/kotlin/dev/msfjarvis/claw/android/paging/LobstersPagingSource.kt

@@ -27,13 +27,14 @@ constructor(
 ) : PagingSource<Int, LobstersPost>() {
 
   override suspend fun load(params: LoadParams<Int>): LoadResult<Int, LobstersPost> {
-    val page = params.key ?: 1
+    val page = params.key ?: STARTING_PAGE_INDEX
     return when (val result = withContext(ioDispatcher) { remoteFetcher.getItemsAtPage(page) }) {
       is Success ->
         LoadResult.Page(
+          itemsBefore = (page - 1) * PAGE_SIZE,
           data = result.value,
-          prevKey = if (page == 1) null else page - 1,
+          prevKey = if (page == STARTING_PAGE_INDEX) null else page - 1,
-          nextKey = page.plus(1)
+          nextKey = page + 1,
         )
       is Failure.NetworkFailure -> LoadResult.Error(result.error)
       is Failure.UnknownFailure -> LoadResult.Error(result.error)
@@ -42,12 +43,19 @@ constructor(
     }
   }
 
-  override fun getRefreshKey(state: PagingState<Int, LobstersPost>): Int {
+  override fun getRefreshKey(state: PagingState<Int, LobstersPost>): Int? {
-    return state.pages.size + 1
+    return state.anchorPosition?.let { anchorPosition ->
+      (anchorPosition / PAGE_SIZE).coerceAtLeast(STARTING_PAGE_INDEX)
+    }
   }
 
   @AssistedFactory
   interface Factory {
     fun create(remoteFetcher: RemoteFetcher<LobstersPost>): LobstersPagingSource
   }
+
+  companion object {
+    const val PAGE_SIZE = 25
+    const val STARTING_PAGE_INDEX = 1
+  }
 }

android/src/main/kotlin/dev/msfjarvis/claw/android/ui/lists/NetworkPosts.kt

@@ -57,7 +57,7 @@ fun NetworkPosts(
       LazyColumn(
         state = listState,
       ) {
-        items(lazyPagingItems) { item ->
+        items(items = lazyPagingItems, key = { it.shortId }) { item ->
           if (item != null) {
             val dbModel = item.toDbModel()
             ListItem(
@@ -70,7 +70,7 @@ fun NetworkPosts(
           }
         }
         if (lazyPagingItems.loadState.append == LoadState.Loading) {
-          item {
+          item(key = "progressbar") {
             ProgressBar(
               modifier =
                 Modifier.fillMaxWidth()

android/src/main/kotlin/dev/msfjarvis/claw/android/viewmodel/ClawViewModel.kt

@@ -15,6 +15,8 @@ import com.slack.eithernet.ApiResult.Failure
 import com.slack.eithernet.ApiResult.Success
 import dev.msfjarvis.claw.android.injection.IODispatcher
 import dev.msfjarvis.claw.android.paging.LobstersPagingSource
+import dev.msfjarvis.claw.android.paging.LobstersPagingSource.Companion.PAGE_SIZE
+import dev.msfjarvis.claw.android.paging.LobstersPagingSource.Companion.STARTING_PAGE_INDEX
 import dev.msfjarvis.claw.android.ui.toLocalDateTime
 import dev.msfjarvis.claw.api.LobstersApi
 import dev.msfjarvis.claw.database.local.SavedPost
@@ -38,9 +40,13 @@ constructor(
   @IODispatcher private val ioDispatcher: CoroutineDispatcher,
 ) : ViewModel() {
   private val hottestPostsPager =
-    Pager(PagingConfig(pageSize = 20)) { pagingSourceFactory.create(api::getHottestPosts) }
+    Pager(PagingConfig(pageSize = PAGE_SIZE), initialKey = STARTING_PAGE_INDEX) {
+      pagingSourceFactory.create(api::getHottestPosts)
+    }
   private val newestPostsPager =
-    Pager(PagingConfig(pageSize = 20)) { pagingSourceFactory.create(api::getHottestPosts) }
+    Pager(PagingConfig(pageSize = PAGE_SIZE), initialKey = STARTING_PAGE_INDEX) {
+      pagingSourceFactory.create(api::getHottestPosts)
+    }
 
   val hottestPosts
     get() = hottestPostsPager.flow