fix: remember scroll position per folder (#104)

feature/folder/src/main/java/com/android/swingmusic/folder/presentation/screen/FoldersAndTracks.kt

@@ -39,6 +39,10 @@ import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshotFlow
+import kotlinx.coroutines.flow.drop
+import kotlinx.coroutines.flow.filter
+import kotlinx.coroutines.flow.first
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.StrokeCap
@@ -90,7 +94,9 @@ private fun FoldersAndTracks(
     onGotoArtist: (hash: String) -> Unit,
     baseUrl: String,
     isManualRefreshing: Boolean,
-    onManualRefreshingChange: (Boolean) -> Unit
+    onManualRefreshingChange: (Boolean) -> Unit,
+    getSavedScroll: (path: String) -> Pair<Int, Int>?,
+    onSaveScroll: (path: String, index: Int, offset: Int) -> Unit
 ) {
     val sheetState = rememberModalBottomSheetState()
     val scope = rememberCoroutineScope()
@@ -134,6 +140,25 @@ private fun FoldersAndTracks(
 
     val lazyColumnState = rememberLazyListState()
     val pathsLazyRowState = rememberLazyListState()
+
+    LaunchedEffect(currentFolder.path) {
+        val saved = getSavedScroll(currentFolder.path)
+        if (saved != null) {
+            snapshotFlow { lazyColumnState.layoutInfo.totalItemsCount }
+                .filter { it > 0 }
+                .first()
+            lazyColumnState.scrollToItem(saved.first, saved.second)
+        }
+
+        snapshotFlow {
+            lazyColumnState.firstVisibleItemIndex to lazyColumnState.firstVisibleItemScrollOffset
+        }
+            .drop(1)
+            .collect { (index, offset) ->
+                onSaveScroll(currentFolder.path, index, offset)
+            }
+    }
+
     Scaffold { it ->
         PullToRefreshBox(
             modifier = Modifier.fillMaxSize(),
@@ -654,7 +679,11 @@ fun FoldersAndTracksScreen(
             onGotoArtist = { hash ->
                 navigator.gotoArtistInfo(hash)
             },
-            baseUrl = baseUrl ?: ""
+            baseUrl = baseUrl ?: "",
+            getSavedScroll = { path -> foldersViewModel.getScrollPosition(path) },
+            onSaveScroll = { path, index, offset ->
+                foldersViewModel.saveScrollPosition(path, index, offset)
+            }
         )
     }
 }

feature/folder/src/main/java/com/android/swingmusic/folder/presentation/viewmodel/FoldersViewModel.kt

@@ -68,6 +68,17 @@ class FoldersViewModel @Inject constructor(
         mutableStateOf(FoldersContentPagingState())
     val foldersContentPaging: State<FoldersContentPagingState> = _foldersContentPaging
 
+    private val folderScrollPositions = mutableMapOf<String, Pair<Int, Int>>()
+
+    private fun normalizeFolderPath(path: String): String = path.trimEnd('/')
+
+    fun saveScrollPosition(path: String, index: Int, offset: Int) {
+        folderScrollPositions[normalizeFolderPath(path)] = index to offset
+    }
+
+    fun getScrollPosition(path: String): Pair<Int, Int>? =
+        folderScrollPositions[normalizeFolderPath(path)]
+
     init {
         getFoldersContentPaging(homeDir.path)
     }