perf(library): reuse collection lookups across selection rebuilds

lib/screens/library_tracks_folder_data.dart

@@ -0,0 +1,25 @@
+import 'package:spotiflac_android/models/track.dart';
+import 'package:spotiflac_android/providers/download_history_provider.dart';
+import 'package:spotiflac_android/providers/library_collections_provider.dart';
+import 'package:spotiflac_android/services/history_database.dart';
+
+/// Collection providers replace their track list when its contents change.
+/// Keep derived data across selection/layout rebuilds of the same snapshot.
+class LibraryTracksFolderData {
+  List<CollectionTrackEntry>? _entries;
+  Set<String> keys = const {};
+  List<Track> tracks = const [];
+  HistoryBatchLookupRequest historyRequest = const HistoryBatchLookupRequest(
+    [],
+  );
+
+  void update(List<CollectionTrackEntry> entries) {
+    if (identical(entries, _entries)) return;
+    _entries = entries;
+    keys = Set.unmodifiable(entries.map((entry) => entry.key));
+    tracks = List.unmodifiable(entries.map((entry) => entry.track));
+    historyRequest = HistoryBatchLookupRequest.snapshot(
+      tracks.map(historyLookupForTrack),
+    );
+  }
+}

lib/screens/library_tracks_folder_screen.dart

@@ -28,6 +28,7 @@ import 'package:spotiflac_android/utils/nav_bar_inset.dart';
 import 'package:spotiflac_android/utils/cover_art_utils.dart';
 import 'package:spotiflac_android/screens/collapsing_header_scroll_mixin.dart';
 import 'package:spotiflac_android/screens/selection_mode_mixin.dart';
+import 'package:spotiflac_android/screens/library_tracks_folder_data.dart';
 import 'package:spotiflac_android/screens/track_metadata_screen.dart';
 import 'package:spotiflac_android/widgets/selection_action_button.dart';
 import 'package:spotiflac_android/widgets/selection_bottom_bar.dart';
@@ -67,6 +68,7 @@ class _LibraryTracksFolderScreenState
         SelectionModeMixin<LibraryTracksFolderScreen>,
         CollapsingHeaderScrollMixin<LibraryTracksFolderScreen> {
   UserPlaylistCollection? playlist;
+  final _folderData = LibraryTracksFolderData();
 
   @override
   void initState() {
@@ -272,7 +274,8 @@ class _LibraryTracksFolderScreenState
         break;
     }
 
-    pruneSelection(entries.map((e) => e.key).toSet());
+    _folderData.update(entries);
+    pruneSelection(_folderData.keys);
 
     final title = switch (widget.mode) {
       LibraryTracksFolderMode.wishlist => context.l10n.collectionWishlist,
@@ -297,16 +300,10 @@ class _LibraryTracksFolderScreenState
       LibraryTracksFolderMode.playlist =>
         context.l10n.collectionPlaylistEmptySubtitle,
     };
-    final folderTracks = entries
-        .map((entry) => entry.track)
-        .toList(growable: false);
-    final historyLookups = folderTracks
-        .map(historyLookupForTrack)
-        .toList(growable: false);
+    final folderTracks = _folderData.tracks;
+    final historyLookups = _folderData.historyRequest.tracks;
     final existingHistoryKeys = ref.watch(
-      downloadHistoryVisibleBatchExistsProvider(
-        HistoryBatchLookupRequest(historyLookups),
-      ),
+      downloadHistoryVisibleBatchExistsProvider(_folderData.historyRequest),
     );
 
     final bottomPadding = MediaQuery.paddingOf(context).bottom;

lib/services/history_database.dart

@@ -48,6 +48,12 @@ class HistoryBatchLookupRequest {
 
   const HistoryBatchLookupRequest(this.tracks);
 
+  /// An immutable request whose hash is computed only once. Use for lists
+  /// retained across widget builds; the legacy constructor remains available.
+  factory HistoryBatchLookupRequest.snapshot(
+    Iterable<HistoryLookupRequest> tracks,
+  ) = _HistoryBatchLookupSnapshot;
+
   @override
   bool operator ==(Object other) {
     if (identical(this, other)) return true;
@@ -65,6 +71,23 @@ class HistoryBatchLookupRequest {
   int get hashCode => Object.hashAll(tracks);
 }
 
+class _HistoryBatchLookupSnapshot extends HistoryBatchLookupRequest {
+  _HistoryBatchLookupSnapshot(Iterable<HistoryLookupRequest> tracks)
+    : super(List.unmodifiable(tracks));
+
+  late final int _hash = Object.hashAll(tracks);
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is HistoryBatchLookupRequest &&
+          hashCode == other.hashCode &&
+          super == other;
+
+  @override
+  int get hashCode => _hash;
+}
+
 class HistoryDatabase {
   // The FTS table is a derived, optional index and is initialized lazily after
   // the existing schema migration. Keep this contract at v13 because the

test/library_tracks_folder_data_test.dart

@@ -0,0 +1,65 @@
+import 'dart:collection';
+import 'package:spotiflac_android/models/track.dart';
+import 'package:spotiflac_android/providers/library_collections_provider.dart';
+import 'package:spotiflac_android/screens/library_tracks_folder_data.dart';
+import 'package:spotiflac_android/services/history_database.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class _ReadCountingList<T> extends ListBase<T> {
+  _ReadCountingList(this.values);
+  final List<T> values;
+  int reads = 0;
+  @override
+  int get length => values.length;
+  @override
+  set length(int value) => throw UnsupportedError('immutable');
+  @override
+  T operator [](int index) {
+    reads++;
+    return values[index];
+  }
+
+  @override
+  void operator []=(int index, T value) => throw UnsupportedError('immutable');
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  test('unchanged collection snapshot does no additional full-list reads', () {
+    final entries = _ReadCountingList(
+      List.generate(
+        4000,
+        (index) => CollectionTrackEntry(
+          key: '$index',
+          addedAt: DateTime.utc(2026),
+          track: Track(
+            id: '$index',
+            name: 'Song $index',
+            artistName: 'Artist',
+            albumName: 'Album',
+            duration: 1,
+          ),
+        ),
+      ),
+    );
+    final data = LibraryTracksFolderData()..update(entries);
+    final request = data.historyRequest;
+    final reads = entries.reads;
+    final hash = request.hashCode;
+    for (var i = 0; i < 100; i++) {
+      data.update(entries);
+      expect(identical(data.historyRequest, request), isTrue);
+      expect(data.historyRequest.hashCode, hash);
+    }
+    expect(entries.reads, reads);
+    expect(data.keys.length, 4000);
+    expect(() => data.tracks.clear(), throwsUnsupportedError);
+    expect(() => request.tracks.clear(), throwsUnsupportedError);
+    expect(request, HistoryBatchLookupRequest(request.tracks));
+    expect(hash, HistoryBatchLookupRequest(request.tracks).hashCode);
+    data.update(entries.take(2).toList());
+    expect(data.keys, {'0', '1'});
+    expect(data.tracks.length, 2);
+    expect(identical(data.historyRequest, request), isFalse);
+  });
+}