Improve how snack bars are queued

app/lib/download_handler.dart

@@ -157,13 +157,13 @@ class _DownloadHandlerWeb extends _DownloadHandlerBase {
     String? parentDir,
   }) async {
     _log.info("[downloadFiles] Downloading ${files.length} file");
-    var controller = SnackBarManager().showSnackBar(SnackBar(
-      content: Text(L10n.global().downloadProcessingNotification),
-      duration: k.snackBarDurationShort,
-    ));
-    controller?.closed.whenComplete(() {
-      controller = null;
-    });
+    SnackBarManager().showSnackBar(
+      SnackBar(
+        content: Text(L10n.global().downloadProcessingNotification),
+        duration: k.snackBarDurationShort,
+      ),
+      canBeReplaced: true,
+    );
     int successCount = 0;
     for (final f in files) {
       try {
@@ -175,7 +175,6 @@ class _DownloadHandlerWeb extends _DownloadHandlerBase {
         ++successCount;
       } on PermissionException catch (_) {
         _log.warning("[downloadFiles] Permission not granted");
-        controller?.close();
         SnackBarManager().showSnackBar(SnackBar(
           content: Text(L10n.global().errorNoStoragePermission),
           duration: k.snackBarDurationNormal,
@@ -186,7 +185,6 @@ class _DownloadHandlerWeb extends _DownloadHandlerBase {
         break;
       } catch (e, stackTrace) {
         _log.shout("[downloadFiles] Failed while DownloadFile", e, stackTrace);
-        controller?.close();
         SnackBarManager().showSnackBar(SnackBar(
           content: Text("${L10n.global().downloadFailureNotification}: "
               "${exception_util.toUserString(e)}"),
@@ -195,7 +193,6 @@ class _DownloadHandlerWeb extends _DownloadHandlerBase {
       }
     }
     if (successCount > 0) {
-      controller?.close();
       SnackBarManager().showSnackBar(SnackBar(
         content: Text(L10n.global().downloadSuccessNotification),
         duration: k.snackBarDurationShort,

app/lib/notified_action.dart

@@ -14,25 +14,22 @@ class NotifiedAction {
   });
 
   Future<void> call() async {
-    ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? controller;
     if (processingText != null) {
-      controller = SnackBarManager().showSnackBar(SnackBar(
-        content: Text(processingText!),
-        duration: k.snackBarDurationShort,
-      ));
+      SnackBarManager().showSnackBar(
+        SnackBar(
+          content: Text(processingText!),
+          duration: k.snackBarDurationShort,
+        ),
+        canBeReplaced: true,
+      );
     }
-    controller?.closed.whenComplete(() {
-      controller = null;
-    });
     try {
       await action();
-      controller?.close();
       SnackBarManager().showSnackBar(SnackBar(
         content: Text(successText),
         duration: k.snackBarDurationNormal,
       ));
     } catch (e) {
-      controller?.close();
       SnackBarManager().showSnackBar(SnackBar(
         content: Text(
             (failureText?.isNotEmpty == true ? "$failureText: " : "") +
@@ -67,16 +64,15 @@ class NotifiedListAction<T> {
 
   /// Perform the action and return the success count
   Future<int> call() async {
-    ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? controller;
     if (processingText != null) {
-      controller = SnackBarManager().showSnackBar(SnackBar(
-        content: Text(processingText!),
-        duration: k.snackBarDurationShort,
-      ));
+      SnackBarManager().showSnackBar(
+        SnackBar(
+          content: Text(processingText!),
+          duration: k.snackBarDurationShort,
+        ),
+        canBeReplaced: true,
+      );
     }
-    controller?.closed.whenComplete(() {
-      controller = null;
-    });
     final failedItems = <T>[];
     for (final item in list) {
       try {
@@ -87,7 +83,6 @@ class NotifiedListAction<T> {
       }
     }
     if (failedItems.isEmpty) {
-      controller?.close();
       SnackBarManager().showSnackBar(SnackBar(
         content: Text(successText),
         duration: k.snackBarDurationNormal,
@@ -95,7 +90,6 @@ class NotifiedListAction<T> {
     } else {
       final failureText = getFailureText?.call(failedItems);
       if (failureText?.isNotEmpty == true) {
-        controller?.close();
         SnackBarManager().showSnackBar(SnackBar(
           content: Text(failureText!),
           duration: k.snackBarDurationNormal,

app/lib/share_handler.dart

@@ -166,21 +166,19 @@ class ShareHandler {
         return;
       }
       _log.info("[_shareAsLink] Share as folder: ${result.albumName}");
-      ScaffoldFeatureController? controller =
-          SnackBarManager().showSnackBar(SnackBar(
-        content: Text(L10n.global().createShareProgressText),
-        duration: k.snackBarDurationShort,
-      ));
-      controller?.closed.whenComplete(() {
-        controller = null;
-      });
+      SnackBarManager().showSnackBar(
+        SnackBar(
+          content: Text(L10n.global().createShareProgressText),
+          duration: k.snackBarDurationShort,
+        ),
+        canBeReplaced: true,
+      );
       clearSelection?.call();
       isSelectionCleared = true;
 
       final c = KiwiContainer().resolve<DiContainer>();
       final path = await _createDir(c.fileRepo, account, result.albumName);
       await _copyFilesToDir(c.fileRepo, account, files, path);
-      controller?.close();
       return _shareFileAsLink(
         account,
         File(

app/lib/snack_bar_manager.dart

@@ -1,3 +1,5 @@
+import 'dart:collection';
+
 import 'package:flutter/material.dart';
 import 'package:logging/logging.dart';
 
@@ -9,7 +11,8 @@ import 'package:logging/logging.dart';
 class SnackBarManager {
   factory SnackBarManager() => _inst;
 
-  SnackBarManager._();
+  @visibleForTesting
+  SnackBarManager.scoped();
 
   void registerHandler(SnackBarHandler handler) {
     _handlers.add(handler);
@@ -19,42 +22,69 @@ class SnackBarManager {
     _handlers.remove(handler);
   }
 
-  /// Show a snack bar if possible
+  /// Queue a snack bar to be shown ASAP
   ///
-  /// If the snack bar can't be shown at this time, return null.
+  /// If the snack bar can't be shown, return null.
   ///
   /// If [canBeReplaced] is true, this snackbar will be dismissed by the next
   /// snack bar
-  ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? showSnackBar(
+  void showSnackBar(
     SnackBar snackBar, {
     bool canBeReplaced = false,
   }) {
-    if (_canPrevBeReplaced) {
-      _prevController?.close();
+    _add(_Item(snackBar, canBeReplaced));
+    _ensureRunning();
+  }
+
+  void _ensureRunning() {
+    if (!_isRunning) {
+      _isRunning = true;
+      _next();
     }
-    _canPrevBeReplaced = canBeReplaced;
+  }
+
+  void _add(_Item item) {
+    _queue.add(item);
+    if (_currentItem?.canBeReplaced == true) {
+      _currentItem!.controller?.close();
+    }
+  }
+
+  Future<void> _next() async {
+    if (_queue.isEmpty) {
+      _isRunning = false;
+      return;
+    }
+    final item = _queue.removeFirst();
+    if (item.canBeReplaced && _queue.isNotEmpty) {
+      _log.info("[_next] Skip replaceable snack bar");
+      return _next();
+    }
+    // show item
     for (final h in _handlers.reversed) {
-      final result = h.showSnackBar(snackBar);
-      if (result != null) {
-        _prevController = result;
-        result.closed.whenComplete(() {
-          if (identical(_prevController, result)) {
-            _prevController = null;
-          }
-        });
-        return result;
+      final controller = h.showSnackBar(item.snackBar);
+      if (controller != null) {
+        item.controller = controller;
+        _currentItem = item;
+        try {
+          final reason = await controller.closed;
+          _log.fine("[_next] Snack bar closed: ${reason.name}");
+        } finally {
+          _currentItem = null;
+        }
+        return _next();
       }
     }
-    _log.warning("[showSnackBar] No handler available");
-    _prevController = null;
-    return null;
+    _log.warning("[_next] No handler available");
+    return _next();
   }
 
   final _handlers = <SnackBarHandler>[];
-  ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? _prevController;
-  var _canPrevBeReplaced = false;
+  final Queue<_Item> _queue = DoubleLinkedQueue();
+  var _isRunning = false;
+  _Item? _currentItem;
 
-  static final _inst = SnackBarManager._();
+  static final _inst = SnackBarManager.scoped();
 
   static final _log = Logger("snack_bar_manager.SnackBarManager");
 }
@@ -63,3 +93,11 @@ abstract class SnackBarHandler {
   ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? showSnackBar(
       SnackBar snackBar);
 }
+
+class _Item {
+  _Item(this.snackBar, this.canBeReplaced);
+
+  final SnackBar snackBar;
+  final bool canBeReplaced;
+  ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? controller;
+}

app/lib/widget/trashbin_viewer.dart

@@ -157,17 +157,16 @@ class _TrashbinViewerState extends State<TrashbinViewer> {
   void _onRestorePressed() async {
     final file = widget.streamFiles[_viewerController.currentPage];
     _log.info("[_onRestorePressed] Restoring file: ${file.path}");
-    var controller = SnackBarManager().showSnackBar(SnackBar(
-      content: Text(L10n.global().restoreProcessingNotification),
-      duration: k.snackBarDurationShort,
-    ));
-    controller?.closed.whenComplete(() {
-      controller = null;
-    });
+    SnackBarManager().showSnackBar(
+      SnackBar(
+        content: Text(L10n.global().restoreProcessingNotification),
+        duration: k.snackBarDurationShort,
+      ),
+      canBeReplaced: true,
+    );
     try {
       await RestoreTrashbin(KiwiContainer().resolve<DiContainer>())(
           widget.account, file);
-      controller?.close();
       SnackBarManager().showSnackBar(SnackBar(
         content: Text(L10n.global().restoreSuccessNotification),
         duration: k.snackBarDurationNormal,
@@ -178,7 +177,6 @@ class _TrashbinViewerState extends State<TrashbinViewer> {
     } catch (e, stacktrace) {
       _log.shout("Failed while restore trashbin: ${logFilename(file.path)}", e,
           stacktrace);
-      controller?.close();
       SnackBarManager().showSnackBar(SnackBar(
         content: Text("${L10n.global().restoreFailureNotification}: "
             "${exception_util.toUserString(e)}"),

app/test/snack_bar_manager_test.dart

@@ -0,0 +1,70 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:nc_photos/snack_bar_manager.dart';
+
+void main() {
+  group("SnackBarManager", () {
+    group("showSnackBar", () {
+      testWidgets("canBeReplaced = true", (tester) async {
+        final manager = SnackBarManager.scoped();
+        await tester.pumpWidget(_TestWidget(manager));
+        manager.showSnackBar(
+          const SnackBar(
+            content: Text("test1"),
+            duration: Duration(seconds: 10),
+          ),
+          canBeReplaced: true,
+        );
+        await tester.pumpAndSettle();
+        expect(find.text("test1"), findsOneWidget);
+
+        manager.showSnackBar(const SnackBar(
+          content: Text("test2"),
+          duration: Duration(seconds: 1),
+        ));
+        await tester.pumpAndSettle();
+        expect(find.text("test1"), findsNothing);
+        expect(find.text("test2"), findsOneWidget);
+      });
+    });
+  });
+}
+
+class _TestWidget extends StatefulWidget {
+  const _TestWidget(this.manager, {Key? key}) : super(key: key);
+
+  @override
+  createState() => _TestWidgetState();
+
+  final SnackBarManager manager;
+}
+
+class _TestWidgetState extends State<_TestWidget> implements SnackBarHandler {
+  @override
+  initState() {
+    super.initState();
+    widget.manager.registerHandler(this);
+  }
+
+  @override
+  build(BuildContext context) {
+    return MaterialApp(
+      scaffoldMessengerKey: _scaffoldMessengerKey,
+      home: Scaffold(
+        body: Container(),
+      ),
+    );
+  }
+
+  @override
+  dispose() {
+    super.dispose();
+    widget.manager.unregisterHandler(this);
+  }
+
+  @override
+  showSnackBar(SnackBar snackBar) =>
+      _scaffoldMessengerKey.currentState?.showSnackBar(snackBar);
+
+  final _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
+}