fix: fix top-level snack bar

Restore with global key.

Author: realth000

lib/app.dart

@@ -489,6 +489,7 @@ class _AppState extends State<App> with WindowListener, LoggerMixin {
                 theme: lightTheme,
                 darkTheme: darkTheme,
                 themeMode: ThemeMode.values[themeModeIndex],
+                scaffoldMessengerKey: snackbarKey,
               );
             },
           ),

lib/instance.dart

@@ -1,5 +1,6 @@
 import 'dart:ui' as ui;
 
+import 'package:flutter/material.dart';
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
 import 'package:get_it/get_it.dart';
 import 'package:talker_flutter/talker_flutter.dart';
@@ -30,3 +31,11 @@ late final FlutterLocalNotificationsPlugin flnp;
 Future<ui.ImmutableBuffer> getPlaceholderImageData() async {
   return ui.ImmutableBuffer.fromAsset(assetPlaceholderImagePath);
 }
+
+/// The global snackbar key.
+///
+/// Because we have global BlocListeners outside of `MaterialApp` that calls `showSnackBar`, use this global key to
+/// access a context with `Scaffold` to show the snack bar.
+///
+/// Do NOT use this global key directly, call `showSnackBar` function instead.
+final GlobalKey<ScaffoldMessengerState> snackbarKey = GlobalKey<ScaffoldMessengerState>();

lib/utils/show_toast.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:tsdm_client/i18n/strings.g.dart';
+import 'package:tsdm_client/instance.dart';
 
 /// Show a snack bar contains message show no more contents.
 void showNoMoreSnackBar(BuildContext context, {bool floating = true}) {
@@ -21,8 +22,6 @@ void showSnackBar({
   required String message,
   bool floating = true,
   SnackBarAction? action,
-}) {
-  ScaffoldMessenger.of(context).showSnackBar(
-    SnackBar(behavior: floating ? SnackBarBehavior.floating : null, content: Text(message), action: action),
-  );
-}
+}) => snackbarKey.currentState?.showSnackBar(
+  SnackBar(behavior: floating ? SnackBarBehavior.floating : null, content: Text(message), action: action),
+);