Stop showing errors if page is invisible

lib/widget/home_albums.dart

@@ -28,6 +28,7 @@ import 'package:nc_photos/widget/archive_viewer.dart';
 import 'package:nc_photos/widget/dynamic_album_viewer.dart';
 import 'package:nc_photos/widget/home_app_bar.dart';
 import 'package:nc_photos/widget/new_album_dialog.dart';
+import 'package:nc_photos/widget/page_visibility_mixin.dart';
 import 'package:tuple/tuple.dart';
 
 class HomeAlbums extends StatefulWidget {
@@ -42,7 +43,8 @@ class HomeAlbums extends StatefulWidget {
   final Account account;
 }
 
-class _HomeAlbumsState extends State<HomeAlbums> {
+class _HomeAlbumsState extends State<HomeAlbums>
+    with RouteAware, PageVisibilityMixin<HomeAlbums> {
   @override
   initState() {
     super.initState();
@@ -307,10 +309,12 @@ class _HomeAlbumsState extends State<HomeAlbums> {
       _transformItems(state.albums);
     } else if (state is ListAlbumBlocFailure) {
       _transformItems(state.albums);
-      SnackBarManager().showSnackBar(SnackBar(
-        content: Text(exception_util.toUserString(state.exception, context)),
-        duration: k.snackBarDurationNormal,
-      ));
+      if (isPageVisible()) {
+        SnackBarManager().showSnackBar(SnackBar(
+          content: Text(exception_util.toUserString(state.exception, context)),
+          duration: k.snackBarDurationNormal,
+        ));
+      }
     } else if (state is ListAlbumBlocInconsistent) {
       _reqQuery();
     }

lib/widget/home_photos.dart

@@ -33,6 +33,7 @@ import 'package:nc_photos/use_case/update_property.dart';
 import 'package:nc_photos/widget/album_picker_dialog.dart';
 import 'package:nc_photos/widget/home_app_bar.dart';
 import 'package:nc_photos/widget/measure.dart';
+import 'package:nc_photos/widget/page_visibility_mixin.dart';
 import 'package:nc_photos/widget/photo_list_item.dart';
 import 'package:nc_photos/widget/popup_menu_zoom.dart';
 import 'package:nc_photos/widget/selectable_item_stream_list_mixin.dart';
@@ -51,7 +52,11 @@ class HomePhotos extends StatefulWidget {
 }
 
 class _HomePhotosState extends State<HomePhotos>
-    with WidgetsBindingObserver, SelectableItemStreamListMixin<HomePhotos> {
+    with
+        WidgetsBindingObserver,
+        SelectableItemStreamListMixin<HomePhotos>,
+        RouteAware,
+        PageVisibilityMixin {
   @override
   initState() {
     super.initState();
@@ -256,10 +261,12 @@ class _HomePhotosState extends State<HomePhotos>
       }
     } else if (state is ScanDirBlocFailure) {
       _transformItems(state.files);
-      SnackBarManager().showSnackBar(SnackBar(
-        content: Text(exception_util.toUserString(state.exception, context)),
-        duration: k.snackBarDurationNormal,
-      ));
+      if (isPageVisible()) {
+        SnackBarManager().showSnackBar(SnackBar(
+          content: Text(exception_util.toUserString(state.exception, context)),
+          duration: k.snackBarDurationNormal,
+        ));
+      }
     } else if (state is ScanDirBlocInconsistent) {
       _reqQuery();
     }

lib/widget/my_app.dart

@@ -24,6 +24,10 @@ import 'package:nc_photos/widget/viewer.dart';
 class MyApp extends StatefulWidget {
   @override
   createState() => _MyAppState();
+
+  static RouteObserver get routeObserver => _routeObserver;
+
+  static final _routeObserver = RouteObserver<PageRoute>();
 }
 
 class _MyAppState extends State<MyApp> implements SnackBarHandler {
@@ -46,6 +50,7 @@ class _MyAppState extends State<MyApp> implements SnackBarHandler {
       themeMode: Pref.inst().isDarkTheme() ? ThemeMode.dark : ThemeMode.light,
       initialRoute: Splash.routeName,
       onGenerateRoute: _onGenerateRoute,
+      navigatorObservers: <NavigatorObserver>[MyApp.routeObserver],
       scaffoldMessengerKey: _scaffoldMessengerKey,
       locale: language_util.getSelectedLocale(context),
       localizationsDelegates: AppLocalizations.localizationsDelegates,

lib/widget/page_visibility_mixin.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/widgets.dart';
+import 'package:nc_photos/widget/my_app.dart';
+
+mixin PageVisibilityMixin<T extends StatefulWidget> on State<T>, RouteAware {
+  @override
+  didChangeDependencies() {
+    super.didChangeDependencies();
+    MyApp.routeObserver.subscribe(this, ModalRoute.of(context));
+  }
+
+  @override
+  dispose() {
+    MyApp.routeObserver.unsubscribe(this);
+    super.dispose();
+  }
+
+  @override
+  didPush() {
+    _isVisible = true;
+  }
+
+  @override
+  didPushNext() {
+    _isVisible = false;
+  }
+
+  @override
+  didPop() {
+    _isVisible = false;
+  }
+
+  @override
+  didPopNext() {
+    _isVisible = true;
+  }
+
+  bool isPageVisible() => _isVisible;
+
+  var _isVisible = true;
+}