fix: synchronously handle popping page events

The callback of `BackButtonListener` is called before `RootLocationCubit`
update its state. This commit moves all popping page process to the
cubit listener to handle popping events synchronously. Now the listener
is purely an app wide interceptor only redirect popping events without
handling them.

Fixes #305

Author: realth000

lib/features/home/view/home_page.dart

@@ -17,13 +17,14 @@ import 'package:tsdm_client/features/local_notice/stream.dart';
 import 'package:tsdm_client/features/notification/bloc/notification_state_auto_sync_cubit.dart';
 import 'package:tsdm_client/features/notification/models/models.dart';
 import 'package:tsdm_client/features/root/bloc/root_location_cubit.dart';
+import 'package:tsdm_client/features/root/models/models.dart';
+import 'package:tsdm_client/features/root/stream/root_location_stream.dart';
 import 'package:tsdm_client/features/settings/bloc/settings_bloc.dart';
 import 'package:tsdm_client/features/settings/repositories/settings_repository.dart';
 import 'package:tsdm_client/i18n/strings.g.dart';
 import 'package:tsdm_client/instance.dart';
 import 'package:tsdm_client/routes/screen_paths.dart';
 import 'package:tsdm_client/shared/providers/storage_provider/storage_provider.dart';
-import 'package:tsdm_client/shared/repositories/forum_home_repository/forum_home_repository.dart';
 import 'package:tsdm_client/utils/logger.dart';
 import 'package:tsdm_client/utils/platform.dart';
 import 'package:tsdm_client/utils/show_dialog.dart';
@@ -34,12 +35,7 @@ const _drawerWidth = 250.0;
 /// Page of the homepage of the app.
 class HomePage extends StatefulWidget {
   /// Constructor.
-  const HomePage({
-    required ForumHomeRepository forumHomeRepository,
-    required this.showNavigationBar,
-    required this.child,
-    super.key,
-  }) : _forumHomeRepository = forumHomeRepository;
+  const HomePage({required this.showNavigationBar, required this.child, super.key});
 
   /// Control to show the app level navigation bar or not.
   ///
@@ -49,8 +45,6 @@ class HomePage extends StatefulWidget {
   /// Child widget, or call it the body widget.
   final Widget child;
 
-  final ForumHomeRepository _forumHomeRepository;
-
   @override
   State<HomePage> createState() => _HomePageState();
 }
@@ -244,39 +238,68 @@ class _HomePageState extends State<HomePage> with LoggerMixin {
               );
             }
 
-            return RepositoryProvider.value(
-              value: widget._forumHomeRepository,
+            // The global listener handles app-wide leave page events.
+            // Every time user intended to leave a certain page, `lastRequestLeavePageTime` is updated and this
+            // listener process check for leave event, decide the page can be popped or not.
+            //
+            // The logic here is to implements double-press before exit app feature. All popping page events are
+            // intercepted by the `BackButtonListener` below, it only triggers the update of `lastRequestLeavePageTime`,
+            // which notifies this listener.
+            //
+            // Every time the pop is allowed, update page locations in `RootLocationCubit` by adding
+            // `RootLocationEventLeave` to stream.
+            return BlocListener<RootLocationCubit, RootLocationState>(
+              listenWhen: (prev, curr) => prev.lastRequestLeavePageTime != curr.lastRequestLeavePageTime,
+              listener: (context, state) async {
+                // Check if fine to pop the current page.
+                final location = state.locations.lastOrNull;
+                if (location != ScreenPaths.homepage &&
+                    location != ScreenPaths.topic &&
+                    location != ScreenPaths.settings.path &&
+                    location != null) {
+                  // Popping current page will not close the app, allow to pop.
+                  rootLocationStream.add(RootLocationEventLeave(location));
+                  return context.pop();
+                }
+
+                // Check for double press exit feature.
+                // From here, if we intend to pop the page, in fact we shall exit the app.
+
+                final doublePressExit = getIt.get<SettingsRepository>().currentSettings.doublePressExit;
+                if (!doublePressExit) {
+                  // Double-press before exit feature is disabled, exit app.
+                  await exitApp();
+                  return;
+                }
+
+                // From here, double-press before exit feature is enabled.
+
+                final tr = context.t.home;
+                final currentTime = DateTime.now();
+                if (lastPopTime == null ||
+                    currentTime.difference(lastPopTime!).inMilliseconds > exitConfirmDuration.inMilliseconds) {
+                  lastPopTime = currentTime;
+                  ScaffoldMessenger.of(context).hideCurrentSnackBar();
+                  showSnackBar(context: context, message: tr.confirmExit);
+                  // Require the second pop event.
+                  return;
+                }
+
+                // A second pop event is here, exit app.
+                await exitApp();
+                // Unreachable
+                return;
+              },
               child: BackButtonListener(
                 onBackButtonPressed: () async {
+                  // App wide popping events interceptor, handles all popping events and notify the listener above.
+                  rootLocationStream.add(const RootLocationEventLeavingLast());
                   if (!context.mounted) {
+                    // Well, leave it here.
                     await exitApp();
                     return true;
                   }
-                  final location = context.read<RootLocationCubit>().current;
-                  if (location != ScreenPaths.homepage &&
-                      location != ScreenPaths.topic &&
-                      location != ScreenPaths.settings.path) {
-                    // Do NOT handle pop events on other pages.
-                    return false;
-                  }
-                  final doublePressExit = getIt.get<SettingsRepository>().currentSettings.doublePressExit;
-                  if (!doublePressExit) {
-                    // Do NOT handle pop events on double press check is disabled.
-                    await exitApp();
-                    return true;
-                  }
-                  final tr = context.t.home;
-                  final currentTime = DateTime.now();
-                  if (lastPopTime == null ||
-                      currentTime.difference(lastPopTime!).inMilliseconds > exitConfirmDuration.inMilliseconds) {
-                    lastPopTime = currentTime;
-                    ScaffoldMessenger.of(context).hideCurrentSnackBar();
-                    showSnackBar(context: context, message: tr.confirmExit);
-                    return true;
-                  }
-                  await exitApp();
-                  // Unreachable
-                  return false;
+                  return true;
                 },
                 child: child,
               ),

lib/features/root/bloc/root_location_cubit.dart

@@ -1,33 +1,60 @@
 import 'dart:async';
 
 import 'package:bloc/bloc.dart';
+import 'package:dart_mappable/dart_mappable.dart';
 import 'package:tsdm_client/features/root/models/models.dart';
 import 'package:tsdm_client/features/root/stream/root_location_stream.dart';
+import 'package:tsdm_client/routes/screen_paths.dart';
 import 'package:tsdm_client/utils/logger.dart';
 
+part 'root_location_cubit.mapper.dart';
+part 'root_location_state.dart';
+
 /// Cubit to store and control current page route.
 ///
 /// State is a list (more exactly, stack) of screen paths that pages ever
 /// enter currently.
-final class RootLocationCubit extends Cubit<List<String>> with LoggerMixin {
+///
+/// Now this class handles page locations. Every page path saved in state is the page we in or nested in. The global
+/// cubit listener is responsible to check pop page logic, ensuring all pop requests are satisfies user settings.
+final class RootLocationCubit extends Cubit<RootLocationState> with LoggerMixin {
   /// Constructor.
-  RootLocationCubit() : super(const []) {
+  RootLocationCubit() : super(const RootLocationState()) {
     _sub = rootLocationStream.stream.listen(
       (event) => switch (event) {
         RootLocationEventEnter(:final path) => () {
+          // Already enter new page.
           debug('enter page $path');
-          emit(state.toList()..add(path));
+          emit(state.copyWith(locations: state.locations.toList()..add(path)));
         }(),
         RootLocationEventLeave(:final path) => () {
+          // Already leave the current page.
           debug('leave page $path');
-          if (state.lastOrNull != path) {
+          if (currentPath != path) {
+            if (path == ScreenPaths.homepage) {
+              // Special case for shelled route.
+              //emit(state.toList()..removeWhere((e) => e == path));
+              return;
+            }
+
             error(
               'failed to leave page non-current path $path, current '
               'page is $state',
             );
             return;
           }
-          emit(state.toList()..removeLast());
+          emit(state.copyWith(locations: state.locations.toList()..removeLast()));
+        }(),
+        RootLocationEventLeavingLast() => () {
+          // Intend to leave current page.
+          debug('leave last page');
+          if (state.locations.isEmpty) {
+            error('location state already empty');
+            return;
+          }
+          // Till now we do not know it it's find to really leave current page.
+          // Instead, update request time to notify the global listener which works on this.
+          emit(state.copyWith(lastRequestLeavePageTime: DateTime.now()));
         }(),
       },
     );
@@ -36,13 +63,13 @@ final class RootLocationCubit extends Cubit<List<String>> with LoggerMixin {
   late final StreamSubscription<RootLocationEvent> _sub;
 
   /// Get the current path;
-  String? get current => state.lastOrNull;
+  String? get currentPath => state.locations.lastOrNull;
 
   /// Currently in page [path] or not.
-  bool isIn(String path) => state.lastOrNull == path;
+  bool isIn(String path) => state.locations.lastOrNull == path;
 
   /// Ever pushed page [path] in route stack or not.
-  bool ever(String path) => state.contains(path);
+  bool ever(String path) => state.locations.contains(path);
 
   @override
   Future<void> close() async {

lib/features/root/bloc/root_location_state.dart

@@ -0,0 +1,21 @@
+part of 'root_location_cubit.dart';
+
+/// The state of root location cubit.
+@MappableClass()
+final class RootLocationState with RootLocationStateMappable {
+  /// Constructor.
+  const RootLocationState({this.lastRequestLeavePageTime, this.locations = const []});
+
+  /// Date time when last request leave page, like an ID of different pop page requests.
+  ///
+  /// Use this field to identity different pop page requests and trigger the listener which handles double-press
+  /// exit app feature.
+  ///
+  /// Literally it is not semantic, but ok.
+  final DateTime? lastRequestLeavePageTime;
+
+  /// Current locations.
+  ///
+  /// Nested route paths.
+  final List<String> locations;
+}

lib/features/root/models/root_location_event.dart

@@ -18,6 +18,8 @@ final class RootLocationEventEnter extends RootLocationEvent with RootLocationEv
 }
 
 /// Leaved (or say popped) a new page with path [path].
+///
+/// When this event is triggered, page at [path] is already popped.
 @MappableClass()
 final class RootLocationEventLeave extends RootLocationEvent with RootLocationEventLeaveMappable {
   /// Constructor.
@@ -26,3 +28,15 @@ final class RootLocationEventLeave extends RootLocationEvent with RootLocationEv
   /// Path of page entered.
   final String path;
 }
+
+/// About to leave the last page in location.
+///
+/// Use this event to let the cubit knows: The last page is going to pop.
+///
+/// When this event is triggered, it does not mean the last page is already popped, only a request to trigger leave
+/// page checking logic for the cubit.
+@MappableClass()
+final class RootLocationEventLeavingLast extends RootLocationEvent with RootLocationEventLeavingLastMappable {
+  /// Constructor.
+  const RootLocationEventLeavingLast();
+}

lib/features/root/view/root_page.dart

@@ -26,12 +26,6 @@ class _RootPageState extends State<RootPage> with LoggerMixin {
     rootLocationStream.add(RootLocationEventEnter(widget.path));
   }
 
-  @override
-  void dispose() {
-    rootLocationStream.add(RootLocationEventLeave(widget.path));
-    super.dispose();
-  }
-
   @override
   Widget build(BuildContext context) {
     return widget.child;

lib/routes/app_routes.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:go_router/go_router.dart';
 import 'package:tsdm_client/extensions/string.dart';
 import 'package:tsdm_client/features/authentication/view/login_page.dart';
@@ -49,7 +48,6 @@ import 'package:tsdm_client/features/update/view/local_changelog_page.dart';
 import 'package:tsdm_client/features/update/view/update_page.dart';
 import 'package:tsdm_client/routes/screen_paths.dart';
 import 'package:tsdm_client/shared/models/models.dart';
-import 'package:tsdm_client/shared/repositories/forum_home_repository/forum_home_repository.dart';
 
 /// App router instance wrapped with global singleton widgets.
 final router = GoRouter(
@@ -62,11 +60,7 @@ final _appRoutes = [
   StatefulShellRoute.indexedStack(
     builder: (context, router, navigator) {
       final hideNavigationBarPages = [ScreenPaths.settingsThreadAppearance.fullPath];
-      return HomePage(
-        forumHomeRepository: RepositoryProvider.of<ForumHomeRepository>(context),
-        showNavigationBar: !hideNavigationBarPages.contains(router.fullPath),
-        child: navigator,
-      );
+      return HomePage(showNavigationBar: !hideNavigationBarPages.contains(router.fullPath), child: navigator);
     },
     branches: [
       StatefulShellBranch(