app: Maintain that the navigator stack is never empty

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/app.dart

@@ -223,7 +223,13 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
         theme: themeData,
 
         navigatorKey: ZulipApp.navigatorKey,
-        navigatorObservers: widget.navigatorObservers ?? const [],
+        navigatorObservers: [
+          if (widget.navigatorObservers != null)
+            ...widget.navigatorObservers!,
+            // This must be the last item to maintain the invariant
+            // that the navigator stack is always non-empty.
+            _EmptyStackNavigatorObserver(),
+          ],
         builder: (BuildContext context, Widget? child) {
           if (!ZulipApp.ready.value) {
             SchedulerBinding.instance.addPostFrameCallback(
@@ -246,6 +252,39 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
   }
 }
 
+/// Pushes a route whenever the observed navigator stack becomes empty.
+class _EmptyStackNavigatorObserver extends NavigatorObserver {
+  void _pushRouteIfEmptyStack() async {
+    final navigator = await ZulipApp.navigator;
+    bool isEmptyStack = true;
+    // TODO: find a better way to inspect the navigator stack
+    navigator.popUntil((route) {
+      isEmptyStack = false;
+      return true; // never actually pops
+    });
+    if (isEmptyStack) {
+      unawaited(navigator.push(
+        MaterialWidgetRoute(page: const ChooseAccountPage())));
+    }
+  }
+
+  @override
+  void didRemove(Route<void> route, Route<void>? previousRoute) async {
+    if (previousRoute == null) {
+      // The route removed is the bottom-most one.
+      _pushRouteIfEmptyStack();
+    }
+  }
+
+  @override
+  void didPop(Route<void> route, Route<void>? previousRoute) async {
+    if (previousRoute == null) {
+      // The route popped is the bottom-most one.
+      _pushRouteIfEmptyStack();
+    }
+  }
+}
+
 class ChooseAccountPage extends StatelessWidget {
   const ChooseAccountPage({super.key});
 

test/widgets/app_test.dart

@@ -4,6 +4,7 @@ import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:zulip/log.dart';
+import 'package:zulip/model/actions.dart';
 import 'package:zulip/model/database.dart';
 import 'package:zulip/widgets/app.dart';
 import 'package:zulip/widgets/home.dart';
@@ -57,6 +58,39 @@ void main() {
     });
   });
 
+  group('_EmptyStackNavigatorObserver', () {
+    late List<Route<void>> pushedRoutes;
+    late List<Route<void>> removedRoutes;
+
+    Future<void> prepare(WidgetTester tester) async {
+      addTearDown(testBinding.reset);
+
+      pushedRoutes = [];
+      removedRoutes = [];
+      final testNavObserver = TestNavigatorObserver();
+      testNavObserver.onPushed = (route, prevRoute) => pushedRoutes.add(route);
+      testNavObserver.onRemoved = (route, prevRoute) => removedRoutes.add(route);
+
+      await tester.pumpWidget(ZulipApp(navigatorObservers: [testNavObserver]));
+      await tester.pump(); // start to load account
+      check(pushedRoutes).single.isA<WidgetRoute>().page.isA<HomePage>();
+      pushedRoutes.clear();
+    }
+
+    testWidgets('push route when removing last route on stack', (tester) async {
+      await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+      await prepare(tester);
+
+      final future = logOutAccount(testBinding.globalStore, eg.selfAccount.id);
+      await tester.pump(TestGlobalStore.removeAccountDuration);
+      await future;
+      check(testBinding.globalStore.takeDoRemoveAccountCalls())
+        .single.equals(eg.selfAccount.id);
+      check(removedRoutes).single.isA<WidgetRoute>().page.isA<HomePage>();
+      check(pushedRoutes).single.isA<WidgetRoute>().page.isA<ChooseAccountPage>();
+    });
+  });
+
   group('ChooseAccountPage', () {
     Future<void> setupChooseAccountPage(WidgetTester tester, {
       required List<Account> accounts,