fix route annoucement for first route and last route (#57339)

packages/flutter/lib/src/widgets/navigator.dart

@@ -2293,6 +2293,12 @@ enum _RouteLifecycle {
 
 typedef _RouteEntryPredicate = bool Function(_RouteEntry entry);
 
+class _NotAnnounced extends Route<void> {
+  // A placeholder for the lastAnnouncedPreviousRoute, the
+  // lastAnnouncedPoppedNextRoute, and the lastAnnouncedNextRoute before any
+  // change has been announced.
+}
+
 class _RouteEntry extends RouteTransitionRecord {
   _RouteEntry(
     this.route, {
@@ -2311,10 +2317,12 @@ class _RouteEntry extends RouteTransitionRecord {
   @override
   final Route<dynamic> route;
 
+  static Route<dynamic> notAnnounced = _NotAnnounced();
+
   _RouteLifecycle currentState;
-  Route<dynamic> lastAnnouncedPreviousRoute; // last argument to Route.didChangePrevious
-  Route<dynamic> lastAnnouncedPoppedNextRoute; // last argument to Route.didPopNext
-  Route<dynamic> lastAnnouncedNextRoute; // last argument to Route.didChangeNext
+  Route<dynamic> lastAnnouncedPreviousRoute = notAnnounced; // last argument to Route.didChangePrevious
+  Route<dynamic> lastAnnouncedPoppedNextRoute = notAnnounced; // last argument to Route.didPopNext
+  Route<dynamic> lastAnnouncedNextRoute = notAnnounced; // last argument to Route.didChangeNext
 
   bool get hasPage => route.settings is Page;
 

packages/flutter/test/widgets/navigator_test.dart

@@ -1840,6 +1840,57 @@ void main() {
     expect(tickCount, 4);
   });
 
+  testWidgets('Route annouce correctly for first route and last route', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/57133.
+    Route<void> previousOfFirst = NotAnnounced();
+    Route<void> nextOfFirst = NotAnnounced();
+    Route<void> popNextOfFirst = NotAnnounced();
+    Route<void> firstRoute;
+
+    Route<void> previousOfSecond = NotAnnounced();
+    Route<void> nextOfSecond = NotAnnounced();
+    Route<void> popNextOfSecond = NotAnnounced();
+    Route<void> secondRoute;
+
+    final GlobalKey<NavigatorState> navigator = GlobalKey<NavigatorState>();
+    await tester.pumpWidget(
+      MaterialApp(
+        navigatorKey: navigator,
+        initialRoute: '/second',
+        onGenerateRoute: (RouteSettings settings) {
+          if (settings.name == '/') {
+            firstRoute = RouteAnnouncementSpy(
+              onDidChangeNext: (Route<void> next) => nextOfFirst = next,
+              onDidChangePrevious: (Route<void> previous) => previousOfFirst = previous,
+              onDidPopNext: (Route<void> next) => popNextOfFirst = next,
+              settings: settings,
+            );
+            return firstRoute;
+          }
+          secondRoute = RouteAnnouncementSpy(
+            onDidChangeNext: (Route<void> next) => nextOfSecond = next,
+            onDidChangePrevious: (Route<void> previous) => previousOfSecond = previous,
+            onDidPopNext: (Route<void> next) => popNextOfSecond = next,
+            settings: settings,
+          );
+          return secondRoute;
+        },
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(previousOfFirst, isNull);
+    expect(nextOfFirst, secondRoute);
+    expect(popNextOfFirst, isA<NotAnnounced>());
+
+    expect(previousOfSecond, firstRoute);
+    expect(nextOfSecond, isNull);
+    expect(popNextOfSecond, isA<NotAnnounced>());
+
+    navigator.currentState.pop();
+    expect(popNextOfFirst, secondRoute);
+  });
+
   group('Page api', (){
     Widget buildNavigator(
       List<Page<dynamic>> pages,
@@ -2561,6 +2612,50 @@ void main() {
   });
 }
 
+typedef AnnouncementCallBack = void Function(Route<dynamic>);
+
+class NotAnnounced extends Route<void> {/* A place holder for not announced route*/}
+
+class RouteAnnouncementSpy extends Route<void> {
+  RouteAnnouncementSpy({
+    this.onDidChangePrevious,
+    this.onDidChangeNext,
+    this.onDidPopNext,
+    RouteSettings settings,
+  }) : super(settings: settings);
+  final AnnouncementCallBack onDidChangePrevious;
+  final AnnouncementCallBack onDidChangeNext;
+  final AnnouncementCallBack onDidPopNext;
+
+  @override
+  List<OverlayEntry> get overlayEntries => <OverlayEntry>[
+    OverlayEntry(
+      builder: (BuildContext context) => const Placeholder(),
+    )
+  ];
+
+  @override
+  void didChangeNext(Route<dynamic> nextRoute) {
+    super.didChangeNext(nextRoute);
+    if (onDidChangeNext != null)
+      onDidChangeNext(nextRoute);
+  }
+
+  @override
+  void didChangePrevious(Route<dynamic> previousRoute) {
+    super.didChangePrevious(previousRoute);
+    if (onDidChangePrevious != null)
+      onDidChangePrevious(previousRoute);
+  }
+
+  @override
+  void didPopNext(Route<dynamic> nextRoute) {
+    super.didPopNext(nextRoute);
+    if (onDidPopNext != null)
+      onDidPopNext(nextRoute);
+  }
+}
+
 class _TickingWidget extends StatefulWidget {
   const _TickingWidget({this.onTick});