Add router instance to OnEnter callback

Provides access to GoRouter within OnEnter callback to support
navigation during early routing stages when InheritedGoRouter
is not yet available in the widget tree.

packages/go_router/example/lib/top_level_on_enter.dart

@@ -55,8 +55,12 @@ class App extends StatelessWidget {
       /// 1. Block navigation and perform actions (return false)
       /// 2. Allow navigation to proceed (return true)
       /// 3. Show loading states during async operations
-      onEnter: (BuildContext context, GoRouterState currentState,
-          GoRouterState nextState) {
+      onEnter: (
+        BuildContext context,
+        GoRouterState currentState,
+        GoRouterState nextState,
+        GoRouter goRouter,
+      ) {
         // Track analytics for deep links
         if (nextState.uri.hasQuery || nextState.uri.hasFragment) {
           _handleDeepLinkTracking(nextState.uri);

packages/go_router/lib/src/configuration.dart

@@ -27,6 +27,7 @@ typedef OnEnter = bool Function(
   BuildContext context,
   GoRouterState currentState,
   GoRouterState nextState,
+  GoRouter goRouter,
 );
 
 /// The route configuration for GoRouter configured by the app.

packages/go_router/lib/src/parser.dart

@@ -32,8 +32,10 @@ class GoRouteInformationParser extends RouteInformationParser<RouteMatchList> {
   GoRouteInformationParser({
     required this.configuration,
     required String? initialLocation,
+    required GoRouter fallbackRouter,
     required this.onParserException,
-  })  : _routeMatchListCodec = RouteMatchListCodec(configuration),
+  })  : _fallbackRouter = fallbackRouter,
+        _routeMatchListCodec = RouteMatchListCodec(configuration),
         _initialLocation = initialLocation;
 
   /// The route configuration used for parsing [RouteInformation]s.
@@ -52,6 +54,25 @@ class GoRouteInformationParser extends RouteInformationParser<RouteMatchList> {
   /// Store the last successful match list so we can truly "stay" on the same route.
   RouteMatchList? _lastMatchList;
 
+  /// The fallback [GoRouter] instance used during route information parsing.
+  ///
+  /// During initial app launch or deep linking, route parsing may occur before the
+  /// [InheritedGoRouter] is built in the widget tree. This makes [GoRouter.of] or
+  /// [GoRouter.maybeOf] unavailable through [BuildContext].
+  ///
+  /// When route parsing happens in these early stages, [_fallbackRouter] ensures that
+  /// navigation APIs remain accessible to features like [OnEnter], which may need to
+  /// perform navigation before the widget tree is fully built.
+  ///
+  /// This is used internally by [GoRouter] to pass its own instance as
+  /// the fallback. You typically don't need to provide this when constructing a
+  /// [GoRouteInformationParser] directly.
+  ///
+  /// See also:
+  ///  * [parseRouteInformationWithDependencies], which uses this fallback router
+  ///    when [BuildContext]-based router access is unavailable.
+  final GoRouter _fallbackRouter;
+
   /// The future of current route parsing.
   ///
   /// This is used for testing asynchronous redirection.
@@ -116,6 +137,7 @@ class GoRouteInformationParser extends RouteInformationParser<RouteMatchList> {
         context,
         currentState,
         nextState,
+        GoRouter.maybeOf(context) ?? _fallbackRouter,
       );
 
       // If navigation was intercepted (canEnter == false):

packages/go_router/lib/src/router.dart

@@ -282,6 +282,7 @@ class GoRouter implements RouterConfig<RouteMatchList> {
       onParserException: parserExceptionHandler,
       configuration: configuration,
       initialLocation: initialLocation,
+      fallbackRouter: this,
     );
 
     routeInformationProvider = GoRouteInformationProvider(

packages/go_router/test/parser_test.dart

@@ -647,8 +647,12 @@ void main() {
 
       final GoRouter router = GoRouter(
         initialLocation: '/',
-        onEnter:
-            (BuildContext context, GoRouterState current, GoRouterState next) {
+        onEnter: (
+          BuildContext context,
+          GoRouterState current,
+          GoRouterState next,
+          GoRouter goRouter,
+        ) {
           onEnterCallCount++;
           capturedCurrentState = current;
           capturedNextState = next;