[go_router] Refactor parser and on_enter for clarity and type safety

Simplified the OnEnterResult class documentation and removed factory constructors from the sealed class. Introduced type aliases for navigation callbacks and route information state in parser.dart, and updated method signatures to use these aliases for improved readability and type safety. Also streamlined fallback logic when navigation is blocked.

packages/go_router/lib/src/on_enter.dart

@@ -7,24 +7,14 @@ import 'dart:async';
 /// The result of an onEnter callback.
 ///
 /// This sealed class represents the possible outcomes of navigation interception.
-/// Being sealed, it can only be extended within this library, ensuring a controlled
-/// set of result types while still allowing construction via factory constructors
-/// and the public concrete subtypes [Allow] and [Block].
+/// This class can't be extended. One must use one of its subtypes, [Allow] or
+/// [Block], to indicate the result.
 sealed class OnEnterResult {
   /// Creates an [OnEnterResult].
   const OnEnterResult({this.then});
 
-  /// Creates an [Allow] result that allows navigation to proceed.
-  ///
-  /// The [then] callback is executed after the navigation is allowed.
-  const factory OnEnterResult.allow({FutureOr<void> Function()? then}) = Allow;
-
-  /// Creates a [Block] result that blocks navigation from proceeding.
-  ///
-  /// The [then] callback is executed after the navigation is blocked.
-  const factory OnEnterResult.block({FutureOr<void> Function()? then}) = Block;
-
-  /// Executed after the decision is committed. Errors are reported and do not revert navigation.
+  /// Executed after the decision is committed.
+  /// Errors are reported and do not revert navigation.
   final FutureOr<void> Function()? then;
 }
 

packages/go_router/lib/src/parser.dart

@@ -26,6 +26,12 @@ typedef ParserExceptionHandler =
       RouteMatchList routeMatchList,
     );
 
+/// The function signature for navigation callbacks in [_OnEnterHandler].
+typedef NavigationCallback = Future<RouteMatchList> Function();
+
+/// Type alias for route information state with dynamic type parameter.
+typedef RouteInfoState = RouteInformationState<dynamic>;
+
 /// Converts between incoming URLs and a [RouteMatchList] using [RouteMatcher].
 ///
 /// Also integrates the top-level `onEnter` guard and then performs legacy
@@ -121,17 +127,11 @@ class GoRouteInformationParser extends RouteInformationParser<RouteMatchList> {
       infoState: infoState,
       onCanEnter: () => _navigate(routeInformation, context, infoState),
       onCanNotEnter: () {
-        // If navigation is blocked, return the last successful match or a fallback.
+        // If navigation is blocked, return the last successful match or empty.
         if (_lastMatchList != null) {
           return SynchronousFuture<RouteMatchList>(_lastMatchList!);
         } else {
-          final Uri defaultUri = Uri.parse(_initialLocation ?? '/');
-          final RouteMatchList fallbackMatches = configuration.findMatch(
-            defaultUri,
-            extra: infoState.extra,
-          );
-          _lastMatchList = fallbackMatches;
-          return SynchronousFuture<RouteMatchList>(fallbackMatches);
+          return SynchronousFuture<RouteMatchList>(RouteMatchList.empty);
         }
       },
     );
@@ -145,7 +145,7 @@ class GoRouteInformationParser extends RouteInformationParser<RouteMatchList> {
   Future<RouteMatchList> _navigate(
     RouteInformation routeInformation,
     BuildContext context,
-    RouteInformationState<dynamic> infoState,
+    RouteInfoState infoState,
   ) {
     // Normalize the URI: ensure it has a valid path and remove trailing slashes.
     Uri uri = routeInformation.uri;
@@ -375,9 +375,9 @@ class _OnEnterHandler {
   Future<RouteMatchList> handleTopOnEnter({
     required BuildContext context,
     required RouteInformation routeInformation,
-    required RouteInformationState<dynamic> infoState,
-    required Future<RouteMatchList> Function() onCanEnter,
-    required Future<RouteMatchList> Function() onCanNotEnter,
+    required RouteInfoState infoState,
+    required NavigationCallback onCanEnter,
+    required NavigationCallback onCanNotEnter,
   }) {
     final OnEnter? topOnEnter = _configuration.topOnEnter;
     // If no onEnter is configured, allow navigation immediately.
@@ -577,7 +577,7 @@ class _OnEnterHandler {
   RouteMatchList? _redirectionErrorMatchList(
     BuildContext context,
     Uri redirectedUri,
-    RouteInformationState<dynamic> infoState,
+    RouteInfoState infoState,
   ) {
     _redirectionHistory.add(redirectedUri);
     if (_redirectionHistory.length > _configuration.redirectLimit) {