feat(navigation): expose the previous route in navigation service (#9)

packages/fluorflow/lib/src/navigation/navigation_service.dart

@@ -55,11 +55,18 @@ class NavigationService {
 
   /// Returns the current route name (or an empty string if no route
   /// matches).
+  /// The whole route object (for more advanced usecases) can be accessed
+  /// via the `currentRoute` property of the observer.
   String get currentRoute => _observer.currentRoute?.settings.name ?? '';
 
   /// Returns the current route arguments (`dynamic` typed).
   dynamic get currentArguments => _observer.currentRoute?.settings.arguments;
 
+  /// Returns the previous route name (or `null` if no previous route exists).
+  /// The whole route object (for more advanced usecases) can be accessed
+  /// via the `previousRoute` property of the observer.
+  String? get previousRoute => _observer.previousRoute?.settings.name;
+
   /// Navigate "back" and return an optional result.
   void back<T>([T? result]) => navigatorKey.currentState?.canPop() == true
       ? navigatorKey.currentState!.pop(result)

packages/fluorflow/lib/src/navigation/navigator_observer.dart

@@ -36,6 +36,19 @@ final class FluorflowNavigatorObserver extends NavigatorObserver {
   Route<dynamic>? get currentRoute =>
       _history.where((r) => r.isCurrent).firstOrNull;
 
+  Route<dynamic>? get previousRoute {
+    if (_history.length < 2) {
+      return null;
+    }
+
+    final current = currentRoute;
+    if (current == null) {
+      return null;
+    }
+
+    return _history[_history.indexOf(current) - 1];
+  }
+
   @override
   void didPush(Route<dynamic> route, Route<dynamic>? previousRoute) {
     _history.add(route);

packages/fluorflow/test/navigation/navigation_service_test.dart

@@ -91,6 +91,27 @@ void main() {
       expect(nav.currentArguments['foo'], 'bar');
     });
 
+    testWidgets('should return no previous route if none exists.',
+        (tester) async {
+      await tester.pumpWidget(_app());
+      await tester.pumpAndSettle();
+
+      const nav = NavigationService();
+
+      expect(nav.previousRoute, null);
+    });
+
+    testWidgets('should return previous route.', (tester) async {
+      await tester.pumpWidget(_app());
+      await tester.pumpAndSettle();
+
+      const nav = NavigationService();
+      nav.navigateTo('/two');
+      await tester.pumpAndSettle();
+
+      expect(nav.previousRoute, '/one');
+    });
+
     testWidgets('should correctly navigate back.', (tester) async {
       await tester.pumpWidget(_app());
       await tester.pumpAndSettle();