{beamer_delegate}: [add] updateListenable

	- closes #467

package/lib/src/beamer_delegate.dart

@@ -17,6 +17,7 @@ class BeamerDelegate extends RouterDelegate<RouteInformation>
     this.initialPath = '/',
     this.routeListener,
     this.buildListener,
+    this.updateListenable,
     @Deprecated(
       'No longer used by this package, please remove any references to it. '
       'This feature was deprecated after v1.0.0.',
@@ -42,6 +43,8 @@ class BeamerDelegate extends RouterDelegate<RouteInformation>
     );
 
     configuration = RouteInformation(location: initialPath);
+
+    updateListenable?.addListener(_update);
   }
 
   /// A state of this delegate. This is the `routeInformation` that goes into
@@ -136,6 +139,10 @@ class BeamerDelegate extends RouterDelegate<RouteInformation>
   /// are updated. it receives a reference to this delegate.
   final void Function(BuildContext, BeamerDelegate)? buildListener;
 
+  /// A Listenable to which an update listener will be added, i.e.
+  /// [update] will be called when listeners are notified.
+  final Listenable? updateListenable;
+
   @Deprecated(
     'No longer used by this package, please remove any references to it. '
     'This feature was deprecated after v1.0.0.',
@@ -979,6 +986,8 @@ class BeamerDelegate extends RouterDelegate<RouteInformation>
     _beamLocationCandidate = location;
   }
 
+  void _update() => update();
+
   void _updateFromParent({bool rebuild = true}) {
     update(
       configuration: _parent!.configuration.copyWith(),
@@ -1009,6 +1018,7 @@ class BeamerDelegate extends RouterDelegate<RouteInformation>
     _parent?.removeListener(_updateFromParent);
     _disposeBeamLocation(currentBeamLocation);
     currentBeamLocation.dispose();
+    updateListenable?.removeListener(_update);
     super.dispose();
   }
 }

package/test/beamer_delegate_test.dart

@@ -619,4 +619,38 @@ void main() {
       expect(delegate.beamingHistory.last.history.length, 1);
     });
   });
+
+  testWidgets('updateListenable', (tester) async {
+    final guardCheck = ValueNotifier<bool>(true);
+
+    final delegate = BeamerDelegate(
+      initialPath: '/r1',
+      locationBuilder: RoutesLocationBuilder(
+        routes: {
+          '/r1': (context, state, data) => Container(),
+          '/r2': (context, state, data) => Container(),
+        },
+      ),
+      guards: [
+        BeamGuard(
+          pathPatterns: ['/r1'],
+          check: (_, __) => guardCheck.value,
+          beamToNamed: (_, __) => '/r2',
+        ),
+      ],
+      updateListenable: guardCheck,
+    );
+    await tester.pumpWidget(
+      MaterialApp.router(
+        routeInformationParser: BeamerParser(),
+        routerDelegate: delegate,
+      ),
+    );
+
+    expect(delegate.configuration.location, '/r1');
+
+    guardCheck.value = false;
+
+    expect(delegate.configuration.location, '/r2');
+  });
 }