Avoid retaining routes when subscriptions are cleared (#88310)

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

@@ -1733,6 +1733,19 @@ abstract class PopupRoute<T> extends ModalRoute<T> {
 class RouteObserver<R extends Route<dynamic>> extends NavigatorObserver {
   final Map<R, Set<RouteAware>> _listeners = <R, Set<RouteAware>>{};
 
+  /// Whether this observer is managing changes for the specified route.
+  ///
+  /// If asserts are disabled, this method will throw an exception.
+  @visibleForTesting
+  bool debugObservingRoute(R route) {
+    late bool contained;
+    assert(() {
+      contained = _listeners.containsKey(route);
+      return true;
+    }());
+    return contained;
+  }
+
   /// Subscribe [routeAware] to be informed about changes to [route].
   ///
   /// Going forward, [routeAware] will be informed about qualifying changes
@@ -1753,9 +1766,15 @@ class RouteObserver<R extends Route<dynamic>> extends NavigatorObserver {
   /// subscribed to multiple types, this will unregister it (once) from each type.
   void unsubscribe(RouteAware routeAware) {
     assert(routeAware != null);
-    for (final R route in _listeners.keys) {
+    final List<R> routes = _listeners.keys.toList();
+    for (final R route in routes) {
       final Set<RouteAware>? subscribers = _listeners[route];
-      subscribers?.remove(routeAware);
+      if (subscribers != null) {
+        subscribers.remove(routeAware);
+        if (subscribers.isEmpty) {
+          _listeners.remove(route);
+        }
+      }
     }
   }
 

packages/flutter/test/widgets/routes_test.dart

@@ -525,6 +525,33 @@ void main() {
       expect(pageRouteAware.didPushCount, 2);
       expect(pageRouteAware.didPopCount, 0);
     });
+
+    test('releases reference to route when unsubscribed', () {
+      final RouteObserver<PageRoute<dynamic>> observer = RouteObserver<PageRoute<dynamic>>();
+      final MockRouteAware pageRouteAware = MockRouteAware();
+      final MockRouteAware page2RouteAware = MockRouteAware();
+      final MockPageRoute pageRoute = MockPageRoute();
+      final MockPageRoute nextPageRoute = MockPageRoute();
+      observer.subscribe(pageRouteAware, pageRoute);
+      observer.subscribe(pageRouteAware, nextPageRoute);
+      observer.subscribe(page2RouteAware, pageRoute);
+      observer.subscribe(page2RouteAware, nextPageRoute);
+      expect(pageRouteAware.didPushCount, 2);
+      expect(page2RouteAware.didPushCount, 2);
+
+      expect(observer.debugObservingRoute(pageRoute), true);
+      expect(observer.debugObservingRoute(nextPageRoute), true);
+
+      observer.unsubscribe(pageRouteAware);
+
+      expect(observer.debugObservingRoute(pageRoute), true);
+      expect(observer.debugObservingRoute(nextPageRoute), true);
+
+      observer.unsubscribe(page2RouteAware);
+
+      expect(observer.debugObservingRoute(pageRoute), false);
+      expect(observer.debugObservingRoute(nextPageRoute), false);
+    });
   });
 
   testWidgets('Can autofocus a TextField nested in a Focus in a route.', (WidgetTester tester) async {