CupertinoDialogRoute leak fix (#148774)

packages/flutter/lib/src/cupertino/route.dart

@@ -1245,23 +1245,7 @@ final Animatable<double> _dialogScaleTween = Tween<double>(begin: 1.3, end: 1.0)
   .chain(CurveTween(curve: Curves.linearToEaseOut));
 
 Widget _buildCupertinoDialogTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
-  final CurvedAnimation fadeAnimation = CurvedAnimation(
-    parent: animation,
-    curve: Curves.easeInOut,
-  );
-  if (animation.status == AnimationStatus.reverse) {
-    return FadeTransition(
-      opacity: fadeAnimation,
-      child: child,
-    );
-  }
-  return FadeTransition(
-    opacity: fadeAnimation,
-    child: ScaleTransition(
-      scale: animation.drive(_dialogScaleTween),
-      child: child,
-    ),
-  );
+  return child;
 }
 
 /// Displays an iOS-style dialog above the current contents of the app, with
@@ -1388,14 +1372,58 @@ class CupertinoDialogRoute<T> extends RawDialogRoute<T> {
     String? barrierLabel,
     // This transition duration was eyeballed comparing with iOS
     super.transitionDuration = const Duration(milliseconds: 250),
-    super.transitionBuilder = _buildCupertinoDialogTransitions,
+    this.transitionBuilder,
     super.settings,
     super.anchorPoint,
   }) : super(
         pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
           return builder(context);
         },
+        transitionBuilder: transitionBuilder ?? _buildCupertinoDialogTransitions,
         barrierLabel: barrierLabel ?? CupertinoLocalizations.of(context).modalBarrierDismissLabel,
         barrierColor: barrierColor ?? CupertinoDynamicColor.resolve(kCupertinoModalBarrierColor, context),
       );
+
+  /// Custom transition builder
+  RouteTransitionsBuilder? transitionBuilder;
+
+  CurvedAnimation? _fadeAnimation;
+
+  @override
+  Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
+
+    if (transitionBuilder != null) {
+      return super.buildTransitions(context, animation, secondaryAnimation, child);
+    }
+
+    if (_fadeAnimation?.parent != animation) {
+      _fadeAnimation?.dispose();
+      _fadeAnimation = CurvedAnimation(
+        parent: animation,
+        curve: Curves.easeInOut,
+      );
+    }
+
+    final CurvedAnimation fadeAnimation = _fadeAnimation!;
+
+    if (animation.status == AnimationStatus.reverse) {
+      return FadeTransition(
+        opacity: fadeAnimation,
+        child: super.buildTransitions(context, animation, secondaryAnimation, child),
+      );
+    }
+    return FadeTransition(
+      opacity: fadeAnimation,
+      child: ScaleTransition(
+        scale: animation.drive(_dialogScaleTween),
+        child: super.buildTransitions(context, animation, secondaryAnimation, child),
+      ),
+    );
+  }
+
+  @override
+  void dispose() {
+    _fadeAnimation?.dispose();
+    super.dispose();
+  }
 }

packages/flutter/test/cupertino/route_test.dart

@@ -2696,6 +2696,36 @@ void main() {
     await tester.tap(find.text('tap'));
     await tester.pumpAndSettle();
   });
+
+  testWidgets('CupertinoDialogRoute does not leak CurveAnimation',
+  // TODO(polina-c): remove when fixed https://github.com/flutter/flutter/issues/145600 [leak-tracking-opt-in]
+  experimentalLeakTesting: LeakTesting.settings.withTracked(classes: <String>['CurvedAnimation']),
+  (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Navigator(
+        onGenerateRoute: (RouteSettings settings) {
+          return PageRouteBuilder<dynamic>(
+            pageBuilder: (BuildContext context, Animation<double> _, Animation<double> __) {
+              return GestureDetector(
+                onTap: () async {
+                  await showCupertinoDialog<void>(
+                    context: context,
+                    useRootNavigator: false,
+                    builder: (BuildContext context) => const SizedBox(),
+                  );
+                },
+                child: const Text('tap'),
+              );
+            },
+          );
+        },
+      ),
+    ));
+
+    // Open the dialog.
+    await tester.tap(find.text('tap'));
+    await tester.pumpAndSettle();
+  });
 }
 
 class MockNavigatorObserver extends NavigatorObserver {