fixes CupertinoModalPopupRoute

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

@@ -1096,7 +1096,7 @@ class CupertinoModalPopupRoute<T> extends PopupRoute<T> {
   @override
   Duration get transitionDuration => _kModalPopupTransitionDuration;
 
-  Animation<double>? _animation;
+  CurvedAnimation? _animation;
 
   late Tween<Offset> _offsetTween;
 
@@ -1142,6 +1142,12 @@ class CupertinoModalPopupRoute<T> extends PopupRoute<T> {
       ),
     );
   }
+
+  @override
+  void dispose() {
+    _animation?.dispose();
+    super.dispose();
+  }
 }
 
 /// Shows a modal iOS-style popup that slides up from the bottom of the screen.
@@ -1238,26 +1244,6 @@ Future<T?> showCupertinoModalPopup<T>({
 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,
-    ),
-  );
-}
-
 /// Displays an iOS-style dialog above the current contents of the app, with
 /// iOS-style entrance and exit animations, modal barrier color, and modal
 /// barrier behavior (by default, the dialog is not dismissible with a tap on
@@ -1382,7 +1368,6 @@ 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,
     super.settings,
     super.anchorPoint,
   }) : super(
@@ -1392,4 +1377,33 @@ class CupertinoDialogRoute<T> extends RawDialogRoute<T> {
         barrierLabel: barrierLabel ?? CupertinoLocalizations.of(context).modalBarrierDismissLabel,
         barrierColor: barrierColor ?? CupertinoDynamicColor.resolve(kCupertinoModalBarrierColor, context),
       );
+
+  CurvedAnimation? _fadeAnimation;
+
+  @override
+  Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
+    _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,
+    ),
+  );
+  }
+
+  @override
+  void dispose() {
+    _fadeAnimation?.dispose();
+    super.dispose();
+  }
 }

packages/flutter/test/cupertino/route_test.dart

@@ -2479,6 +2479,61 @@ void main() {
 
     await tester.pump(const Duration(milliseconds: 400));
   });
+
+  testWidgets('CupertinoModalPopupRoute and 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 showCupertinoModalPopup<void>(
+                    context: context,
+                    semanticsDismissible: true,
+                    builder: (BuildContext context) => const SizedBox(),
+                  );
+                },
+                child: const Text('tap'),
+              );
+            },
+          );
+        },
+      ),
+    ));
+
+    // Push the route.
+    await tester.tap(find.text('tap'));
+    await tester.pumpAndSettle();
+
+    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 {